Topic: Critical phenomena in Optics - Graduiertenkolleg 695

Graduiertenkolleg 695Nichtlinearitäten optischer MaterialienArbeits- und Ergebnisbericht01.01.2001 – 31.03.2003Teil 1(Anhänge sind im Teil 2 zu finden)Gefördert vonder Deutschen Forschungsgemeinschaftund dem Land Niedersachsen1

1. Umsetzung der Zielsetzung und Konzeption des Kollegs1.1 Kurzberichte zu den Forschungsbeiträgen der beteiligten HochschullehrerProf. Dr. Klaus Bärwinkel,Apl. Prof. Dr. Heinz-Jürgen Schmidt,Priv.-Doz. Dr. Jürgen SchnackForschungsübersichtNeben den bisherigen Arbeitsgebieten (Transporttheorie, Quantenthermostaten,Thermodynamik kleiner Quantensysteme, Fermionische Molekulardynamik, Relativitätstheorie)ist seit 1999 die theoretische Behandlung kleiner Spinsysteme und magnetischerMoleküle ein neuer Schwerpunkt der Arbeitsgruppe. Hier werden mit exaktenund approximativen Methoden die Eigenschaften des Energiespektrums undthermodynamische Eigenschaften einschließlich Spin-Spin-Korrelationsfunktionenuntersucht. Wichtige Teilergebnisse sind z. B. Regeln für die k-Quantenzahlen vonrelativen Grundzuständen in Spinringen, die Entdeckung spezieller exakter Grundzustände(independent magnon states), die zu makroskopischen Magnetisierungssprüngenführen sowie die Erklärung von Rotationsbändern im Spektrum vieler Spinsysteme.Forschung im KollegSeit 2001 werden im Zusammenhang mit dem Graduiertenkolleg 695 zusätzlich optischeund magnetische Solitonen untersucht, wobei durchaus inhaltliche und methodischeZusammenhänge mit den oben genannten Themen bestehen. Für die Untersuchungvon magnetischen Solitonen (Stipendiat Pavlo Shchelokovskyy) ist dies offensichtlich:Hier sollen quantenmechanische Analoga zu den bekannten klassischenSolitonen in Spinringen gefunden und deren experimentelle Realisierung diskutiertwerden.Ein zweites Projekt (Stipendiat Felix Homann) ist der Entwicklung von Näherungsmethodenfür die Beschreibung von Solitonen gewidmet, die auch in solchen Fällen Ergebnisseliefert, in denen die exakten Methoden der inversen Streutheorie versagen.Hier werden Ideen aus der Fermionischen Molekulardynamik und Methoden der analytischenMechanik Hamiltonscher Systeme verwendet.Kooperationen im KollegNeben der engen Zusammenarbeit innerhalb der Arbeitsgruppe gab es Berührungspunkteund Diskussionen mit den von Prof. Schürmann betreuten Projekten und Kollegiat(inn)ensowie mit der Arbeitsgruppe von Prof. Ringhofer zum Thema „Metamaterialien“.Publikationen im Zusammenhang mit dem Graduiertenkolleg (ab 2000)K. Bärwinkel, H.-J. Schmidt, J. Schnack, Energy bounds for n-partite spin systems,Eur. Phys. J. B (2003) submitted4

H.-J. Schmidt, J. Schnack, Symmetric polynomials in physics,Plenary talk at the G24 conference, Paris, 2002, contribution to the proceedings 2003H.-J. Schmidt, M. Luban, Classical ground states of symmetrical Heisenberg spinsystems, J. Phys. A: Math. Gen. (2003) submittedM. Exler, J. Schnack, Evaluation of the low-lying energy spectrum of magnetic Kepleratemolecules with DMRG, Phys. Rev. B (2003), submittedH.-J. Schmidt, Linear energy bounds for Heisenberg spin systems, J. Phys. A: Math.Gen. 35 (2002) 6545-6555J. Schulenburg, A. Honecker, J. Schnack, J. Richter, H.-J. Schmidt, Macroscopicmagnetization jumps due to independent magnons in frustrated quantum spin lattices,Phys. Rev. Lett. 88 (2002) 167207H.-J. Schmidt, J. Schnack, Partition functions and symmetric polynomials,Am. J. Phys. 70 (2002) 53-57J. Schnack, H.-J. Schmidt, J. Richter, J. Schulenburg, Independent magnon states onmagnetic polytopes, Eur. Phys. J. B 24 (2001) 475J. Schnack, M. Luban, R. Modler, Quantum rotational band model for the Heisenbergmolecular magnet Mo_72Fe_30, Europhysics Letters 56 (2001) 863D. Mentrup, J. Schnack, Isothermal quantum dynamics: Nosé-Hoover method forcoherent states, in Advances in Quantum Many-Body Theory, Proceedings of "The11th International Conferences on Recent Progress in Many-Body Theories",Manchester, edited by Raymond F. Bishop, Tobias Brandes, Klaus A. Gernoth, NielsR. Walet, and Yang Xian (UMIST, Manchester, UK, 2001), World ScientificJ. Schnack, M. Luban, R. Modler, Rotational band structure of low-lying excitations insmall Heisenberg systems, in Advances in Quantum Many-Body Theory, Proceedingsof "The 11th International Conferences on Recent Progress in Many-BodyTheories", Manchester, edited by Raymond F. Bishop, Tobias Brandes, Klaus A.Gernoth, Niels R. Walet, and Yang Xian (UMIST, Manchester, UK, 2001), World ScientificH.-J. Schmidt, J. Schnack, M. Luban, Heisenberg exchange parameters of molecularmagnets from the high-temperature susceptibility expansion, Phys. Rev. B 64 (2001)224415A. Müller, M. Luban, C. Schröder, R. Modler, P. Kögerler, M. Axenovich, J. Schnack,P.C. Canfield, S. Bud'ko, and Neil Harrison, Classical and Quantum Magnetism inGiant Keplerate Magnetic Molecules, Chem. Phys. Chem. 2 (2001) 517D. Mentrup, J. Schnack, Nose-Hoover dynamics for coherent states, Physica A 297(2001) 337-347H.-J. Schmidt, J. Schnack, M. Luban, Bounding and approximating parabolas for thespectrum of Heisenberg spin systems, Europhysics Letters 55 (2001) 105 –1115

H.-J. Schmidt, M. Luban, Continuous families of isospectral Heisenberg spin systemsand the limits of inference from measurements, J. Phys. A: Math. Gen. 34 (2001)2839-2858J. Schnack, M. Luban, Rotational modes in molecular magnets with antiferromagneticHeisenberg exchange, Phys. Rev. B 63 (2001) 014418J. Schnack, Properties of the first excited state of nonbipartite Heisenberg spin rings,Phys. Rev. B 62 (2000) 14855-14859H.-J. Schmidt, F. Homann, Photon Stars, General Relativity and Gravitation (GRG)Vol. 32, No. 5 (May 2000) 919 – 931H. Feldmeier, J. Schnack, Molecular Dynamics for Fermions, Rev. Mod. Phys. 72(2000) 655-688K. Bärwinkel, H.-J. Schmidt, J. Schnack, Ground state properties of antiferromagneticHeisenberg spin rings, Journal of Magnetism and Magnetic Materials 220 (2000) 227D. Mentrup, H.-J. Schmidt, J. Schnack, M. Luban, Transition from quantum to classicalHeisenberg trimers: Thermodynamics and time correlation functions,Physica A 278 (2000) 214-221Y. Furukawa, M. Luban, F. Borsa, D.C. Johnston, A.V. Mahajan, L.L. Miller, D. Mentrup,J. Schnack, A. Bino, Spin dynamics of the magnetic cluster[Cr_4S(O_2CCH_3)_8(H_2O)_4](NO_3)_2H_2O, Phys. Rev. B 61 (2000) 8635K. Bärwinkel, H.-J. Schmidt, J. Schnack, Structure and relevant dimension of theHeisenberg model and applications to spin rings, Journal of Magnetism and MagneticMaterials 212 (2000) 240-2506

Publikationen im Zusammenhang mit dem GraduiertenkollegK. Betzler, H. Hesse, R. Jaquet, D. Lammers: Optical second-harmonic generation inlead formate. J. Appl. Physics 87, 22 (2000).D. Xue, K. Betzler, H. Hesse, D. Lammers, S. Zhang: Theoretical studies of nonlinearoptical properties of compounds K 4 Ln 2 (CO 3 ) 3 F 4 (Ln=Pr, Nd, Sm, Eu, Gd). J. Appl.Physics 87, 2849 (2000).D. Xue, K. Betzler, H. Hesse: Structural characteristics and second order nonlinearoptical properties of borate crystals. Trends in Optics and Photonics Series 34, 542(2000).D. Xue, K. Betzler, H. Hesse: Dielectric constants of binary rare-earth compounds. J.Phys.: Condens. Matter 12, 3113 (2000).D. Xue, K. Betzler, H. Hesse, D. Lammers: Nonlinear optical properties of boratecrystals. Solid State Communications 114, 21 (2000).D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the second order nonlinearoptical behavior of Mg-doped lithium niobate. J. Phys.: Condens. Matter, 12, 6245(2000).D. Xue, K. Betzler, H. Hesse: Dielectric properties of lithium niobate-tantalate crystals.Solid State Communications, 115, 581 (2000).D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the second order nonlinearoptical behavior of Zn-doped lithium niobate. Optics Communications, 182, 167(2000).D. Lammers, K. Betzler, D. Xue, J. Zhao: Optical Second-Harmonic Generation inBenzophenone. physica status solidi (a) 180/2, R5 (2000).D. Xue, K. Betzler, H. Hesse: Dielectric properties of I-III-VI 2 type chalcopyrite semiconductors.Physical Review B15, 62, 13546 (2000).D. Xue, K. Betzler, H. Hesse, D. Lammers: Temperature dependence of the dielectricresponse of lithium niobate. J. Phys. Chem. Solids 62, 973 (2001).D. Xue, K. Betzler, H. Hesse: Second order nonlinear optical properties of In-dopedlithium niobate. J. Appl. Phys. 89, 849 (2001).D. Xue, K. Betzler, H. Hesse: Induced Li-site vacancies and nonlinear optical behaviorof doped lithium niobate crystals. Optical Materials 16, 381 (2001).D. Xue, K. Betzler: Influence of optical-damage resistant dopants on the nonlinearoptical properties of lithium niobate. Applied Physics B 72, 641 (2001).D. Xue, K. Betzler, H. Hesse, H. Ratajczak: Theoretical analysis of the chemicalbonding behavior and the dielectric properties of different phases of ice. Bulletin ofthe Polish Academy of Sciences Chemistry 49, 289 (2001).D. Xue, K. Betzler, H. Hesse, D. Lammers: Linear and nonlinear optical susceptibilitiesof orthorhombic rare earth molybdates RE 2 (MoO 4 ) 3 . J. Phys. Chem. Solids 63,359 (2002).D. Xue, K. Betzler, H. Hesse, H. Ratajczak: Chemical bond analysis on second ordernonlinear optical properties of Na 2 SeO 4·H 2 SeO 4·H 2 O. Bulletin of the Polish Academyof Sciences Chemistry 50, 289 (2002).D. Xue, K. Betzler, H. Hesse: Chemical bond analysis of the nonlinear optical propertiesof the borate crystals LiB 3 O 5 , CsLiB 6 O 10 , and CsB 3 O 5 . Applied Physics A 74,779 (2002).K.-U. Kasemir, K. Betzler: Detecting ellipses of limited eccentricity in images withhigh noise levels. Image and Vision Computing, 21, 223 (2002).Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:Composition dependence of the ultraviolet absorption edge in lithium tantalate.J. Appl. Physics, in print (2003).8

Prof. Dr. Peter HertelWegen der Übernahme des Amtes des Vizepräsidenten der Universität Osnabrückim Herbst 2000 konnte das Graduiertenkolleg lediglich durch die Vorlesung „Nonlinearresponse theory“ im WS 2001/02 sowie durch Diskussionen und Beratungenunterstützt werden.11

Prof. Dr. Siegmar KapphanForschungsübersichtDie Arbeitsgruppe "Laseroptik" beschäftigt sich mit störstellenrelevanten Problemen,insbesondere in oxidischen Kristallen die für photorefraktive und laseroptische Anwendungenvon Interesse sind. Mit Fourier-IR Spektroskopie UV-VISAbsorptionsmessungen, Lumineszenz und optischer Frequenzverdopplung werdenin einem großen Temperatur- und Spektralbereich die physikalischen Eigenschaftenund das Zusammenspiel von Störstellen und Wirtsgitter untersucht. Vor allem dieFourier IR-Spektroskopie hat sich in den letzten Jahren für die Untersuchung vonlichtinduzierten polaronischen Zuständen als sehr nützlich erwiesen, da diesePolaronen charakteristische optische Übergänge in Nah-Infrarot Bereich besitzen.Forschung im KollegDas Forschungsprojekt im Kolleg konzentriert sich auf lichtinduzierte Absorptionseffektein Sr x Ba 1-x Nb 2 O 6 (SBN) und in Ba 1-y Ca y TiO 3 (BCT) Kristallen. Diese Kristallekönnen in der Kristallzucht (Dr. Pankrath) in einer kongruenten Zusammensetzung(x = 0,61 für SBN und y = 0,23 für BCT) gezogen werden. In dieser Zusammensetzungsind große homogene Kristalle hoher Qualität möglich, die für Anwendungengute Voraussetzungen bietet. Die photorefraktiven Eigenschaften können durch passendeDotierungen (z. B. Cer und Chrom) erhöht und optimiert werden. Ein lichtinduzierterLadungstransport von den Dotierungsstörstellen zu polaronischen Zentren,der vereinfacht als Cr 3+ + Nb 5+ ↔ Cr 4+ + Nb 4+ (z.B. für SBN:Cr) skizziert werdenkann, ist als erster Schritt in der Kette identifiziert worden die zum Aufbau von Raumladungsfeldernführt, die den Brechungsindex modifizieren. Die Nb 4+ -Elektronenpolaronen (Ti 3+ -Polaronen in BCT) besitzen charakteristische breite Absorptionenim NIR deren Eigenschaften und nichtlineare Abhängigkeiten von Lichtintensität,Dotierung und Temperaturen es zu beschreiben gilt. Neben den NIR-Absorptionen treten weitere breite Absorptionen in VIS-Bereich auf, deren Natur sichnoch in der Diskussion befindet. Beide Zentrenarten zeigen bei Raumtemperatur einehohe thermisch-induzierte Beweglichkeit, die zu einer raschen Abnahme der Zentrenkonzentrationin den beleuchteten Zonen führt. Dieses thermisch-induzierte "Hopping"ist bei tiefen Temperaturen unterdrückt, so dass im Tieftemperaturbereich hinreichendgroße polaronische Zentrenkonzentration spektroskopisch untersucht werdenkönnen. Mit Frau Inna Kislova konnten wir kürzlich (2002) qualitativeUntersuchungen zu einer erstmals beobachteten Kr + -Laser induzierten Dissoziationvon VIS-Zentren durchführen, die als Dissoziationsprodukt freie Nb 4+ (bzw. Ti 3+ )Polaronen-Absorptionen ergab und damit erste experimentelle Hinweise auf dieNatur dieser VIS-Zentren. Frau Kislova beendete ihren Aufenthalt in Deutschland auspersönlichen, familiären Gründen und das Promotionsvorhaben musste dahervorzeitig abgebrochen werden (Okt. 2002).Die Untersuchungen zu diesem Zentren und insbesondere zu der Dissoziation derVIS-Zentren sollen mit Herrn A. Goubaev fortgesetzt (Beginn 12/02) und mit Methodender Photoleitfähigkeit und des Photo-Hall-Effektes ergänzt werden, um eine Klärungder lichtinduzierten Vorgänge und ihres Einflusses auf Aufbau (Einschreiben)und Speicherung von photorefraktiven Prozessen in diesen Materialien zu erzielen.Kooperationen im KollegEine enge Zusammenarbeit besteht insbesondere mit der Abteilung Kristallzüchtung(H. Hesse, R. Pankrath), die die benötigten dotierten Kristalle in hoher Qualität herzustellenvermögen. Darüber hinaus bestehen enge Kontakte mit den Forschungsgruppenvon E. Krätzig, K. Betzler und M. Wöhlecke für die zum Teil auch Messun-12

gen im IR Bereich mit dem Fourierspektrometer durchgeführt werden. Eine Kooperationzur theoretischen Modellbetrachtung von polaronischer Zentren und zu dennichtlinearen Eigenschaften optischer Materialien besteht mit Prof. Dr. V. Vikhnin undProf. V. Trepakov, Ioffe Inst., RAS, St. Petersburg, RusslandPublikationen im Zusammenhang mit dem GraduiertenkollegM. Gao, S. Kapphan, R. Pankrath, J. Zhao, ‘NIR-Absorption of Nb 4+ -Polarons in ReducedSBN-Crystals’, phys. stat. sol. (b), Vol. 217, 999 (2000)V. Vikhnin, S. Kapphan, H. Liu, W. Jia, V. Trepakov, L. Jastrabik, ‘Polaron andCharge Transfer Vibronic Exciton Phenomena in Ferroelectrics’, Ferroelectrics, 237,81-88 (2000)I. I. Tupitsyn, A. Deineka, V. Trepakov, L. Jastrabik and S. Kapphan, ‘Li-Doping Effecton the Energy Structure of KTaO 3 ’, Ferroelectrics, Vol. 237, 9-16 (2000)V. Trepakov, A. Skvortsov, S. Kapphan, L. Jastrabik and V. Vorlíček, ‘ComparativeStudies of Luminescence in Congruent and Stoichiometric VTE-Treated LiNbO 3 :Cr’,Ferroelectrics, Vol. 239, 297-304, 1167 - 1174 (2000)Ming Gao, S. Kapphan, R. Pankrath, Xiqi Feng, Yuanfen Tang, V. Vikhnin, ‘LightinducedVIS-absorption and light-induced charge transfer in pure and doped SBNcrystals’, J. of Phys. Chem. Sol., 61, 1775-1787 (2000)M. Gao, S. Kapphan, R. Pankrath, ‘Photoluminescence and thermoluminescence inSBN:Cr crystals’, J. of Phys. Chem. Sol., 61, 1959-1971 (2000)M. Gao, S.Kapphan, R. Pankrath, J. Zhao, ‘NIR Absorption of Nb 4+ Polarons in reducedSBN crystals’, Ferroelectrics, 239, 251-256, 1121 - 1126 (2000)S. A. Basun, A. A. Kaplyanskii, A. B. Kutsenko, V. Dierolf, T. Tröster, S. E. Kapphan,and K. Polgar, ‚Dominant Cr 3+ Centers in LiNbO 3 under Hydrostatic Pressure’ Phys.of Sol. State, Vol. 43, No. 6, 1043-1051 (2001)V. S. Vikhnin, R. I. Eglitis, E. A. Kotomin, S. Kapphan, G. Borstel, ‚New Polaronic-Type Excitons in Ferroelectric Oxides: INDO-Calculations and Experimental Manifestation’,Mat. Res. Soc. Symp. Proc., Vol. 677, AA 4.15.1 (2001)Zhao Jian-Lin, Wang Bin, Wu Jian-Jun, Yang De-Xing, S. Kapphan, R. Pankrath, ‚Investigationof photorefractive Two-Wave Coupling in Cr-doped Strontium BariumNiobate Crystal’, Chin. Phys. Soc., Vol. 10, 739 – 742 (2001)V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, E. A. Kotomin, G. Borstel, ‚A new phase inferroelectric oxides: The phase of charge transfer vibronic excitons’, Europhys. Lett.,56, 702 – 708 (2001)S. A. Basun, A. A. Kaplyanskii, A. B. Kutsenko, V. Dierolf, T. Troester, S. E. Kapphan,K. Polgar, ‚Optical characterization of Cr 3+ centers in LiNbO 3 ’, Appl. Phys. B,73, 453 – 461 (2001)Zhao Jianlin, Wu Jianjun, Wang Bin, Yang Dexing, S. Kapphan, R. Pankrath, ‚ImageEdge-Enhancement using photorefractive Two-Wave Coupling in Cr:SBNCrystal’,Acta Optica Sinica, Vol. 21, No. 11, 1343 (2001)M. Wierschem, T. Lindemann, R. Pankrath, S. E. Kapphan, ‚NIR Absorption andlight-induced charge transfer in photorefractive Ba 0.77 Ca 0.23 TiO 3 crystals doped withiron’, Ferroelectrics, Vol. 264, pp. 315-324 (2001)13

V. S. Vikhnin, S. E. Kapphan, ‚Local Configuration instability as an origin of Relaxor-Type Properties of Ferroelectric solid solutions SBN, SCT and KLTN, FerroelectricsLetters’, Vol. 28(5-6), pp. 123-133 (2001)V. S. Vikhnin, A. A. Kaplyanskii, A. B. Kutsenko, G. K. Liu, J. V. Beitz, S. E. Kapphan,,"Charge transfer-lattice" clusters induced by charged impurities’, Journal of Luminescence94-95, 775-779 (2001)V. S. Vikhnin, I. Kislova, A. B. Kutsenko, S. E. Kapphan, ‚Charge transfer vibronicexcitons and excitonic-type polaron states: photoluminescence in SBN’, Solid StateComm., 121, 83 – 88 (2002)V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, G. Borstel, E. A. Kotomin, ‚Polaronic-typeexcitons in ferroelectric oxides: Microscopic calculations and experimental manifestation’,Physical Review B. Vol. 65, 104304 (2002)V. S. Vikhnin, R. I. Eglitis, S. E. Kapphan, ,Charge Transfer Vibronic Excitons in IncipientFerroelectrics and Related Problems’, Ferroelectrics, Vol. 265, pp. 177 –178(2002)R. I. Eglitis, V. S. Vikhnin, E. A. Kotomin, S. E. Kapphan, G. Borstel, ,TheoreticalPrediction and Experimental Confirmation of Charge Transfer Vibronic Excitons andTheir phase in ABO 3 Perovskite Crystals’, Mat. Res. Soc. Symp. Proc., Vol. 718(2002)I. L. Kislova, M. Gao, S. E. Kapphan, R. Pankrath, A. B. Kutsenko, V. S. Vikhnin,,Photo- and Thermoluminescence in congruent SBN Crystals doped with Ce and Cr’,Ferroelectrics, Vol. 273, pp. 187-192 (2002)V. S. Vikhnin, S. Avanesyan, H. Liu, S. E. Kapphan, ,An origin of light induced centersin the visible range in ferroelectric oxides: possible role of the states of chargetransfer vibronic excitons’, Journal of Physics and Chemistry of Solids, Vol. 63, 1677-1683 (2002)Z. Bryknar, V. Trepakov, Z. Potucek, L. Jastrabik, S. Kapphan, ‘PhotoluminescenceSpectroscopy of Chromium doped Cd 2 Nb 2 O 7 ’, Ferroelectrics, Vol. 272, 363 - 368(2002)V. S. Vikhnin, R. Blinc, R. Pirc, S. E. Kapphan, I. L. Kislova, P. A. Markovin, ‘A Modelof Polar Clusters in Ferroelectric Relaxors of PMN-Type: Polaronic and ChargeTransfer Effects’, Ferroelectrics, Vol. 268, 257 – 262 (2002)R. I. Eglitis, E. A. Kotomin, V. A. Trepakov, S. E. Kapphan and G. Borstel, ‚Quantumchemical modelling of electron polarons and ‚green’ luminescence in PbTiO 3perovskite crystals’, J. Phys.: Condens. Matter, 14, L 647 – L 653 (2002)D. Millers, L. Grigorjeva, V. Pankratov, V. A. Trepakov, S. E. Kapphan, ‘Pulsed electronbeam excited transient absorption in SrTiO 3 ’, NIM B, 194, 469 – 473 (2002)A. G. Badalyan, P. G. Baranov, V. A. Trepakov, C. B. Azioni, P. Gabinetto, M. C.Mozzati, L. Jastrabik, J. Rosa, M. Hrabovský, ‘Recent researches of the Copper Centresin Potassium Tantalate Single Crystals’, Ferroelectrics, Vol. 272, 205 – 210(2002)14

Prof. Dr. Detlef KipForschungsübersichtNach der Habilitation in der Arbeitsgruppe Elektrooptik von E. Krätzig im Jahre 1999hat Detlef Kip im Sommer 2002 einen Ruf auf eine Professur für Optische Technologienan die TU Clausthal angenommen. Die Forschung der noch in Osnabrückdurchgeführten Arbeiten sowie der in Clausthal im Aufbau befindlichen Abteilung OptischeTechnologien beschäftigt sich mit verschiedenen Bereichen der Photonik. Ineinem ersten Schwerpunkt geht es um die Entwicklung und Optimierung oxidischerKristalle. Für Anwendungen im Bereich der Integrierten Optik entwickeln wir Wellenleiter,z.B. in den Substratmaterialien Lithiumniobat, Lithiumtantalat und Strontium-Barium-Niobat. Unser besonderes Interesse gilt hier der Entwicklung von schmalbandigenintegriert-optischen Wellenlängenfiltern für Anwendungen in der optischenNachrichtentechnik und der effizienten Frequenzverdopplung in optischen Wellenleitern.In einem weiteren Schwerpunkt geht es um die Erzeugung und Untersuchungvon optischen räumlichen Solitonen. Konkret untersuchen wir hierbei die Existenzbereicheund Wechselwirkungseigenschaften solcher räumlicher Solitonen in photorefraktivenKristallen, die in diesem Zusammenhang ein ausgezeichnetes, experimentellvergleichsweise leicht zugängliches Modellsystem für die universellen Eigenschaftenvon Solitonen darstellen.Forschung im KollegIm Graduiertenkolleg wird von uns das Themengebiet der optischen Frequenzverdopplung(SHG) in oxidischen Wellenleitern bearbeitet. Hierbei konzentrieren wir unsauf die Kristalle LiNbO 3 und LiTaO 3 , die kommerziell als Wafer in hervorragenderoptischer Qualität erhältlich sind. In beiden Materialien ist für eine effiziente Frequenzverdopplungund der Nutzung des größten SHG-Koeffizienten d 33 eine Quasi-Phasenanpassung durch räumlich periodische Modulation der ferroelektrischen Domänennotwendig. Das Schalten der Domänen geschieht mit Hilfe von Hochspannungspulsenund lithographisch strukturierten Fingerelektroden mit Gitterperiodenvon einigen Mikrometern. Die Wellenleiterherstellung erfolgt durch Eindiffusion vonTitanstreifen in das Substratmaterial. Besonderes Augenmerk gilt der Vermeidungvon photorefraktiven Effekten im Wellenleiter. Hierzu werden Substratmaterialien mitveränderter, nahezu stöchiometrischer Zusammensetzung durch VTE-Behandlung(Vapor Transport Equilibration) untersucht. Nähere Informationen zu den laufendenund geplanten Arbeiten enthält der entsprechende Bericht.Kooperationen im KollegDie Untersuchungen in diesem Projekt werden in enger Zusammenarbeit mit denForschungsgruppen von E. Krätzig, M. Imlau und K. Buse (jetzt Universität Bonn)sowie mit der Abteilung Kristallzüchtung (H. Hesse, R. Pankrath) durchgeführt.Publikationen in Zusammenhang mit dem GraduiertenkollegV. Shandarov, M. Wesner, J. Hukriede und D. Kip: „Observation of dark spatial photovoltaicsolitons in planar waveguides in lithium niobate“. J. Opt. A: Pure Appl. Opt.2, 500 (2000)15

J. Hukriede, D. Kip und E. Krätzig: “Investigation of titanium- and copper-indiffusedchannel waveguides in lithium niobate and their application as holographic filters forinfrared light”. J. Opt. A: Pure Appl. Opt. 2, 481 (2000)M. Wesner, C. Herden, D. Kip und P. Moretti: “Photorefractive steady-state solitonsup to telecommunication wavelengths in planar SBN waveguides”. Opt. Commun.188, 69 (2001)M. Wesner, C. Herden und D. Kip: “Electrical fixing of waveguide channels in srontium-bariumniobate crystals”. Appl. Phys. B 72, 733 (2001)M. Wesner, C. Herden, R. Pankrath, D. Kip und P. Moretti: “Temporal development ofphotorefractive solitons up to telecommunication wavelengths in SBN”. Phys. Rev. E64, 36613 (2001)J. Hukriede, D. Kip und E. Krätzig: "Permanent narrow-band reflection holograms forinfrared light recorded in LiNbO 3 :Ti:Cu channel waveguides". Appl. Phys. B 72, 749(2001)D. Kip, C. Herden und M. Wesner: “All-optical signal routing using interaction of mutuallyincoherent spatial solitons”. Ferroelectrics 274, 135 (2002)J. Xu, V. Shandarov, M. Wesner und D. Kip: “Observation of two-dimensional spatialsolitons in iron-doped barium-calcium titanate crystals”. phys. stat. sol. (a) 189, R4(2002)J. Imbrock, A. Wirp, D. Kip und E. Krätzig: “Photorefractive properties of lithium andcopper in-diffused lithium niobate crystals”. J. Opt. Soc. Am. B 19, 1822 (2002)16

Prof. Dr. Eckhard KrätzigForschungsübersichtUnserer Gruppe „Angewandte Physik: Elektrooptik“ beschäftigt sich mit den Schwerpunkten„Photorefraktive Effekte“ und „Integrierte Optik“. Photorefraktive Effektebraucht man zur Aufzeichnung von Volumenphasenhologrammen, die etwa zur Speicherungvon Information, zur optischen Phasenkonjugation, zur Lichtverstärkung undOszillation, zur Bild- und Signalverarbeitung, zur holographischen Interferometrie o-der zur Filterung herangezogen werden können. Die Integrierte Optik zielt darauf ab,möglichst viele optische Komponenten auf einem gemeinsamen Substrat zu vereinen.In elektrooptischen Materialien kann man mit verschiedenen Methoden - derEindiffusion, der Ionenimplantation oder dem Ionenaustausch - Wellenleiter mit relativgeringer Dämpfung erzeugen. Im Berichtszeitraum haben wir folgende Themenbearbeitet (http://www.physik.uni-osnabrueck.de/elektrooptik):•••••••Der lichtinduzierte Ladungstransport in elektrooptischen KristallenHolographische Streuung, Lichtverstärkung und OszillationPhotorefraktives Schreiben mit IR-Licht und HologrammstabilisierungReduktion lichtinduzierter BrechungsindexänderungenRaumladungswellen in photorefraktiven KristallenPhotorefraktive WellenleiterSolitonen in photorefraktiven KristallenForschung im KollegIm Kolleg behandeln wir die Projekte „Raumladungswellen in photorefraktiven Kristallen“und „Nichtlineare optische Eigenschaften photorefraktiver SBN-Kristalle“. Beimersten Projekt (Stipendiat F. Rahe) liegt der Schwerpunkt auf der Erforschung nichtlinearerWechselwirkungen der Raumladungswellen in photorefraktiven Kristallen, diein Analogie zur „Nichtlinearen Optik“ zur Frequenzverdopplung und zur optischenGleichrichtung führen. Daneben treten aber auch Effekte auf, die in der „NichtlinearenOptik“ nicht bekannt sind, nämlich räumliche Gleichrichtung ohne zeitlicheGleichrichtung und räumliche Verdopplung ohne zeitliche Verdopplung. Im zweitenProjekt (Kollegiatin M. Wesner) geht es um die Untersuchung des attraktiven photorefraktivenMaterials Strontium-Barium-Niobat in Bezug auf aktuelle Fragestellungender nichtlinearen Optik und deren Anwendungen. Interessant sind insbesondere dieErzeugung optischer Solitonen bis zu Wellenlängen von 1.5 µm und eine neue Methodedes elektrischen Fixierens. - Weitere Informationen sind in den Berichten derKollegiat(inn)en zu finden.Kooperationen im KollegObige Projekte werden in Zusammenarbeit mit den Gruppen von K. H. Ringhofer sowieK. Betzler, S. Kapphan und M. Wöhlecke durchgeführt. Weiter arbeiten wir in engerKooperation mit den früheren Gruppenmitgliedern K. Buse (jetzt UniversitätBonn) und D. Kip (jetzt Universität Clausthal). Die Kristalle erhalten wir von derGruppe von H. Hesse und R. Pankrath, die vielfach Proben auf unseren Wunsch hinmodifiziert haben. Weitere Kooperationen gibt es mit M. P. Petrov (Ioffe Institute,St.Petersburg, Russland), P. Moretti (Universität Lyon, Frankreich), S. Odoulov (Academyof Sciences, Kiev, Ukraine), V. Shandarov (Universität Tomsk, Russland) undJinjun Xu (Universität, Tianjin, VR China).17

Publikationen in Zusammenhang mit dem Graduiertenkolleg• S, Odoulov, B. Sturman, E. Krätzig, Seeded and Spontaneous Light Hexagons inLiNbO 3 :Fe, Appl. Phys. B 70, 645 (2000)• M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Spatial Rectificationof the Electric Field of Space Charge Waves, Phys. Rev. Lett. 84, 5114(2000)• M. P. Petrov, V. V. Bryksin, V. M. Petrov, S. Wevering, E. Krätzig, Spectra ofSpace Charge Waves in Photorefractive Crystals, Technical Digest CLEOEurope, 121 (2000)• J. Hukriede, D. Kip und E. Krätzig: “Investigation of titanium- and copperindiffusedchannel waveguides in lithium niobate and their application as holographicfilters for infrared light”. J. Opt. A: Pure Appl. Opt. 2, 481 (2000)• M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Spatial Rectificationof the Electric Field of Space Charge Waves in Sillenites, Technical DigestCLEO Europe, 377 (2000)• M. Goul'kov, S. Odoulov, O. Shinkarenko, E. Krätzig, R. Pankrath, Threshold ofOscillation in a Ring-Loop Phase Conjugator as a Second Order Optical PhaseTransition, Appl. Phys. B 72, 187 (2001)• M. Wesner, C. Herden, D. Kip, E. Krätzig, P. Moretti, Photorefractive Steady StateSolitons up to Telecommunication Wavelengths in Planar SBN Waveguides, OpticsCommun. 188, 69 (2001)• M. P. Petrov, A. P. Paugurt, V. V. Bryksin, S. Wevering, E. Krätzig, Dynamic ElectroopticEffect Induced by Space Charge Waves in Sillenites, Optical Materials 18,99 (2001)• J. Hukriede, D. Kip und E. Krätzig: "Permanent narrow-band reflection hologramsfor infrared light recorded in LiNbO 3 :Ti:Cu channel waveguides". Appl. Phys. B 72,749 (2001)• M. P. Petrov, V. V. Bryksin, S. Wevering, E. Krätzig, Nonlinear Interactions andScattering of Space Charge Waves in Sillenites, Appl. Phys. B 73, 669 (2001)• M. Goulkov, S. Odoulov, Th. Woike, J. Imbrock, M. Imlau, E. Krätzig, C. Bäumer,H. Hesse, Holographic Light Scattering in Photorefractive Crystals with Local Response,Phys. Rev. B 65, 195111 (2002)• M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Optically InducedNonlinear Wave Processes in Photorefractive Crystals, Technical Digest IQEC2002, 375 (2002)• S. Schwalenberg, F. Rahe, E. Krätzig, Recording Mechanisms of AnisotropicHolographic Scattering Cones in Photorefractive Crystals, Optics Commun. 209,467 (2002)• M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Overall Rectification andSecond Harmonic Generation of Space Charge Waves, Phys. Rev. B 66, 085107(2002)• J. Imbrock, A. Wirp, D. Kip, E. Krätzig, Photorefractive Properties of Lithium andCopper In-diffused Lithium Niobate Crystals, J. Opt. Soc. Am. B 19, 1822 (2002)• Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke,Composition Dependence of the Ultraviolet Absorption Edge in Lithium TantalateJ. Appl. Phys., in print (2003)• M. P. Petrov, V. V. Bryksin, F. Rahe, C. E. Rüter, E. Krätzig, Space Charge RectificationEffects in Photorefractive Bi 12 TiO 20 Crystals, Optics Commun., submitted18

Dr. Rainer PankrathForschungsübersichtDie Arbeitsgruppe Kristallzucht beschäftigt sich mit der Herstellung oxidischer Kristalleaus Schmelzen oder Schmelzlösungen für photorefraktive Anwendungen sowie fürdie optische Frequenzverdopplung (SHG). Dabei lag der Schwerpunkt im Berichtszeitraumauf:• Züchtung von Ba x Ca 1-x TiO 3 mit verschiedenen Dotierungen für Grundlagenuntersuchungenund photorefraktive Anwendungen.• Züchtung von LiTaO 3 mit verschiedenen Dotierungen für photorefraktive Anwendungensowie von reinen Kristallen für die SHG.• Vapour transport equilibration-Behandlungen dieser Kristalle mit dem Ziel, das„optical damage“ zu minimieren.• Herstellung von Gläsern im System Bi 2 O 3 -B 2 O 3 mit verschiedenen Dotierungen(z.B.: Er, Nd).• Optimierung von Sr 2 FeMoO 6 -Keramiken für Untersuchungen der magnetischenEigenschaften.• Züchtung von Sr x Ba 1-x Nb 2 O 6 (SBN).Forschung im KollegIm Rahmen des Projektes „Growth and characterization of Sr x Ba 1-x Nb 2 O 6 crystalswith x ranging from 0.2 to 0.8“ (Stipendiat M. Ulex) werden undotierte Sr x Ba 1-x Nb 2 O 6 -Mischkristalle über den gesamten Bereich der Mischkristallbildung hergestellt. Dabeigeht es unter anderem darum, den Existenzbereich der Mischkristalle abzugrenzenund das Phasendiagramm des Systems zu bestimmen. Zu diesem Zweck wurde einbereits vorhandener Ofen modifiziert, in dem Liquidustemperaturen bestimmt werdenkönnen. Die Zusammensetzung der gezüchteten Kristalle wird mit der Röntgenfluoreszenzbestimmt.Während Sr x Ba 1-x Nb 2 O 6 mit kongruent schmelzender Zusammensetzung (x=0.61) inunseren Anlagen mit hoher optischer Qualität gezüchtet werden kann, ist die optischeQualität von Kristallen mit höherer und niedrigerer Sr-Konzentration in der Regeldeutlich vermindert. Ursache ist die An- oder Abreicherung bestimmter Komponentenan der Phasengrenze zwischen Kristall und Schmelze. Durch Variation derRotationsgeschwindigkeit und der vertikalen Temperaturgradienten in der Schmelzewird versucht, die Inhomogenitäten zu verringern.Die Charakterisierung der gezüchteten Kristalle erfolgt in anderen Arbeitsgruppen imHause und in Kooperation mit Arbeitsgruppen an anderen Universitäten (siehe Kooperationenim Kolleg).Kooperationen im KollegDas Projekt wird in enger Zusammenarbeit mit den Gruppen von K. Betzler und M.Wöhlecke durchgeführt. Eine Kooperation zur Bestimmung der Gitterkonstanten undder Sr,Ba-Verteilung zwischen den entsprechenden kristallographischen Positionenbesteht mit Prof. Dr. Schmahl (Ruhr-Universität Bochum). Die spezifische Dichte derKristalle wird in Zusammenarbeit mit Prof. Dr. Bohaty (Universität zu Köln) bestimmt.19

Publikationen in Zusammenhang mit dem Graduiertenkolleg• J. Dec, W. Kleemann, Th. Woike, R. Pankrath: Phase transitions inSr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : I. Susceptibility of clusters and domains. Eur. Phys. J.B14, 627-632 (2000)• J. Dec, W. Kleemann, Th. Woike, R. Pankrath: Phase transitions inSr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : II. Linear birefringence studies of spontaneous andprecursor polarization. Eur. Phys. J. B14, 633-637 (2000)• Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. Pankrath:Refractive Indices of congruently melting Sr 0.61 Ba 0.39 Nb 2 O 6 . phys. stat. sol. (a)186, R13 (2001)• T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.Kleemann: Influence of pinning effects on the ferroelectric hysteresis in cerium-dopedSr 0.61 Ba 0.39 Nb 2 O 6 . Phys. Rev. B 6317, art. no. 174101 (2001).• T. Woike, U. Dörfler, L. Tsankov, G. Weckwerth, D. Wolf, M. Wöhlecke,T.Granzow, R. Pankrath, M. Imlau, W. Kleemann: Photorefractive properties ofCr-doped Sr 0.61 Ba 0.39 Nb 2 O 6 related to crystal purity and doping concentration.Appl. Phys. B-Lasers Opt. 72, 661-666 (2001).• W. Kleemann, P. Licinio, T. Woike, R. Pankrath: Dynamic light scattering atdomains and nanoclusters in a relaxor ferroelectric. Phys. Rev. Lett. 86, 6014-6017 (2001).• T. Woike, T. Granzow, U. Dörfler, C. Pötsch, M. Wöhlecke, R. Pankrath: Refractiveindices of congruently melting Sr 0.61 Ba 0.39 Nb 2 O 6 . Phys. Status Solidi(a) 186, R13-R15 (2001).• J.L. Zhao, B. Wang, J.J. Wu, D.X. Yang, S. Kapphan, R. Pankrath: Investigationof photorefractive two-wave coupling in Cr-doped strontium barium niobatecrystal. Chin. Phys. 10, 739-742 (2001).• J. Dec, W. Kleemann, V. Bobnar, Z. Kutnjak, A. Levstik, R. Pirc, R. Pankrath:Random-field Ising-type transition of pure and doped SBN from the relaxor intothe ferroelectric state. Europhys. Lett. 55, 781-787 (2001).• M. Wesner, C. Herden, R. Pankrath, D. Kip, P. Moretti: Temporal developmentof photorefractive solitons up to telecommunication wavelengths in strontiumbariumniobate waveguides. Phys. Rev. E 6403, art. no. 036613 (2001).• T. Volk, L.Ivleva, P. Lykov, N. Polozkov, V. Salobutin, R. Pankrath, M.Wöhlecke: Effects of rare-earth impurity doping on the ferroelectric andphotorefractive properties of strontium-barium niobate crystals. Opt. Mater. 18,179-182 (2001).• R. Blinc, A. Gregorovic, B. Zalar, R. Pirc, J. Seliger, W. Kleemann, S.G. Lushnikov,R. Pankrath: Nb-93 NMR of the random-field-dominated relaxor transitionin pure and doped SBN. Phys. Rev. B 6413, art. no. 134109 (2001).• V.V. Gladkii, V.A. Kirikov, E.V. Pronina, T.R. Volk, R. Pankrath, M. Wöhlecke:Anomalies in the slow polarization kinetics of a ferroelectric relaxor in the temperatureregion of a diffuse phase transition. Phys. Solid State 43, 2140-2145(2001).• P. Lehnen, E. Beckers, W. Kleemann, T. Woike, R. Pankrath: Ferroelectricdomains in the uniaxial relaxor system SBN:Ce, Cr and Co. Ferroelectrics 253,567-575 (2001).• W. Kleemann, V. Bobnar, J. Dec, P. Lehnen, R. Pankrath, S.A. Prosandeev:Relaxor properties of dilute and concentrated polar solid solutions. Ferroelectrics261, 707-716 (2001).20

• P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomainsin the uniaxial relaxor system Sr 0.61-x Ba 0.39 Nb 2 O 6 :Ce -x (3+) . Phys. Rev. B 6422,art. no. 224109 (2001).• T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.Kleemann: Local electric-field-driven repoling reflected in the ferroelectric polarizationof Ce-doped Sr 0.61 Ba 0.39 Nb 2 O 6 . Appl. Phys Lett. 80, 470-472 (2002).• W. Kleemann, J. Dec, P. Lehnen, R. Blinc, B. Zalar, R. Pankrath: Uniaxial relaxorferroelectrics: The ferroic random-field Ising model materialized at last.Europhys. Lett. 57, 14-19 (2002).• T. Granzow, U. Dörfler, T. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.Kleemann: Evidence of random electric fields in the relaxor-ferroelectricSr 0.61 Ba 0.39 Nb 2 O 6 . Europhys. Lett. 57, 597-603 (2002).• P. Lehnen, J. Dec, W. Kleemann, T. Woike, R. Pankrath: Domain responsefeatures of SBN:Ce. Ferroelectrics 268, 533-538 (2002).• W. Kleemann, J. Dec, R. Blinc, B. Zalar, R. Pankrath: Random fields at transitionsfrom relaxor to glassy and ferroelectric states. Ferroelectrics 267, 157-164 (2002).• W. Kleemann, J. Dec, S. Miga, T. Woike, R. Pankrath: Non-Debye domainwall-induceddielectric response in Sr 0.61-x Ce x Ba 0.39 Nb 2 O 6 . Phys. Rev. B 65,art. no. 220101 (2002).• I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B. Kutsenko, V.S. Vikhnin:Photo- and thermoluminescence in congruent SBN crystals doped with Ce andCr. Ferroelectrics 273, 2565-2570 (2002).• H.L. Zhao, Q.T. Xu, W.M. Zhou, D.S. Yang, S. Kapphan, R. Pankrath:Photorefractive edge-enhancement joint transform correlator. Opt. Commun.212, 287-292 (2002).• S. Kapphan, B. Pedko, V. Trepakov, M. Savinov, R. Pankrath, I. Kislova:Variation of doping-dependent properties in photorefractive Sr x Ba 1-x Nb 2 O 6 :Ce,Cr, Ce+Cr. Radiat. Eff. Defects Solids 157, 1033-1037 (2002).• I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B. Kutsenko, V.S. Vikhnin:Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 doubly doped with Ce and Cr: Photo- and thermoluminescenceinvestigations. Radiat. Eff. Defects Solids 157, 1015-1020(2002).21

Prof. Dr. Klaus Ringhofer, Dr. Maxim GorkounovForschungsübersichtDie Arbeitsgruppe Theoretische Optik beschäftigt sich mit der Beschreibung nichtlineareroptischer Effekte in photorefraktiven Kristallen. In solchen Kristallen verursachteine Intensitätsmodulation, die von zwei interferierenden Lichtstrahlen erzeugtwird, ein moduliertes elektrisches Raumladungsfeld, das eine entsprechende Modulationdes Brechungsindex hervorruft. Wir modellieren derartige nichtlineare Wechselwirkungenoptischer Strahlen in Kristallen mit unterschiedlichen Symmetrien unterverschieden externen Bedingungen. Außerdem beteiligen wir uns an der Forschungelektromagnetischer Eigenschaften der neuen Metamaterialen, die als Analoga optischerKristalle im Mikrowellenfrequenzbereich gelten. Die mikroskopischen Eigenschaftensolcher künstlich hergestellten Materialen sind relativ leicht kontrollierbar,während die makroskopischen Eigenschaften alle praktischen Anwendungenbestimmen. Unser Ziel ist es, den Zusammenhang zu beschreiben und optimale Metamaterialienvorzuschlagen.Forschung im KollegIm Kolleg werden die beiden Projekte „Vectorial beam coupling in fast photorefractivecrystals with AC-enhanced response (Oleg Filippov)“ und „Microwave interactions innonlinear metamaterials (Mikhail Lapine)“ durchgeführt (s. Berichte der Stipendiaten).Kooperationen im KollegDie Projekte werden in enger Zusammenarbeit mit B. Sturman (International Institutefor Nonlinear Studies, Novosibirsk, Russland) und E. Shamonina (Universität Oxford,England) bearbeitet. Außerdem gibt es Kooperationen mit den Gruppen von K. Betzlerund E. Krätzig.Publikationen in Zusammenhang mit dem Graduiertenkolleg• V. P. Kamenov, Yi Hu, E. Shamonina, K. H. Ringhofer, and V. Ya. Gayvoronsky,“Two-wave mixing in (111)-cut Bi 12 SiO 20 and Bi 12 TiO 20 crystals: Characterizationand comparison with the general orientation”, Phys. Rev. E 62, 2863 (2000).• E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. Pavlyuk, K. V. Shcherbin, V. Ya.Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, „Attractors and autooscillationsfor feedback controlled photorefractive beam coupling“, Opt. Comm.192, 399 (2001).• E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. Pavlyuk, K. V. Shcherbin, V. Ya.Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, „Dynamics of feedback controlledphotorefractive beam coupling“, Phys. Rev. A 63, 053805 (2001).• V.P. Kamenov , E. Shamonina, K.H. Ringhofer, E. Nippolainen, V.V. Prokofiev,and A.A. Kamshilin, „Photorefractive light scattering families in (111)-cut Bi 12 TiO 20crystals with an external electric ac field“, Phys. Rev. E 63 (1), 016607 (2001).• M.V. Gorkunov, E.V. Podivilov and B.I.Sturman, “Critical enhancement of nonlinearresponse in fast photorefractive crystals”, JETP 94, 470-481 (2002).22

• E.V. Podivilov, B.I. Sturman, M.V. Gorkunov, V.P. Kamenov, and K.H. Ringhofer,“Theory of critical enhancement of photorefractive beam coupling”, Phys. Rev. E,65 046623 (2002).• E. Shamonina E, V.A. Kalinin, K.H. Ringhofer, L. Solymar, „Magneto-inductivewaveguide“, Electronics Lett. 38 (8), 371 (2002).• E. Shamonina, V.A. Kalinin, K.H. Ringhofer, and L. Solymar, „Magnetoinductivewaves in one, two, and three dimensions“, J. Appl. Phys. 92, 6252 (2002).• Gorkunov M., Lapine M., Shamonina E., Ringhofer K.H., “Effective magneticproperties of a composite material with circular conductive elements”, Eur. Phys. J.B 28, 263 (2002).• B. I. Sturman, V. Kamenov, M. Gorkunov, and K. H. Ringhofer, “Formation ofmoving light domains during photorefractive feedback-controlled beam coupling”,Opt. Comm. 216, 225 - 231 (2003)Leider verlor Klaus Ringhofer im Dezember 2002 endgültig den Kampf gegen denKrebs. Die Betreuung der Stipendiaten im Kolleg wird von M. Gorkounov, K. Betzlerund E. Krätzig weitergeführt.23

Prof. Dr. Eckart Rühl,Dr. Roman FleschForschungsübersichtDas Ziel der Forschungsvorhaben im Graduiertenkolleg bestand darin, neue Quellenzur Erzeugung von kurzwelliger Strahlung im Bereich des Vakuum-UV zu entwickeln.Dies sollte mit Hilfe von Clustern und Aerosolen erfolgen, die als Medium für nichtlineareoptische Prozesse dienen. Ausgangspunkt waren Arbeiten zur Erzeugung derdritten Harmonischen in atomaren Gasen, wie z. B. Edelgasen [1, 2]. KomplementäreArbeiten erfolgten im Rahmen des Graduiertenkollegs zur Erzeugung von hochenergetischerXUV-Strahlung aus einer Laser-Plasma-Quelle.Forschung im KollegDie Experimente zur nichtlinearen Optik, die im Rahmen des Graduiertenkollegsdurchgeführt wurden, hat Herr Dr. A. Pramann maßgeblich vorangetrieben (s. Berichtvon Dr. A. Pramann). Ebenso war Herr J. Plenge im Graduiertenkolleg tätig, der aufbauendauf ersten Arbeiten zur Frequenzverdreifachung an atomaren Gasen eineXUV-Plasmaquelle unter Nutzung metallischer Targets zur Erzeugung von durchstimmbarerXUV-Strahlung aufgebaut und genutzt hat [3-9] (vgl. Bericht von J. Plenge).Während der Beschäftigungszeit von Herrn Dr. Pramann von November 2001 bisJanuar 2003 hat er erfolgreich eine gepulste Gasexpansion zur Erzeugung von freienClustern in Verbindung mit nichtlinearen optischen Effekten aufgebaut. Herrn Pramannist es vor allem gelungen, einen kompakten Versuchsaufbau zu realisieren, deres ermöglicht, ohne jegliche Reflexionsoptiken kurzwellige Strahlung zu erzeugenund nachzuweisen. Dies geht über vorhergehende Arbeiten hinaus, in denen einkomplizierter und verhältnismäßig ineffizient arbeitender Aufbau genutzt wurde, indem lange Wege und zahlreiche Reflexionen mit steilem Einfallswinkel sowie ein Vakuum-UV-Monochromatorgenutzt wurden [2]. Das Experiment von Herrn Pramannhat erste Funktionstests bestanden. Er konnte zunächst anhand von atomaren Gasenzeigen, dass es Frequenzverdreifachung in bisher unbekannten spektralen Regionengibt. Der nächste Schritt besteht in der Kühlung der Düsenstrahlexpansion, damit effizientCluster entstehen und nichtlineare optische Prozesse gemäß dem Projektantraguntersucht werden können.Kooperationen im KollegDie vorgeschlagenen Experimente sind als komplementär zu den übrigen Vorhabendes Graduiertenkollegs anzusehen, die im Projektbereich Frequenzkonversion angesiedeltsind. An Stelle von kristallinen Festkörpern standen Cluster und flüssige Partikelim Vordergrund der Untersuchungen. Ebenso hatte das Vorhaben als einziges zurAufgabe, sehr kurzwellige Strahlung im XUV zu erzeugen. Daher fand eine Kooperationinnerhalb des Kollegs primär auf der Ebene eines intensiven Erfahrungsaustauschessowie während der gemeinsamen Seminare statt. Zur Intensivierung der Diskussionwurden gezielt Vortragende aus dem Umfeld des Arbeitsgebietes in das Seminardes Graduiertenkollegs eingeladen, wie Prof. Dr. L. Wöste (Berlin) und Prof.Dr. B. Wellegehausen (Hannover).24

Literatur1. R. Flesch, B. Wassermann, B. Rothmund und E. Rühl, J. Phys. Chem. 98, 6263(1994).2. R. Flesch, M.C. Schürmann, J. Plenge, M. Hunnekuhl, H. Meiss, M. Bischof und E.Rühl, Phys. Chem. Chem. Phys. 1, 5423 (1999).3. R. Flesch, M.C. Schürmann, M. Hunnekuhl, H. Meiss, J. Plenge und E. Rühl, Rev.Sci. Instrum. 71, 1319 (2000).4. R. Flesch, M.-C. Schürmann, H. Meiss, J. Plenge, M. Hunnekuhl und E. Rühl,Phys. Rev. A 62, 52723 (2000).5. J. Plenge, R. Flesch, M.-C. Schürmann und E. Rühl, J. Phys. Chem. A 105, 4844(2001).6. R. Flesch, J. Plenge, M.-C. Schürmann, S. Kühl, M. Klusmann und E. Rühl, Surf.Rev. Lett. 9, 105 (2002).7. R. Flesch, J. Plenge, S. Kühl, M. Klusmann und E. Rühl, J. Chem. Phys. 117, 9663(2002).8. J. Plenge, R. Flesch, S. Kühl, B. Vogel, R. Müller, F. Stroh und E. Rühl, Geophys.Res. Lett., zur Veröffentlichung eingereicht (2002).9. J. Plenge, Dissertation, Universität Osnabrück (2002).Publikationen in Zusammenhang mit dem Graduiertenkolleg1. R. Flesch, M.C. Schürmann, M. Hunnekuhl, H. Meiss, J. Plenge und E. Rühl, Rev.Sci. Instrum. 71, 1319 (2000).2. R. Flesch, M.-C. Schürmann, H. Meiss, J. Plenge, M. Hunnekuhl und E. Rühl,Phys. Rev. A 62, 52723 (2000).3. J. Plenge, R. Flesch, M.-C. Schürmann und E. Rühl, J. Phys. Chem. A 105, 4844(2001).4. R. Flesch, J. Plenge, M.-C. Schürmann, S. Kühl, M. Klusmann und E. Rühl, Surf.Rev. Lett. 9, 105 (2002).5. R. Flesch, J. Plenge, S. Kühl, M. Klusmann und E. Rühl, J. Chem. Phys. 117, 9663(2002).6. J. Plenge, R. Flesch, S. Kühl, B. Vogel, R. Müller, F. Stroh und E. Rühl, Geophys.Res. Lett., zur Veröffentlichung eingereicht (2002).6. J. Plenge, Dissertation, Universität Osnabrück (2002).E. Rühl folgte im Herbst 2002 einem Ruf an die Universität Würzburg. Seine Projekteim Graduiertenkolleg ‚Nonlinear optical processes in atomic and molecular clusters’und ‘Frequency conversion, nonlinear optical processes in atomic and molecularclusters’ schloss er im Januar 2003 erfolgreich ab (Berichte J. Plenge und A. Pramann).25

Prof. Dr. Hans Werner SchürmannForschungEntsprechend dem Einrichtungsantrag sind untersucht worden:Existenz- und Stabilitätskriterien für Solitonenlösungen der nichtlinearen Schrödinger-GleichungStreuung am nichtlinearen Film mit unterschiedlichen DielektrizitätsfunktionenNichtlineare Wellenleitung bei ortsabhängiger DielektrizitätsfunktionElliptische Lösungen der Kadomtsev-Petviashvili-GleichungSemianalytische Lösungen der Wellengleichung bei photorefraktiver NichtlinearitätDie Untersuchungen fanden in Kooperation mit den Professoren Serov und Shestopalov(Lomonosov Universität Moskau) sowie innerhalb des Kollegs statt; die Ergebnissesind publiziert und auf internationalen Konferenzen vorgestellt worden.Publikationen im Zusammenhang mit dem GraduiertenkollegH. W. Schürmann, V. S. Serov, Criteria for existence and stability of soliton solutionsof the cubic-quintic nonlinear Schrödinger equation, Phys. Rev. E 62,2, pp. 2821-2826 (2000).H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Reflection and transmission ofa plane TE-wave at a lossless nonlinear dielectric film, Physica D, Vol. 158, pp. 197-215 (2001).H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Solutions to the Helmholtzequation for TE-guided waves in a three-layer structure with Kerr-type nonlinearity, J.Phys. A: Math. Gen., 35, 10789 – 10801 (2002).H. W. Schürmann, V. S.. Serov and Y. V. Shestopalov, On the theory of TE polarizedwaves guided by a lossless nonlinear three-layer structure, Proc. Progress in ElectromagneticsResearch Symposium (PIERS), Osaka, Japan, July 18-22, 2001, p.670.H. W. Schürmann, V. S. Serov and Y. V. Shestopalov, Waves in three-layer structureswith Kerr-type nonlinearity and variable permittivity, Proc. Conf. on MathematicalModelling of Wave Phenomena, Växjö University, Sweden, November 3 – 8, 2002(in press; available on www.masda.vxu.se/bni/waephenomena.htm)26

Apl. Prof. Dr. Manfred WöhleckeForschungsübersichtDie Arbeitsgruppe beschäftigt sich mit den optischen und dielektrischen Eigenschaftenvon Oxiden mit Niob-Sauerstoff oder Tantal-Sauerstoff Oktaederbausteinenunter besonderer Berücksichtigung des Einflusses der Phasenübergänge mit Relaxor-Charakterund arbeitet eng mit der Gruppe Nichtlineare Optik (Betzler) zusammen.Ferroelektrische Phasenübergänge können in sehr engen (LiNbO 3 und LiTaO 3 )und breiten (SBN) Temperaturbereichen auftreten und beeinflussen über eine starkeÄnderung der Dielektrizitätskonstante alle damit zusammenhängenden physikalischenEigenschaften. Da die kongruente Schmelze der Kristalle nicht der stöchiometrischenZusammensetzung entspricht, werden viele Eigenschaften durch die aktuelleKristallzusammensetzung bestimmt. Im Berichtszeitraum wurden untersucht:• Einfluss der Dotierung auf die fotorefraktiven und ferroelektrischen Eigenschaftenvon SBN• Dynamik ferroelektrischer Relaxoren• Grundlegende optische Parameter wie Brechungsindex und Bandkante• OH - - Streckschwingung in OxidenForschung im KollegIm Rahmen des Kollegs wurden verschiedene computergesteuerte Messplätze neuaufgebaut oder aktualisiert. Dazu zählen eine Anordnung zur Raman-Streuung, einezur Dotierung von Kristallen mit Wasserstoff und ein Messplatz zur Bestimmung derDielektrizitätskonstanten und der pyroelektrischen Koeffizienten bei verschiedenenTemperaturen. Eine systematische Untersuchung der Dotierung von SBN unterschiedlicherZusammensetzung mit Wasserstoff erlaubt eine Interpretation des rechtunstrukturierten OH-Streckschwingungsspektrums auf der Ebene von Details derKristallstruktur. Die Bandkante hängt in Sr x Ba 1-x Nb 2 O 6 für x= 0,25 - 0,8 nur geringfügigvon der Zusammensetzung ab. Dagegen wird für LiTaO 3 eine ausgeprägte Abhängigkeitgefunden, die sich sehr gut zur zerstörungsfreien Bestimmung der Zusammensetzungdes Kristalls eignet. Dielektrische Charakterisierungen vonSr x Ba 1-x Nb 2 O 6 liegen für x-Werte oberhalb x=0,45 vor, für solche unterhalb muss derTemperaturbereich des Messplatzes erweitert werden.Kooperationen im KollegDie Projekte werden in enger Zusammenarbeit mit den Gruppen von K. Betzler undR. Pankrath (SBN-Kristalle) durchgeführt. Die LiTaO 3 Kristalle unterschiedlicher Zusammensetzungwurden von Ch. Bäumer (Kristallzucht H. Hesse) präpariert. Eineintensive Zusammenarbeit besteht mit der Gruppe Th. Woike (Universität zu Köln).Kooperationen im Rahmen eines INTAS-Projekts gibt es mit T. Volk und L. Ivleva(Russian Academy of Science, Moscow). Ein bilaterales Projekt existiert mit L.Kovács (Hungarian Academy of Sciences, Budapest).27

Publikationen in Zusammenhang mit dem GraduiertenkollegZeitschriftenartikel• Ch. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. WöhleckeComposition dependence of the ultraviolet absorption edge in lithium tantalateJ. Appl. Phys. March 1, 2003 issue• T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W.KleemannLocal electric-field-driven repoling reflected in the ferroelectric polarization ofCe-doped Sr 0.61 Ba 0.39 Nb 2 O 6 .Appl. Phys. Letters 80, 470 - 472 (2002)• T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W.KleemannEvidence of random electric fields in the relaxor-ferroelectric Sr 0.61 Ba 0.39 Nb 2 O 6Europhysics Letters 57, 597 - 603 (2002)• T. Granzow, Th. Woike, M. Wöhlecke, M. Imlau, W. KleemannPolarization-Based Adjustable Memory Behavior in Relaxor FerroelectricsPhys. Rev. Lett. 89, 127601 (2002)• V. V. Gladkii, V. A. Kirikov, E. V. Pronina, T. R. Volk, R. Pankrath, M.WöhleckeAnomalies in the Slow Polarisation Kinetics of a Ferroelectric relaxor in theTemperature Region of a Diffuse Phase TransitionPhysics of the Solid State 43, 2140-2145 (2001)• Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. PankrathRefractive Indices of congruently melting Sr 0.61 Ba 0.39 Nb 2 O 6phys. stat. sol. (a) 186, R13 (2001)• T. Volk, L. Ivleva , P. Lykov, D. Isakov, V. Osiko, M. WöhleckeModification of the optical and photorefractive properties of Ce-doped strontium-bariumniobate by co-doping with a nonphotorefractive La impurityAppl. Phys. Letters 79, 854 (2001)• T. Volk, L. Ivleva, P. Lykov, N. Pollok, V. Salobutin, R. Pankrath, M. WöhleckeEffects of rare-earth impurity doping on the ferroelectric and photorefractiveproperties of strontium-barium niobate crystalsOptical Materials 18, 179 (2001)• T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W.KleemannInfluence of pinning effects on the ferroelectric hysteresis in cerium-dopedSr x Ba 1-x Nb 2 O 6Phys. Rev B 63, 174101 (2001)• Th. Woike, U. Dörfler, L. Tsankov, G. Weckwerth, D. Wolf, M. Wöhlecke, T.Granzow, R. Pankrath, M. Imlau, W. KleemannPhotorefractive properties of Cr-doped Sr 0.61 Ba 0.39 Nb 2 O 6 related to crystal purityand doping concentrationAppl. Phys. B 72, 661 (2001)28

1.2 Einzelberichte der in der vergangenen Periode geförderten Kollegiat(inn)enDipl.-Phys. Calin Adrian DavidTopic: Dielectric and optical properties of doped SBNAbstractThe subject of the project has been extended to members of the ferroelectric lithiumniobate family, because such crystals were available when the project started with adelay of nine months. In the course of the project new experimental set-ups havebeen designed and realized, existing arrangements have been updated or partiallyrenewed including modern computer controlling using C++ or MatLab. Optical anddielectric properties were measured in compounds of different composition of SBNand LiTaO 3 . Special emphasis was placed on the optical band edge in SBN andLiTaO 3 with various composition and the OH-stretching vibration in SBN as well asthe phase transition properties.Construction and rebuilding of set-upsIn the group existed an old double-grating spectrometer (Spex 14018) with some mechanicaldeficiency and an obsolete controlling system, but new uninstalled holographicgratings and freshly coated mirrors.After mounting the new gratings in the old holders and pre-aligning them, the mirrorswere installed and then the whole system was aligned according to the instructions ofthe manufacturer until a resolution better than that guaranteed by the Spex companyhas been achieved. For this task basic procedures of a Visual C++ programme wereused to control the spectrometer and allow simple signal detection with a photomultiplier.Later on the programme has been considerably extended to perform Ramanscattering including data accumulation and standard viewing of the spectrum.The system will be used in the next future to measure the Raman scattering of variousundoped Sr x Ba 1-x Nb 2 O 6 crystals with x varying between 0.25 and 0.8.We used the basics of a set-up to measure the spontaneous polarization as a functionof temperature, which was designed in the group of Th. Woike (Cologne), to buildan improved version with state of the art computer controlling. The system is quiteversatile and can be equipped with different meters to measure the dielectric constantand the conductivity. The set-up is built up of an electrometer charge measuringdevice (Keithley 6514), a temperature controller (PRO800 from Profile), a high voltageamplifier (610C from Trek). Again a Visual C++ programme with an IEEE 488interface card was installed. With this setup poling of the sample, measuring the conductivityand hysteresis like properties is possible.In the near future we will collaborate with the group of M. Imlau to extend the frequencyrange down to the sub-Hertz regime. Preliminary results in Sr x Ba 1-x Nb 2 O 6indicated that an extension of the set-up to more than 300 °C is necessary.29

Determination of the band edge of LiTaO 3 of various compositions andSr x Ba 1-x Nb 2 O 6LiTaO 3 has like LiNbO 3 a congruently melting composition which does not coincidewith the stoichiometric one, but shows a Li-deficiency. The more perfect lattice ofstoichiometric samples minimizes the line broadening in many spectral features and,from an application point of view even more important, makes the material less susceptibleto optical damage. Furthermore, a reduction of the coercive field is obtained,which is a significant parameter for the production of periodically poled nonlinear opticaldevices [1]. All available reports on composition controlled features of LiTaO 3suffer from their very limited compositional resolution. A set of well characterizedplates of LiTaO 3 with various compositions have been prepared by Ch. Bäumer. Thepolarized absorption was measured with a Bruins Instruments Omega 10 spectrometerwith a wavelength accuracy of 0.1 nm, using mercury emission lines for the wavelengthcalibration. Polished y- and z-cut samples of about 0.5 mm thickness with adensity of scratches not exceeding 1 % of the illuminated area were measured at22 °C with high spectral resolution (0.1 nm). The absorption data have been correctedfor reflection losses. The index of refraction has been taken from [2]. We neglectedthe variation of the index of refraction with composition and temperature, becauseits influence on the reflection is very weak.As in the case of LiNbO 3 , the position of the absorption edge is a very sensitivemeasure for the composition of LiTaO 3 crystals and thus can be used to determinethe composition of a crystal with a non-destructive method. In addition, the experimentaldata show that the Li-content is limited to 50.0 mol %, indicating that regularLi sites can be occupied by Ta ions, but no Li can substitute regular Ta ions. The dependenceof the polarized absorption can be described by an exponential fit functionwith three parameters and interpreted with a simple model calculation using an appropriateoverlay of an additional near ultraviolet absorption, caused by tantalum antisiteions, and the base absorption. The concentration of tantalum antisite ions increasesfor Li concentrations below 50 %.Similar measurements have been carried out with ordinarily polarized light forSr x Ba 1-x Nb 2 O 6 over the whole x-range. Only a very weak non-monotonic dependenceof the band edge energy for a given absorption was found (see Figure 1), thus indicatingthat for this light polarisation even for larger absorption coefficients no convenientcomposition determination will work.The situation may change for extraordinary light polarization, because the index ofrefraction data vary for this light polarization, see the report of A. Tunyagi. Suchmeasurements will be performed in the near future after preparing suitable a-cutsamples.30

Figure 1: Wavelength dependence of the absorption as afunction of composition for certain absorption coefficients.OH-stretching modes in Sr x Ba 1-x Nb 2 O 6Hydroxyl ions are often present in as-grown oxide crystals [3]. Their stretching vibrationalmode at about 3495 cm -1 can be detected by infrared (IR) absorption spectroscopy.As-grown SBN crystals contain, however, only a small amount of hydroxyl ions.Thus we had to use temperature treatments under wet atmosphere to increase thehydroxyl ions. We adopted procedures reported in the literature [4,5] and optimizedthem by treating the samples at about 900 °C for 10 h with oxygen flowing through awater bottle held at 80 °C. These parameters guarantee a strong doping but avoidsignificant reduction resulting in disturbing polaron absorption. Treating ten samplesof different composition (0.3 < x 0.8) in this manner yields a strong dependence ofthe maximum absorption on composition. Sr-rich samples accept three times morehydrogen than Ba-rich ones.The IR absorption of the stretching mode had been measured in pure and Ce-dopedcongruent (Sr 0.61 Ba 0.39 Nb 2 O 6 ) crystals [4,5]. A relatively broad absorption band hasbeen detected which is composed of a main line at about 3495 cm -1 and a broadshoulder at the low energy side expanding up to about 3000 cm -1 . The shoulder wasassumed to contain of least two [5] or three [4] bands due to the existence of differenthydrogen environments in the unfilled tungsten bronze structure. The aim of ourstudy was to investigate the OH band shape in SBN crystals with different compositionsin order to obtain more information about the relation between the band componentsand the crystal structure.We observed a significant influence of the composition on the OH-stretch mode absorptionspectra. With rising x, the absorption of the main band at about 3495 cm -1increases, the low energy shoulder decreases and an additional broad absorption isbuilt up. For a decomposition and comparison the spectra were normalized with re-31

spect to the area. This clearly shows that the high energy wing of the main absorptiondoes not depend on the composition, see Figure 2.Figure 2: Normalized absorption of the OH-absorption inSr x Ba 1-x Nb 2 O 6 for (0.3 < x 0.8).Spectra for compositions with x above the value of the congruently melting composition(0.61) intersect at about 3380 cm -1 , while those for x below 0.61 at about3430 cm -1 . Sr-rich compositions cause spectra consisting of an almost symmetricalmain band and a broad low energy feature. In Ba-rich compositions this feature isweak, but the main band is highly asymmetric. Extensive decomposition trials haveshown, that the spectra can be very well described with three transitions, if we neglecta weak transition at about 3222 cm -1 . The transition I describing the main bandvaries so weakly with composition that we fixed it at 3492.7 cm -1 , the same is true forsecond transition II at 3454.5 cm -1 , while the third transition III shifts more than 50cm -1 with composition. The three transitions can be ascribed to hydrogen bond tooxygen in three different environments. Transition I is probably caused by a hydrogenvibrating towards an oxygen which belongs to a niobium octahedron and is located inthe ab-plane. Transition II may be due to an OH-stretching mode influenced by Ba,while type III is a mode related to Sr, which changes its position with respect to x thuscausing a frequency shift.References[1] K. Kitamura, Y. Furukawa, K. Niwa, V. Gopalan, T. E. Mitchell, Applied PhysicsLetters 73, 3073-3075, (1998)[2] K. S. Abedin, H. Ito, J. Appl. Physics 80, 6561-6563 (1996)32

[3] M. Wöhlecke, L. Kovács,Critical Reviews in Solid State and Materials Sciences, 26(1), 1 - 86 (2001)[4] S. Hunsche, A. Gröne, G. Greten, S. Kapphan, R. Pankrath, J. Seglins, PhysicaStatus Solidi A-Applied Research 148, 629 (1995).[5] M. Lee, H. S. Lee, R. S. Feigelson, J. Applied Physics 84, 1558 (1998).PublicationsCh. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:Composition dependence of the ultraviolet absorption edge in lithium tantalite:J. Appl. Physics 93, in print (2003).C. David, A. Tunyagi et al.: OH stretching modes in Sr x Ba 1-x Nb 2 O 6 (in preparation)Attended lecturesWS 01/02 : P. Hertel: Linear response theory.SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity.WS 02/03: H.-J. Schmidt: Nonlinear wave equationsWS 01/02: V. Trepakov: Optics and Spectroscopy of semiconductors and insulatorsWorkshop “Photorefractive Nonlinearities” (October 2001, Osnabrück).Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrück).Workshop “SBN: Crystal Growth and Details of the Structure” (July 2002, Universityof Osnabrück).Seminars of the Graduate College 695 (WS 01/02 SS 02 WS 02/03).Seminars of the Research Group “Optical Materials” (WS 01/02, SS 02, WS 02/03 ).Contribution to the SeminarsSeminary Talk on 27.01.2003 “Optical Properties of as grown and Hydrogen dopedSr x Ba 1-x Nb 2 O 6 ”Various short talks in the seminar of the research groupExternal research stayIR-absorption Measurements performed on SZFKI institute in Budapest (08.07.2002– 19.07.2002)Duration of the dissertation: Start 01.10.2001, termination expected 30.09.2004Period of support in the College: 01.10.2001 – 31.12.2003Supervisor: apl. Prof. Dr. Manfred Wöhlecke33

Dipl.-Phys. Oleg FilippovTopic: Vectorial beam coupling in fast photorefractive crystals with ACenhancedresponseResultsUp to now, the main results of our investigations can be divided into two parts:“Polarization properties of light-induced scattering in Bi 12 TiO 20 ” (see Section I),“Photorefractive AC-enhanced nonlinear response in sillenites” (see Section II). Theresults obtained are of interest for the use of fast photorefractive materials for variousapplications such as grating recording, linear detection of weak signals, and for characterizationpurposes. The work has been performed in close cooperation with Dr.B.I.Sturman, International Institute for Nonlinear Studies, Novosibirsk, Russia.I. Polarization properties of light-induced scattering in Bi 12 TiO 20Cubic crystals of the sillenite family (Bi 12 SiO 20 , Bi 12 TiO 20 , and Bi 12 GeO 20 ) andsemiconductors like GaAs, CdTe, InP are the fastest photorefractive materials, whichmakes them attractive for numerous applications. Some techniques are used to enhancethe weak nonlinear response of these materials. The most appropriate forpractical proposes is the AC-technique and therefore the photorefractive responseunder this technique was considered.Strong spatial amplification achieved in sillenite crystals manifests itself in pronouncedlight-induced (nonlinear) scattering [1,2]. The underlying mechanism of thisphenomenon is not complicated: Weak seed waves, arising due to the surface andbulk crystal imperfections, experience then a strong spatial amplification at the expenseof the pump.Due to the vectorial character of beam coupling in cubic crystals it is not possibleto separate the spatial changes of the light energy and polarization. Furthermore, thelight-induced scattering in this case is highly sensitive to the orientation of the electricfield about the principal crystal directions. This means that one must use the vectorialtheory of beam coupling for the description of scattering light phenomenon in such asystem [3].In contrast to the angular intensity distribution, the polarization states of the scatteredlight in cubic crystals were not yet analyzed theoretically. We have applied thevectorial theory of beam coupling [3] to describe the polarization properties of smallangle,light-induced scattering in cubic AC-biased BTO crystals for different polarizationstates of the incident pump beam [A1]. The diagonal geometry, distinguished bythe strongest vectorial coupling, was chosen for comparison between theory and experiment.We have found the angular intensity distribution for several representative casesof the pump beam. We have revealed that in the case of a horizontal pump beampolarization the scattered light has a pronounced horizontal right lobe. The maximumrate of spatial amplification (increment) is Γ max ≈ 48 cm -1 . The light-induced scatteringis strongest in this case. For the vertical polarization of the pump beam the intensitydistribution is quite different: it possesses one tilted left lobe at the azimuth angle ψ ≈150 0 . The maximum value of the increment is noticeably smaller here,Γ max ≈ 27 cm -1 .To analyze the effect of pump polarization on the scattering characteristics in moredetail, we have considered the cases of right and left circular pump polarization and34

also two cases of linear polarization with a polarization angle ±45 0 . Our theoreticalprediction is that the increment is almost the same for these four cases. We haveshown that the value of the increment in these cases can be represented as a halfsumof the increments for the cases of horizontal and vertical pump beam polarization.Hence the angular distribution consists of two lobes: the strongest one is tiltedby ≈15 0 to the horizontal axis, the weakest lobe is tilted ≈160 0 . The maximum valuesof the increment are Γ max ≈ 30 cm -1 and Γ max ≈ 8 cm -1 , respectively.Coming to the polarization properties of scattered light we have found that they aremore sensitive to the choice of the experimental and material parameters than theintensity distributions. This is especially true with respect to the weakest lobes.In cases of the horizontal and vertical pump beam polarization we have shown thatfor the horizontal lobe the scattering polarization has to be horizontal and for the tiltedlobe is vertical. Experimental polarization measurements confirm this prediction withhigh accuracy.For mixed pump polarization we have obtained that the polarization of the scatteredlight depends on the azimuth angle and does not depend on the polar angle.Since the most important propagation directions correspond to the maximum of theincrement we have determined the scattering light polarization for the azimuth angles,which correspond to the maximal intensity of the scattered light. For the main(strongest) scattering lobe, the polarization is almost horizontal for right-, left-circularand ±45 0 -pump polarizations. Experimental polarization measurements confirm thisresult. For the weakest lobe the intensity ratio of vertical/horizontal components of thescattered light is sufficiently small and therefore cannot be considered as big enoughto expect a quasi-vertical polarization of the scattering lobe. This theoretical predictionhas found only a qualitative experimental confirmation. Experiments show thatthe polarization of the weakest lobe is vertical for the cases of mixed pump polarization.II. Photorefractive AC-enhanced nonlinear response in sillenitesDuring the last decade, the enhancement of photorefractive response in fastphotorefractive crystals by the application of external AC fields is the subject of numerousexperimental and theoretical studies. Large applied fields (up to 50 kV/cm)have become available for AC-experiments. It was established that the low-contrastrange, where the fundamental component of the space-charge field grows linearlywith light contrast (m), is very narrow. Furthermore, the region of large light contrasthas become important in connection with the soliton propagation problem. Lastly, anumber of applications of fast photorefractive materials, such as detection of weaksignals and grating recording, are relevant to high-contrast effects.Particularly, it was found that a square-wave shape of the AC-field provides for thebest enhancement [4]. The enhancement properties are closely related to the presenceof weakly damped low-frequency eigenmodes (space-charge waves) and spatialsubharmonics generation. The results of numerical simulations of the largecontrasteffects in the AC-biased sillenites [5,6] are in good agreement with the experiments,but the understanding of physical background lacked.An analytical approach has been employed recently for the AC-enhancement description[7]. It is based on averaging over the fast AC-oscillations. Using this procedureit is possible to come to a rather general equation for the space-charge field profile.It was shown that the low and high contrast effects in AC-enhanced spacecharge formation could be uniformly described by a simple differential equation for35

the space-charge field. This equation was used recently for the description of beampropagation effects [7].We have applied this promising approach to the analysis of space-charge fieldformation during grating recording [A2]. We have obtained the contrast dependenceof the fundamental harmonic E 1 , the second E 2 and third E 3 harmonics of the spacechargefield in the whole range of light contrast. It was found that the photorefractiveresponse remains non-local within the whole contrast range. We have revealed thatthe quality factor Q (introduced in the low-contrast limit) determines the photorefractivenonlinear response in the whole contrast range and the dependence of this responseon Q is saturated for Q >> 1. This means that for different values of modelparameters corresponding to the same value of Q, the contrast dependence of thefundamental harmonic E 1 (m) is the same within good accuracy. The limiting value ofthe fundamental harmonic (for m close to unity) is also quite universal E 1 ≈ 0.64E 0(where E 0 is the amplitude of the external AC-field).For application purposes one should distinguish between low and high contrastregions. The region of small light contrast (m < 0.05) is the best for spatial amplification.For larger contrast, the amplification becomes smaller but this region is the mostappropriate one for grating recording. Redistribution of the light pattern is relativelyharmless here.Apart from the fundamental harmonic E 1 , responsible for beam-coupling effects,the first higher harmonics E 2 and E 3 are of practical interest. These harmonics can bemeasured with the help of auxiliary Bragg-matched light beams. They are very importantfor characterization purposes. We have found that higher harmonics of thespace-charge field become sufficiently large (up to 0.2-0.4E 0 ) already for relativelysmall values of light contrast. Since they are not weak, their direct measurementsshould not be difficult. We have revealed that the contrast dependence of the secondharmonic peaks at m ≈ 0.5 and then turns to zero at m = 1. We have also found thatthe contrast dependences of higher harmonics of the space-charge field correspondto the formation of the step-like field profile with increasing m.III. Future plansOur investigation of the space-charge field formation in the low- and the highcontrastregions allows to generalize on the whole contrast range the theory of vectorialbeam coupling, which was developed up to now only for the low-contrast case [3].The theory to be developed will describe fully the two-beam coupling in fast photorefractivecrystals under AC-enhancement in the whole contrast range. Unlike the scalartheory, the vectorial one allows to define in addition to intensity distributions ofbeams also their polarization properties. This is especially important for the caseswhere the optical activity essentially influences the beam coupling process. Theplaned generalization of the beam coupling theory would allow to find new conservationlaws, including the polarization degrees of freedom, and new regimes of polarization-dependentbeam coupling in cubic crystals of the sillenite family or in semiconductors.A prominent application of such effects is the linear detection of weak oscillatingsignals by means of polarization filtering. This new effect is feasible exclusivelydue to the vectorial character of beam coupling in cubic crystals. Its efficiency is expectedto gain because of the saturation of the space-charge field fundamental harmonicin the region of large contrasts. On the basis of the generalized vectorial theoryit is possible to optimize the detection technique.36

Another apparent manifestation of the AC-enhanced beam coupling is thelight-induced (nonlinear) scattering in sillenites. Application of the vectorial theory tothe analysis of scattering characteristics was restricted to the low-contrast limit. Ourapproach would allow explaining the effects of saturation, which are clearly seen inexperiments but are missing in existing theoretical considerations.In summary, the main directions of our future investigations are:1. (2003) The generalization of the vectorial beam coupling theory on the wholelight contrast range. Explanation of the saturation effects in photorefractivescattering in sillenites.2. (2004) Theoretical study of the optimization of the linear detection technique.References[1] E. Raita, A. A. Kamshilin, and T. Jaaskelainen, ”Polarization properties offanning light in fiberlike bismuth titanium oxide crystals”, Opt. Lett. 21, 1897-1899 (1996).[2] A. A. Kamshilin, V. V. Prokofiev, and T. Jaaskelainen, ”Beam fanning anddouble phase conjugation in a fiber-like photorefractive sample”, IEEE J.Quant. Electron. 31, 1642-1647 (1995).[3] B. I. Sturman, E. V. Podivilov, K. H. Ringhofer, E. Shamonina, V. P.Kamenov, E. Nippolainen, V. V. Prokofiev, and A. A. Kamshilin, ”Theory ofphotorefractive vectorial wave coupling in cubic crystals”, Phys. Rev. E 60,3332-3352 (1999).[4] S. I. Stepanov and M. P. Petrov, Opt. Commun. 53, 292, 1985.[5] J. E. Millerd, E. M. Garmire, M. B. Klein, B. A. Wechsler, F. P. Strohkendl,and G. A. Brost, J. Opt. Soc. Am. B ,1449, 1992.[6] G. I. Brost, J. Opt. Soc. Am. B 9, 1454, 1992.[7] G. F. Calvo, B. I. Sturman, F. Agull-Lpez, and M. Carrascosa, Phys. Rev.Lett. 84, 3839, 2000.Publications:[A1] O. Filippov, K. H. Ringhofer, M. Shamonin, E. Shamonina, A. A. Kamshilin, E.Nippolainen, B. I. Sturman, “Polarization properties of light-induced scatteringin Bi 12 TiO 20 crystals: Theory and experiment for the diagonal geometry”, acceptedby JOSA B.[A2] O. Filippov, K. H. Ringhofer, B. I. Sturman, “Photorefractive ac-enhancednonlinear response of sillenites: Low- and high-contrast effects”, accepted byEuropean Physical Journal D.37

Attended lectures, conference visits, research stays:1.2.3.P. Hertel: Linear response theoryE. Krätzig, K. Ringhofer: The photorefractive nonlinearityH.-J. Schmidt: Nonlinear wave equationsSeminars and workshops of the Graduate College 695Duration of the dissertation: Start 15.11.2001, termination expected 15.11.2004Period of support in the College: 15.11.2001 - 31.12.2003Supervisors: Prof. Dr. K. H. Ringhofer , Dr. M. Gorkounov, Prof. Dr. E. Krätzig38

Dipl.-Phys. Andreas GeislerTopic: Properties of one-dimensional spatial solitons in photorefractive mediaResults39

Attended lecturesWS 01/02: P. Hertel, Linear response theorySS 02: E. Krätzig/K. Ringhofer, The photorefractive nonlinearityWS 02/03: H.-J. Schmidt, Nonlinear wave equations- Workshop "Photorefractive Nonlinearities" (October 2001, Osnabrück)- AMOP (March 2002, Osnabrück)Duration of the dissertation: start WS 2000, termination expected SS 2004Period of support in the college: -Supervisor: Prof. Dr. H. W. Schürmann43

Dipl.-Phys. Airat GubaevTopic: Light-Induced absorption changes in the visible and infrared range inferroelectric crystals.ResultsIntroductionThe photorefractive crystals Sr x Ba 1-x Nb 2 O 6 (SBN) and Ba 1-y Ca y TiO 3 (BCT)can be grown in a congruent composition (melt and crystal have the same compositionin SBN for x-0,61 and in BCT for y=0,23), which allows to produce large, homogeneoussamples ideally suited for applications. The photorefractive properties canbe enhanced by doping with polyvalent ions like Ce, Cr etc.. The light-induced chargetransport from the doping ions and trapping in shallow polaronic states has beenidentified by photo EPR [1] and optical experiments [2] to constitute the underlyingprocess. The trapping of those photo-induced charge carriers in specific centers canbe studied especially well at low temperatures, where the centers display a ratherlong lifetime and the build-up of the resulting space charge field, which is modifyingthe refractive index, can be investigated with various spectroscopic techniques. Thepolaronic NIR absorption centers have been studied already in some detail [3],whereas the thermally more stable VIS centers are more difficult to describe by theoreticalmodels and need further experimental investigation to clarify the situation.Experimental techniquesFor the detailed spectral measurements a Fourier spectrometer (Bruker IFS120HR) and a Beckman Acta VII grating spectrometer are used to cover the spectralrange from UV to FIR. A liquid helium bath cryostat (Leybold) is employed in the absorptionmeasurements and the crystals are immersed in superfluid helium (at 2K) orin helium exchange gas.As illumination source, we use a Ar + - and a Kr + - laser (spectra physics 171).Experimental results.In continuation of the work of I.Kislova we studied the light-induced (Ar + -laser) absorption charges in SBN:Ce to yield a quantitative description of the nonlinearintensity and temperature dependence of the relevant physical parameters. Thespecific cerium–related FIR bands in SBN:Ce have been investigated at first to yielda quantitative description of the Cerium-center properties and its concentration andtemperature dependence.As a next step we plan to investigate the light-induced dissociation (underKr + - laser light) of VIS centers in SBN:Ce. This dissociation under simultaneous buildupof a transient NIR-Polaron population has qualitatively be seen and described byI.Kislova [4], but it needs further quantitative work to elaborate the underlying processes.Besides additional optical absorption measurements we plan to investigatethe photoconductivity and the photo-Hall effect under Kr + - laser illumination, to getnew information about the charge carriers created. We hope that these results willthen yield clear evidence for the nature of the VIS centers, which are currently beingdiscussed as either bipolarons (in analogy to such centers in LiNbO 3 ) or polaronstrapped at charged centers, or charge transfer vibronic exitons (CTVE) being trappedat charged centers [5].[1] A.Mazur, C.Veber, O.F.Schirmer, C.Kuper, and H.Hesse. J.Appl.Phys. 85, 6751(1999)44

[2] M.Gao, R.Pankrath, S.Kapphan, V.Vikhnin. Appl.Phys. B. 68, 849 (1999)[3] M.Gao, S.Kapphan,R.Pankrath, X.Feng, Y.Tang, V.Vikhnin. J.Phys.Chem..Sol.,61, 1775 (2002)[4] I.Kislova report to Grad.Koll 695 and S.Kapphan, I.Kislova, M.Wierschem,T.Lindemann, M.Gao, R.Pankrath, V.Vikhnin, A.Kutsenko. Rad.Eff. and Defects inSolids, 2003 (in print)[5] V.S.Vikhnin, S.Avanesyan, H.Liu, S.E.Kapphan. J.Phys. and Chem. of Solids, 63,1677 (2002)Duration of the dissertation: Start 05.12.02 , termination expected end 2005Period of support in the College: 05.12.02 - 31.12.03Supervisor: Prof. Dr. S. KapphanA. Gubaev continues the project of I. Kislova who returned to her home country Russiafor personal reasons. Hence he started only in December 2002.45

Dr. Vladimir KamenovTopic: Critical phenomena in OpticsResultsThe investigations are divided into two main fields: “Critical enhancement of photorefractiveresponse” (see Section I) and “Critical phenomena for feedback-controlledphotorefractive beam coupling” (see Section II).I. Critical enhancement of photorefractive responseThe cubic crystals of the sillenite family [e.g., Bi 12 SiO 20 (BSO), Bi 12 TiO 20 (BTO), andBi 12 GeO 20 (BGO)] are attractive because of their fast photorefractive response. Themain disadvantage of sillenites is their weak photorefractive response. There are twoconvenient methods (DC and AC) for enhancement of the photorefractive responseof cubic crystals [1, 2]. These methods employ a DC or AC external electric field anda proper frequency detuning between the interacting light beams. The enhanced exponentialgain factor reaches the values of a few tens of cm -1 .It is well known that the enhanced two-beam coupling is often accompanied by subharmonicgeneration owing to a parametric excitation of weakly damped lowfrequencyspace-charge waves (SCWs) [3, 4]. In the most important case, the fundamentalspace-charge grating with grating vector K, recorded by a pair of pumpbeams, becomes unstable against the spontaneous growth of a SCW with the spatialfrequency K/2 (the subharmonic grating). This instability is a threshold phenomenon:the light contrast m has to exceed a certain threshold value mth≈ 3/ Q, where Q is thequality factor of the fundamental SCW. An important feature of the subharmonic generationis that the K/2-grating becomes very pliable to any driving force when approachingthe threshold of subharmonic generation.Recently, it was proposed to use this pliancy of the subharmonic grating for an additional(critical) enhancement of the photorefractive response [5]. In contrast with theabove enhancement methods [1,2], the exponential gain factor for the critical enhancementcan basically be arbitrary large. Approaching the threshold of the subharmonicgeneration, the gain factor grows infinitely. This novel critical effect hasbeen missed in the previous studies because some important terms related to theeffect of the material nonlinearity had been omitted in the initial equations.Unfortunately, the model considered in [5] does not include such important attributesof the coupling experiments in cubic photorefractive crystals as the vectorial characterof the beam coupling and the longitudinal inhomogeneity of the pump intensityowing to light absorption. The first factor produces spatial oscillations of the couplingstrength and the second one makes the resonance value of the frequency detuningdependent on the propagation coordinate (i.e., broadens the resonance) [6,7]. Therefore,there was a gap between the basic idea of the critical enhancement expressedin cite [5] and the capability of the theory to indicate the necessary conditions for detectionand possible utilization of this novel phenomenon.Our work [A1, A2] aims for an extended analysis of the critical enhancement by takinginto account the above attributes. This includes the formulation of a vectorial52

model of the critical enhancement incorporating the effect of spatial inhomogeneity,an analytical treatment of this model, and a numerical characterization of the criticalspatial amplification. We have shown that the real attributes of subharmonic experimentsaffect considerably the apparent characteristics of the critical enhancement butdo not suppress this effect. Our analytical and numerical results have allowed to optimizethe conditions for detection of the critical enhancement in BSO crystals and topredict the main observable features including polarization, spectral, and orientationproperties. The possibility to achieve a very strong spatial amplification in thin crystals( d ≈1mm ) and to avoid in this way numerous extraneous effects is an importantprediction of our theory.II.Critical phenomena for feedback-controlled photorefractive beam couplingWhen phase volume holograms are recorded in photorefractive crystals, a 100% diffractivityof the recorded grating is often desirable. It has been shown [8] that whenan active feedback stabilization is applied to LiNbO 3 crystals, a diffraction efficiencyof unity can be achieved for a wide range of experimental parameters. This factopens new possibilities for thermal fixing [9] and for reducing the light scattering [10].The main function of the feedback loop is to keep the phase difference between thetransmitted signal beam and the diffracted pump beam in the direction of the signalbeam equal to π /2. This is realized by a proper phase modulation of the input signalbeam.In our work [A3, A4], we investigate the dynamics of the feedback-assisted beamcoupling. We show that two qualitatively different modes of operation are possiblewhen feedback stabilization is applied to photorefractive crystals with local response.If the initial intensity ratio, β > 1, is bigger than some threshold value, βth, the feedbackchanges the phase of the signal beam linearly in time. The corresponding diffractionefficiency of the photorefractive grating is less than 100%. For β < βth, theinitial signal phase consists of an oscillation periodic in time superimposed on lineargrowth. In this case, the diffractivity of the recorded photorefractive grating is 100%.We show that the transition between these two modes of operation is similar to aphase transition, with a critical slowing down of the periodic phase variations. For thecase with periodic phase variations of the signal beam, the system undergoes severaladditional phase transitions: we have found a variety of qualitatively different periodicmodes and non-trivial transitions between them. Good qualitative agreementbetween theory and experiment is obtained for LiNbO 3 crystals.[1] P. Refregier, L. Solymar, H. Rajenbach, and J. P. Hiugnard, „Two-beam couplingin photorefractive Bi 12 SiO 20 crystals with moving grating: theory and experiment”,J. Appl. Phys. 58, 45-57 (1985).[2] S. I. Stepanov and M. P. Petrov, „Efficient unstationary holographic recordingin photorefractive crystals under alternating electric field” Opt. Commum. 53,292-295 (1985).[3] B. I. Sturman, M. Mann, J. Otten, and K. H. Ringhofer, “Space-charge wavesin photorefractive crystals and their parametric excitation”, J. Opt. Soc. Am. B10, 1919-1932 (1993).53

[4] L. Solymar, D. J. Webb, and A. Grunnet-Jepsen, “The Physics and Applicationsof Photorefractive Materials “, Claredon, Oxford, 1996.[5] E. V. Podivilov, B. I. Sturman, H. C. Pedersen, and P. M. Johansen, “Criticalenhancement of photorefractive beam coupling”, Phys. Rev. Lett. 85, 1867-1870 (2000).[6] D. J. Webb and L. Solymar, “The effects of optical activity and absorption ontwo-wave mixing in Bi 12 SiO 20 ”, Opt. Commun. 83, 287-294 (1991).[7] B. I. Sturman, A. I. Chernykh, V. P. Kamenov, E. Shamonina, and K. H. Ringhofer,“Resonant vectorial wave coupling in cubic photorefractive crystals“ J.Opt. Soc. Am. B 17, 985-996 (2000).[8] A. A. Freschi and J. Frejlich, “Stabilized photorefractive modulation recordingbeyond 100% diffraction efficiency in LiNbO 3 :Fe crystals”, J. Opt. Soc. Am. B11, 1837-1841 (1994).[9] S. Breer, K. Buse, K. Peithmann, H. Vogt, and E. Krätzig, “Stabilized recordingand thermal fixing of holograms in photorefractive lithium niobate crystals”,Ref. Sci. Instrum. 68, 1591-1594 (1998).[10] P. M. Garcia, A. A. Freschi, J. Frejlich, and E. Krätzig, “Scattering reduction forhighly diffractive holograms in LiNbO 3 crystals”, Appl. Phys. B 63, 207-208(1996).Publications[A1] E. V. Podivilov, B. I. Sturman, K. H. Ringhofer, M. V. Gorkunov, and V. P.Kamenov, “Theory of critical enhancement of the photorefractive response”,Phys. Rev. E, 65 046623 (2002).[A2] E. V. Podivilov, B. I. Sturman, K. H. Ringhofer, M. V. Gorkunov, V. P.Kamenov, H. C. Pedersen, and P. M. Johansen “Modeling of critical enhancementof photorefractive response in cubic crystals”, OSA TOPS 62, p. 230(2000).[A3]E. V. Podivilov, B. I. Sturman, S. G. Odoulov, S. M. Pavlyuk, K. V. Shcherbin,V. Ya. Gayvoronsky, K. H. Ringhofer, and V. P. Kamenov, “Wealth of dynamicregimes for feedback-controlled photorefractive beam coupling”, OSA TOPS 62,p. 221 (2000).[A4] K. V. Shcherbin, S. M. Pavlyuk, S. G. Odoulov, K. H. Ringhofer, V. P.Kamenov, E. V. Podivilov, and B. I. Sturman, “Critical phenomena for feedbackassistedphase grating recording”, OSA TOPS 62, p. 616 (2000).Duration of the dissertation: PostdocPeriod of support in the College: 01.01.01 - 30.08.2001Supervisors: Prof. Dr. K. H. Ringhofer, E. KrätzigDr. Kamenov left the Graduate College 30.08.01 to start an activity at the Carl ZeissAG, Oberkochen.54

Dipl.-Phys. Inna KislovaTopic: Light-induced absorption changes in ferroelectric crystalsI. ResultsIntroductionOur research project is aimed at investigating the optical and dielectric properties ofthe crystals Sr x Ba 1-x Nb 2 O 6 (SBN, x=0.61) pure, doped with Ce, Cr ions or doublydoped with Ce and Cr and of the Ba 1-y Ca y TiO 3 (BCT, y=0,23) crystals doped with Fe.Both promising photorefractive crystal systems Ba 1-Y Ca Y TiO 3 and Sr x Ba 1-x Nb 2 O 6possess a congruently melting mixture (for SBN x=0,61 and for BCT y=0,23) [1,2].This allows to grow large, homogeneous crystals of excellent optical quality, which isthe basis for a wide range of optical applications [3]. Due to the statistical distributionof the constituents and a partially unfilled (tungsten bronze) structure for SBN, theferroelectric phase transition (T c ≈373 K for congruent BCT pure and T c ≈353 K forcongruent SBN pure) shows a relaxor type character with polar contributions wellabove T c . The electro-optical coefficients of the pure crystals are already large andcan be enhanced considerably by suitable doping with polyvalent ions like thosementioned above [4,5]. For some of the dopants (like Ce and Cr in SBN) a majoritycharge state 3+ has been determined [6,7], however with an individual site occupancy,Ce 3+ replacing Sr 2+ ions and Cr 3+ sitting on the Nb 5+ sites [8,9]. These dopantscan be identified by their broad impurity induced absorption bands in the visiblerange, a shift of the UV- absorption edge to longer wavelength in the case of Cr dopingand additional Far-IR bands (near 2000cm -1 ) in the case of Ce-doping [6]. A lightinducedcharge transport from these doping ions and trapping in shallow polaronicstates (Ti 3+ in BCT respectively Nb 4+ in SBN) has been identified by photo-EPR [10]and optical experiments [11] to constitute the underlying processes for the enhancedphotorefractive properties in doped crystals. The majority of photo-excited chargecarriers have been determined by laser beam coupling experiments [12] and Halleffect[13] measurements to be electrons. The trapping of these photo-inducedcharge carriers in certain centers can be considered as the first step in the build-up ofspace charge fields which modify the refractive index and are the basis of thephotorefractive effect under non-uniform spatial illumination. The properties and thephysical nature of the centers created under illumination have been identified so faronly to some extent and are investigated further in this study with several techniques.Experimental techniquesA Fourier spectrometer (Bruker IFS 120 HR) and a Beckman Acta VII grating spectrophotometerwere used to measure the absorption spectra from the UV to the FIRregion. A Helium bath cryostat (Leybold) was employed in absorption measurementsand the crystals were immersed in superfluid helium (2 К) or in Helium exchange gas.A Ar + - and Kr + - laser (spectra physics 171) were used as illumination sources.Photoluminescence and excitation spectra were measured using a photon countingsystem. A high pressure Xenon lamp was used as the excitation source. A closedcycle cryostat (Leybold) was used for the Photoluminescence and Thermoluminescencemeasurements.Dielectric susceptibility near Tc was measured in a temperature variable set-up with aHP 4270A automatic bridge.55

Experimental resultsa) Variation of doping-dependent properties in photorefractive Sr x Ba 1-x Nb 2 O 6 : Ce, Cr,Ce+CrDoping SBN crystals with Ce and Cr induces broad dichroic absorption bands. Theabsorption coefficients in the visible region (at 514 nm) for SBN single crystals increaselinearly with the Ce or Cr (up to ~ 20 000 ppm., p.f.u., (per Nb 2 )) concentration.In Ce-doped crystals the integral FIR absorption of the Ce 3+ bands near 2000cm -1 also vary linearly with the Ce concentration in the crystal, providing an independentmethod to estimate the Ce 3+ - content in double doped crystals even wherethe UV-VIS absorption bands of Cr and Ce in SBN overlap. Comparison of individualconcentrations determined in double doped SBN:Ce+Cr and of single doping casesshows no increase in the respective built-in coefficients of Ce and Cr for co-doping,giving no evidence of a self compensation of Cr by Ce centres. For the Cr-dopedcrystals a shift of the UV-absorption edge to longer wavelength with increasing Crdopingis observed as well.Fe 2+/ 3+ centers in BCT have been detected in photo-EPR experiments with absorptionbands at 2eV and 3.5eV, respectively [10].b) Dielectric measurements in the SBN crystals doped with Cr and CeThe dielectric measurements show for both Ce and Cr doping about the same shift ofthe phase transition temperature Tc, decreasing with increasing dopant concentration.The concentration dependence of the transition temperature for co-doped SBN:Ce+Cr appears to be nearly the same as for single doping cases taking into accountthe total impurity centre concentrations [publ.3]. For values of about 20000 ppm(p.f.u.) Tc reaches about room temperature. The width at half maximum of the dielectricpermitivity (ε 33 ) versus temperature increases considerably with increasing concentration.c) Photo- and thermoluminescence in the SBN crystalsOne can observed a broadband green (λ em about 490nm) and a redphotoluminescence(red-PL) band (λ em about 765nm) with UV excitation (λex=350nm). The green-photoluminescence (green-PL) can be excited only at the UV-bandedge, whereas the red-PL can be excited also at longer wavelength. The red(765nm) emission intensity is increasing linearly with the Cr-doping for concentrationsup to about 5000 ppm Cr, with increasing deviations at higher concentrations. Theexcitation spectra of the red luminescence, follow closely the shape of the Cr or Ceinducedabsorption band and the Cr-induced shift of the UV-band edge. The timedependence of the decay of the PL emission after excitation shut-off is nearly monoexponential.The decay time constant is about 3msec at 20 K, getting shorter athigher temperatures. In single doped Cr,Ce SBN as well as in SBN:Ce+Cr two wellseparated Thermoluminescence-peaks can be observed at about 90 K and at about220 K, after low temperature (10K) excitation with a Xenon-lamp and a subsequentwaiting period of about 10 minutes before measurement with a heating rate of 5K/minfrom 10 to 320 K. The spectral distribution of the line shape of the PL and of the TLemission are at first sight the same, indicating a similar emission process after theliberation of the respective charge carriers. However, a closer inspection yields aspectral fine-structure with at least three strong emission subbands at 766nm, 775nmand at 830nm. These sub bands decay with different time constants (λ em = 766nmwith τ=3,7ms and λ em= 775nm with τ=4,4 ms at 10K). The sub band at 766nm can bepreferentially excited in two spectral regions λ ex =350nm and λ ex =650nm, whereas56

the subband at 775nm is more prominent for excitations at wavelength λ ex =470nm.This longer wavelength subband also is getting more intensive with increasing Crdoping in the crystals. Both Thermoluminescence emission peaks, at 90 K and at 220K, show roughly the same spectral distribution in agreement with the Photoluminescenceemission band, pointing at only slightly different recombination processes evenfor the doubly doped SBN:Ce+Cr crystals.d) Light-induced absorption changesUnder illumination with Ar + - laser light (488 nm) at low temperature (2K, crystal immersedin superfluid liquid He) two broad dichroitic light-induced absorption bandscan be observed in BCT:Fe and similarly in SBN:Cr,Ce. The first absorption band(VIS centers) is observed around 2eV and the second in the NIR around 0.7eV (6000cm-1). The NIR absorption has been identified previously by photo-EPR as belongingto Ti 3+ small polarons in BCT or to Nb 4+ small polarons in SBN [10]. The centers responsiblefor the VIS absorption have not been identified yet, but obviously are producedsimultaneously with the NIR polarons. Both, the VIS and the NIR light-inducedabsorption bands depend on polarization and nonlinearly on illumination intensity.The temperature dependence in the production of these centers shows a steepchange at about 100K for the NIR polarons in SBN (at about 40 K for BCT) and atabout 200K for the VIS centers (at about 80 K for BCT). These characteristic temperaturesare also revealed in thermoluminescence studies of SBN:Ce, Cr as intensitypeaks, where charge carriers (electron-polarons) are thermally excited in shallowtraps, followed by a hopping mobility of the liberated polarons till radiative recombinationwith deep trapping centers is occurring. The steady state of the light-induced absorptionunder illumination and the kinetics of its decay after a switch-off of the illumination, can be described by a simple model of charge transport from doping centers(Fe 2+ + Ti 4+ ↔ Fe 3+ + Ti 3+ in BCT, Ce 3+ + Nb 5+ ↔Ce 4+ + Nb 4+ in SBN) with subsequentrecombination as reported previously for SBN [11, 14].e) Light-induced dissociation of VIS centersThe Ar + light-induced VIS centers are rather stable at 2 K, whereas the NIR centersdecay rather fast and disappear completely within less than 50 sec in BCT:Fe (100sec in SBN:Ce) [14, 15]. This allows to perform experiments in the following way. Aftercreating a sizeable population of NIR polarons and of VIS centers at 2 K, the Ar + -laser was switched-off. After waiting about 7 min. to let the NIR-polarons decay completely,then a Kr + - laser was switched-on. First a build-up of NIR polaron absorptionand then a transient decay of this NIR absorption (depending strongly on the Kr + -laser intensity) with a simultaneous decay in the VIS-absorption is observed.After switching-off the Kr + -laser, the NIR polaron absorption decays with its own,characteristic recombination decay time. This clearly demonstrates the dissociation ofthe VIS centers into small polarons and has been observed both, in BCT:Fe and inSBN:Ce.ConclusionsThe nature of the NIR centers as small polaron centers is well established in crystalslike BaTiO 3 [16] or LiNbO 3 [17], and similarly in SBN [11] and BCT [10]. Their characteristicNIR-absorption exhibits in SBN and BCT a temperature- and intensity dependentbehaviour, the details of which are not fully understood yet and warrant furtherstudies.The VIS-centers are discussed as possibly being either bipolarons (in analogy tosuch centers in LiNbO 3 ), or polarons trapped at charged centers, or charge transfer57

vibronic excitons (CTVE) being trapped at charged centers [15]. The present experimentsdo not yet allow to draw unambiguous conclusions – but one of the dissociationproducts must be an electron polaron.References1.C.Kuper, R.Pankrath, H.Hesse, Appl.Phys. A65, 301 (1997)2.R.Neurgaonkar, W.Cory, J.Oliver, H.Ewbank, W.Hall, Opt.Eng.26, 392 (1987)3.P.Guenter, J.P.Huignard, Top. In Appl. Phys.:Photorefr. Mat. 61/62(Springer-Berlin)(1988)4.C.Kuper, K.Buse, U.v.Stevendaal, M.Weber, T.Leidlo, H.Hesse, E.Krätzig, Ferroelectrics,208/209,213(1998)5.Y.Tomita, A.Suzuki, Appl.Phys., A 59, 579 (1994)6.G.Greten, S.Hunsche, U.Knuepfer, R.Pankrath, U.Siefker, N.Wittler, S.Kapphan,Ferroelectrics 185,289 (1996)7.R.Niemann, K.Buse, R.Pankrath, M.Neuman, Sol. St. Commun.98, 209(1996)8.T.Woike, G.Wekwerth, H.Palme, R.Pankrath, Solid St. Comm.102, 743 (1997)9.T.Woike, U.Doerfler, L.Tsankov, G.Weckwerth, D.Wolf, M.Woelecke, T.Granzow,R.Pankrath, M.Jmlau, W.Kleemann, Appl.Phys.B72, 661 (2001)10.A.Mazur, C.Veber, O.Schirmer, C.Kuper, H.Hesse, J.Appl.Phys.,85,6751 (1999)11.M.Gao, R.Pankrath, S.Kapphan, V.Vikhnin, Appl.Phys. B68, 849 (1999)12.M.Ewbank, R.Neurgaonkar, W.Cory, J.Feinberg, J.Appl.Phys.62, 374 (1987)13.A.Gerwens, K.Buse, E.Kraetzig. J. Opt. Soc. Am.B15, 2143 (1998)14.M.Wierschem, T.Lindemann, R.Pankrath, S.Kapphan, Ferroelectrics 264, 315(2001)15.M.Gao, S.Kapphan, R.Pankrath, X.Fenq, Y.Tang, V.Vikhnin, J. Phys. Chem. Sol.61, 1775 (2002)16. S.Koehne, O.F.Schirmer et.al., J.Supercond., 12, 19 (1999)17.E.Krätzig, O.F.Schirmer in “Photorefractive Materials and their Applications”(Ed.P.Guenter,J.P.Huignard) Topics in Appl.Phys., Vol. 61, Springer Berlin (1988)III.Publications1. “Photo- and thermoluminescence in congruent SBN crystals doped with Ce andCr.” I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A. B. Kutsenko, V. S.Vikhnin.Ferroelectrics, 2002, Vol.273, pp.187-192.2.”Charge transfer vibronic excitons and excitonic-type polaron states: photoluminescencein SBN.” Vikhnin V.S., Kislova I., Kutsenko A.B., Kapphan S.E.Solid State Communications 121 (2002) 83-88.3. “Variation of doping-dependent properties in photorefractive Sr x Ba 1-x Nb 2 O 6 : Ce,Cr, Ce+Cr.” S. Kapphan, B. Pedko, V. Trepakov, M. Savinov, R. Pankrath and I. Kislova.Rad. Effects and Defects in Solids, 2002 (in print).4. “Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 doubly doped with Ce and Cr: photo- and thermoluminescenceinvestigations.” I.L. Kislova, M. Gao, S.E. Kapphan, R. Pankrath, A.B.Kutsenko, V.S.Vikhnin. Rad. Effects and Defects in Solids, 2002 (in print).5. “Light-induced plaronic absorption at low temperature in pure and (Fe, Ce, Cr)doped Sr x Ba 1-x Nb 2 O 6 or Ba 1-y Ca y TiO 3 crystals and photodissociation of VIS centersinto small polarons.” S. E. Kapphan, I. Kislova, M. Wierschem, T. Lindemann, M.Gao, R. Pankrath, V. S. Vikhnin ,A. B. Kutsenko. Rad. Effects and Defects in Solids,2002 (in print).58

IV.Attended lecturesWS 01/02 : P. Hertel: Linear response theory.SS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearity.WS 01/02: V.Trepakov: Optics and Spectroscopy of semiconductors and insulatorsWorkshop “Photorefractive Nonlinearities” (October 2001, Osnabrueck).Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrueck).Seminars “Laser Optics” (WS 01/02 SS 02).Seminars of the Graduate College 695 (WS 01/02 SS 02).V. Conference visits1. 10 th International Meeting on Ferroelectricity (IMF-10) (September 2001, Madrid,Spain). (2 posters).2. 9 th Europhysical conference on defects in insulating materials (EURODIM 2002,July 2002, Wroclaw, Poland). (2 posters and 1 oral contribution).3. 16 th Russian Conference on Physics of Ferroelectrics (September 2002. Tver,Russia). (1 report).VI.Duration of the dissertation: Start 01.08.01 - The candidate was leaving forpersonal reasons 31.10.02 (Serious illness of her father in Tver, Russia).VII. Period of support in the College: 01.08.01 - 31.10.02Supervisor: Prof. Dr. Siegmar Kapphan59

Dipl.-Phys. Mikhail LapineTopic: Microwave interactions in nonlinear metamaterialsResultsThe work was performed in close collaboration with Prof. L. Solymar and Dr.E. Shamonina from the Dept. of Engineering Science of the University of Oxford.Metamaterials are artificial structures, composed as a regular lattice of identicalelements. They attracted growing interest in the recent years. This was motivatedby increasing attention to the microwave range in electromagnetics, as metamaterialsoffer new possibilities for manipulations with microwaves.Common principles of structural organization make metamaterials similar tocrystals. However, the scale is different, and this shifts the applicable range of electromagneticradiation to microwaves.In most cases the suggested applications of metamaterials (e.g., magneticfield guides [1] are concerned with the linear properties. Analogy between crystalsand metamaterials encourages us to consider also the nonlinear properties.The most promising metamaterial among the suggested ones [2], which wasalso experimentally studied [3], is based on circular conductive elements. However,in the current literature no proper theory describing metamaterials was given andprior to the analysis of nonlinear properties we had to develop a linear theory for theresponse of a similar metastructure, which allows for analytical consideration [A1, A2,A3]. The metamaterial we consider is assembled as a regular lattice of split conductiverings. The developed theory is based on the same principles as the theory of opticallinear response in crystal optics. The microscopic problem on the level of structureelements is solved with the help of the impedance matrix, assuming that the responseis local and the mutual interaction is described in the quasi-static limit. Thenmacroscopic averaging yields the effective parameters. The obtained permeabilityshows frequency dispersion with the resonance frequency of the metamaterial beingshifted from the resonance of a single element. This shift depends on the lattice constantsand type. The effect is very remarkable, but it was not taken into account byother authors due to rather rough approximations and doubtful assumptions they followed.Above the resonance the real part of permeability is negative. The frequencyrange of negative values depends strongly on the lattice type. We found that thisrange is most extended for a hexagonal arrangement of rings in a plane with theneighboring layers being maximally shifted with respect to each other. We supportedthe analytical consideration with numerical calculations. These were performed by abinitio solving Maxwell's equations for a finite structure consisting of a few thousandelements and subsequent numerical averaging. The permeability obtained in this wayis independent of the shape of the sample and appears to be in an excellent agreementwith the analytical results.In order to provide nonlinearity to the response of the structure element it wassuggested to use diode inclusions [4]. The arising multi-wave interactions allow toaffect the wave propagation directly in a convenient “all-optical” manner, i.e., withoutconversion into electronic signals.To calculate the nonlinear susceptibility of the metamaterial with the diode insertionswe generalize the approach, which we developed for the linear case. Weconsider [A4] a weak nonlinearity, for which the current-voltage characteristic of adiode includes a quadratic nonlinear term. This leads to the coupling of the Fouriercomponents of the fields and currents at different frequencies. A detailed analysisshows that a three-wave interaction occurs. We finally obtain the magnetization of the60

metamaterial in a form analogous to the polarization of an optical medium with aquadratic dielectric nonlinearity, and we derive an analytical expression for the quadraticnonlinear susceptibility. It is determined by the properties of a single element aswell as by the linear properties of the metamaterial. Like the optical nonlinearity, thenonlinearity of the magnetic metamaterial increases resonantly as one of the frequenciesinvolved approaches the resonance of the linear susceptibility.The general symmetry of Maxwell's equations with respect to the magneticfield - electric field transposition allows to expect that one can deal with the nonlinearinteraction of electromagnetic waves in the proposed metamaterial using the welldevelopedapparatus of nonlinear optics. The whole variety of known nonlinear opticalprocesses can have the corresponding analogues in metamaterials.For practical estimations we consider an example of metastructure made ofrings with radius r 0 = 2mm, arranged with the density n ~ r 0 -3 . Choosing backwarddiodes as nonlinear insertions, as they possess the best sensitivity and the highestnonlinearity, we estimate that a nonlinear contribution to the susceptibility of the orderof 0.001 can be achieved. However, this is accompanied by significant losses. Tomake use of a nonlinear metamaterial we have to ensure that the ratio of the nonlinearcontribution to the damping (the latter being determined essentially by the imaginarypart of the linear susceptibility) is as high as possible. This figure of merit appearsto be proportional to the ratio of the impedance of the diode to its resistance,|Z(ω)|/R. For backward diodes it is of the order of unity, and their usage can be limitedby losses. A promising opportunity is offered by varactors. For varactors the capacitiveimpedance can be much higher than the resistance and it is only necessaryto ensure that this condition is fulfilled in the desired frequency range.It is clear that nonlinear metamaterials open vast possibilities for the applicationstaking the advantage of “all-optical” manipulations with microwaves. The developedtheory covers an important particular case of weak nonlinearity, which allowsfor comprehensible theory, analogous to nonlinear optics. However, the use of diodesin the mode of strong nonlinearity is quite desirable for the applications. Thiscase cannot be described in a way similar to optics, and requires an extended theory,which we plan to develop in 2003. The detailed analysis of practically interestingnonlinear processes with microwaves, such as parametric amplification, frequencyconversion, phase conjugation, etc., will be carried out in 2004.[1] E. Shamonina, V. A. Kalinin, K. H. Ringhofer and L. Solymar, J. Appl. Phys. 92,6252 (2002).[2] J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, IEEE Trans. MicrowaveTheory Techn. 47, 2075 (1999).[3] R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 77 (2001).[4] V. A. Kalinin and V. V. Shtykov, Sov. J. Commun. Technol. Electron. 36, 96(1991).Publications:[A1] M. Lapine, M. Gorkunov, E. Shamonina, and K. H. Ringhofer, “Permeability of ametamaterial made of conductive rings”, Proc. of 9th Int. Conf. on Electromagneticsof Complex Media, 65 (2002).61

[A2] M. Gorkunov, M. Lapine, E. Shamonina, and K. H. Ringhofer, “Effective magneticproperties of a composite material with circular conductive elements”, Eur.Phys. J. B, 28, 263 (2002).[A3] E. Shamonina, L. Solymar, V. A. Kalinin, M. Lapine, and K. H. Ringhofer, “Fluxdistributions in a non-resonant magnetic metamaterial”, Proceedings of theProgress in Electromagnetics Research Symposium PIERS 2002, July 1-52002, Cambridge, Massachusetts, USA, p. 249[A4] M. Lapine, M. Gorkunov, and K. H. Ringhofer, “Nonlinearity of a metamaterialarising from diode insertions into resonant conductive elements”, (submitted toPhys. Rev. E, 2003).Visited conferences9th Int. Conf. on Electromagnetics of Complex Media (“Bianisotropics-2002”),8-11 May 2002, Marrakech, Morocco (poster presentation).Attended lectures- P. Hertel: Linear response theory (WS 01/02)- E. Krätzig, K. Ringhofer: The photorefractive nonlinearity (SS 02)- H.-J. Schmidt: Nonlinear wave equations (WS 02/03)- Seminars of the Graduate College 695- Workshops of the Graduate College 695Duration of the dissertation: Start 01.11.01, termination expected 31.10.04.Period of support in the College: 01.11.01 - 31.12.03Supervisors: Prof. Dr. Klaus Ringhofer, Prof. K. Betzler, Dr. M. Gorkounov62

Dipl.-Phys. Manfred MüllerImprovement of lithium niobate crystals for frequency conversionLithium niobate (LiNbO 3 ) is the material of choice for many integrated-optical devices.Due to large nonlinear-optical effects it is also now widely used for second-harmonicgeneration(SHG), for optical parametrical oscillation (OPO), as well as for optical parametricalamplification (OPA). This has been made possible through quasi phasematching (QPM) with periodically-poled lithium niobate (PPLN), which allows frequencyconversion over a wide range of light wavelengths and enables utilization of the nonlinearoptical coefficient d 33 , which is especially high in LiNbO 3 [1].However, most nonlinear-optical devices using LiNbO 3 crystals operate in the near tomiddle IR region. Extension of this technology to smaller wavelengths is impeded by theemergence of photo-induced refractive index and absorption changes (so called “opticaldamage”) and the difficulty to produce PPLN with an adequately short period length.Periodically-poled components were be fabricated with period lengths down to 4 µmusing special lithographic techniques [2]. An alternative method for fabrication of domainswith shorter period lengths was presented for lithium tantalate crystals (LiTaO 3 ).There the direct transfer of a light pattern into a domain structure is demonstrated. Afterreversal of the domains the coercive field is transiently reduced. In LiTaO 3 it was shownthat illumination can accelerate the recovery of the coercive field to the original value.Thus illumination with a light pattern causes for some time a spatially modulated coercivefield, and application of a homogeneous external electrical field of proper strengthduring this time yields the desired domain pattern [3,4]. However, no such effects havebeen reported for LiNbO 3 , although LiNbO 3 and LiTaO 3 are isomorphic.Within the scope of this project Dipl.-Phys. Manfred Müller has investigated methods toavoid optical damage as well as the poling characteristics of LiNbO 3 crystals while illuminatingthem with intense laser light over a wide spectral range. Most of the resultsthat are obtained so far are related to the properties and physics behind "domain engineering"of LiNbO 3 . The goal is to find a way to control optically the resulting domainstructure and hence the nonlinear optical properties. Furthermore, since the domainstructure of LiNbO 3 is not directly visible, new techniques were developed that enableimproved monitoring of the poling process.Experimental setup: Figure 1 shows the standard setup used in the experiments. Theelectric field is applied to the crystal with transparent liquid electrodes (water), whichallow the necessary illumination. During the experiments an external electric field is continuouslyincreased with a rate of 30 V/(s mm) up to values well above the coercive field(about 20 kV/mm). The displacement current due to the change of the spontaneous polarizationis used to monitor the poling process in time. To get spatially-resolved informationthe holder is integrated into a Mach-Zehnder interferometer. In LiNbO 3 the63

HVLiquidelectrodesDMPump lightLiNbO crystal3c-axisBSorientation of a ferroelectric domaindetermines the sign of the electroopticcoefficient and therefore, if ahomogeneous electric field is applied,the sign of the electro-optic refractiveindex change. This leads to a noticeablediscontinuity in the interferencepattern.Test lightGuardringBSFused silica slabsDMO ringFig. 1. Schematic representation of the poling setup(BS: beam splitter, DM: dielectric mirror)All LiNbO 3 crystals described in thisreport are congruently melting, undoped,z-cuts with a thickness of 0.5mm (supplier: Crystal TechnologyInc.).Influence of illumination on the poling characteristics of lithium niobate crystals: LiNbO 3shows like LiTaO 3 a transient reduction of the coercive field immediately after a polingevent. However, full recovery of the coercive field takes only 20-30 s and is thus muchfaster than in LiTaO 3 . It was found that unlike in LiTaO 3 the relaxation of the coercivefield in LiNbO 3 is independent of illumination (except for light-induced thermal effects).20.0Coercive field [kV/mm]19.519.018.518.017.517.0λ = 351 nm λ = 351 nmI = 3 W/ cm 2 I = 6 W/cm 2Inside laser beamOutside laser beamλ = 334 nmI = 3 W/cm 20 10 20 30 40 50 60Number of poling cycleFig. 2. Coercive field versus number of poling cycles for the forward poling direction measured interferometricallyinside (circles) and outside (triangles) the illuminated area. For the time periods indicatedby the gray bars the sample is illuminated by a laser beam with wavelength λ and intensity I.There is always a 6 min waiting time between two poling processes to avoid measurement of transienteffects. Illumination at the wavelength λ = 351 nm changes the coercive field only temporarilybecause the crystal temperature increases. Illumination at the wavelength λ = 334 nm, however,yields a strong change of the coercive field, which is significant even after one hour without illumination.Missing data points indicate that the interferometer couldn’t clearly resolve the phase jump duringpoling.64

Fig. 3. Photo-induced domain pattern illuminated with twoapproximately 40 µm wide stripes of UV-light as indicatedon the right side.However, it was also found that if the crystal is not illuminated between but during thepoling events with light of the wavelength 334 nm or shorter a considerable reductionof the coercive field occurs (see Fig. 2). Even after the laser illumination is stopped, asignificant quasi-permanent decreaseof the coercive field ofabout 800 V/mm persists andremains for hours without appreciablechange. Therefore it canbe ruled out that the observedchange of the coercive field is ofthermal origin. Furthermore, thiseffect is present only if illuminationtakes place during the polingprocess. Illumination before orafter the poling has no impact onthe coercive field. The origin ofthe effect is still under investigation.Possibly the intense UV illuminationinduces defects in thecrystal that assist domain nucleationor otherwise lower the coercivefield.The observed effect is used torealize light-controlled domain patterning in lithium niobate. A crystal is illuminatedthrough a binary grating for four poling cycles. The laser is turned off, and a followingforward poling process is aborted immediately after the domains start to switch. Etchingof the crystal with hydrofluoric acid reveals the presence of a domain pattern,which is approximately a replica of the illumination pattern. Figure 3 shows a magnifiedphotograph of the crystal: it can clearly be seen that ferroelectric domains havebegun growing in the illuminated areas where the coercive field is lowest.By optimization of the effect one should be able to generate domain patterns on themicrometer scale utilizing an interferometrical light pattern and homogeneous electricfields. In doing so it is especially important to time the abortion of the final polingprocess precisely, so that the domains in the illuminated regions had time to enoughto coalesce but that no extension into the unilluminated parts of the crystal occurred.In order to do so, a monitoring technique able to resolve even such small domainsizes must to be developed.Monitoring of the poling process through light diffraction at domain boundaries: Tomonitor the poling process, the crystal is placed into the holder and is illuminatedalong the z-axis with a plane wave of light from an Ar + laser. The generated light patternis observed on a screen that is positioned behind the crystal holder. Figures 4 a-h show for ultraviolet light (wavelength λ = 351.1 nm) the light pattern that is observedduring the various stages of the poling process. Well below the coercive field,only diffuse scattering is present. When the poling starts (i.e. a displacement currentarises) a distinct ring structure at an 8° opening angle appears. With increasing voltagethe ring turns into 6 dots, which transform into a star with a 6-fold symmetry. Insidethe star a fine structure is distinctly visible. The star disappears abruptly with65

completion of the poling process, and only the simple transmitted plane wave remainspresent.Fig. 4. Light pattern observed on a screen behind the crystal during various stages of the poling processwhile the applied field was increased at a rate of 30 V/(s mm). The pictures were taken at a)E = 19.08 kV/mm; b) E = 19.30 kV/mm; c) E = 19.38 kV/mm; d) E = 19.53 kV/mm; e) E = 19.57 kV/mm; f) E = 19.67 kV/ mm; g) E = 19.69 kV/ mm; h) E = 19.72 kV/mm, respectively.If the poling process is aborted and the voltage turned off while the star is visible, thedomain pattern is frozen. Therefore, it is possible to study the resulting star patternthat arises from the same domain pattern for different light wavelengths, different incidentangles of the light and different external electrical fields. For example Figures5 b and c show the wavelength dependence of the star for the same external electricfield, while Fig. 5 a shows a part of the corresponding domain pattern. In accordancewith the crystal symmetry of LiNbO 3 , domain boundaries appear along six preferentialdirections with multiples of 60° between them.To determine whether diffraction at domain boundaries is responsible for this effect, asingle domain wall of macroscopic length is illuminated through a pin hole. Figure 6shows the light pattern that is observed on a screen behind the sample. Diffraction bythe wall is clearly present with a maximum diffraction angle similar the one observedfor the star. However, a surprising feature is that the direction of the diffracted beamsdepends on the sign of the applied field.Fig. 5. a) Frozen domain pattern revealed after etching the crystal in 48 % hydrofluoric acid for 90 minb) The corresponding light pattern for an external electric field of -14 kV/mm seen at a wavelength of351.1 nm and c) seen at a wavelength of 501.7 nm66

0,14sin( α )0,120,100,08350 400 450 500λ [nm]Fig. 6 Light pattern created by illumination of a singledomain boundary through a circular aperture for variouselectrical fields E a) E = -12 kV/mm; b) E = -8 kV/mm;c) E = -4 kV/mm; d) E = 0 kV/mm; e) E = 4 kV/mm;f) E = 8 kV/mm; g) E = 12 kV/mm.Fig. 7. Sine of the opening angle α of thestar versus wavelength λ. The line serves asa guide for the eye.This diffraction effect, together with the hexagonal symmetry of the domain structure,can qualitatively explain the emergence of the star pattern during the poling process,although the physical reason behind the diffraction at the domain boundaries is stillunclear. We suppose that the domain walls have a certain thickness and that thephase jump of the transmitted wave is continuous and not abrupt. Therefore shortwavelengthlight, where the phase jump is larger, should be diffracted with a largerangle, which agrees with the experimental observation (see Fig. 7).However, it is clear that the star gives us information about the directions of the domainboundaries, irrespective of the domain size. This can be a very valuable tool formonitoring of the light-controlled poling process explained above. — Based on theachievements we are pretty optimistic that the PhD thesis of Dipl.-Phys. ManfredMüller will help to realize improved optical parametric oscillators.1. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, "Quasi-phased-matchedsecond harmonic generation: tuning and tolerances", IEEE J. Quant. Electron. 28,2631-2654 (1992)2. R. G. Batchko, V. Y. Shur, M. M. Fejer, and R. L. Byer, "Backswitch poling in lithiumniobate for high-fidelity domain patterning and efficient blue light generation",Appl. Phys. Lett. 75, 1673-1675 (1999)3. S. Chao and C. Hung, "Large photoinduced ferroelectric coercive field increaseand photodefined domain pattern in lithium-tantalate crystal", Appl. Phys. Lett. 69,3803-3805 (1996)4. P. Brown, G. Ross, R. Eason, and A. Pogosyan, "Control of domain structures inlithium tantalate using interferometric optical patterning", Opt. Comm. 163, 310-316 (1999)67

Publications1. I. Nee, M. Müller, K. Buse, E. Krätzig, "Role of iron in lithium-niobate crystals forthe dark-storage time of holograms", J. Appl. Phys. 88, 4282-4286 (2000)2. I. Nee, M. Müller, and K. Buse, "Development of thermally fixed photorefractiveholograms without light", Appl. Phys. B 72, 195-200 (2001)3. M. Wengler, M. Müller, E. Soergel, and K. Buse, "Dynamics of ferroelectric domainreversal in lithium niobate crystals", Appl. Phys. B, accepted4. M. Müller, E. Soergel, M. Wengler, and K. Buse, "Star-shaped light diffractionfrom ferroelectric boundaries", submitted5. M. Müller, E. Soergel, M. Falk, J. Hukriede, and K. Buse, "Reduction of opticaldamage in lithium niobate crystals by hydrogen loading", in preparationAttended lectures, conference visits, research staysConferences:1. 10 th European Conference on Integrated Optics (ECIO), Paderborn04.-06.04.20012. 4 th Annual Meeting of the COST Action P2, Budapest, 16.-19.05.20013. Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Osnabrück,04.-08.03.20024. Conference on Lasers and Electro-Optics '03, Baltimore, 01.-06.06.20035. 9 th International Conference on Photorefractive Effects, Materials, and Devices,Nice, 17.-21.06.2003Lectures:• Atomphysikalisches Kolloquium (4 courses)• Physikalisches Kolloquium (4 courses)• Seminar über angewandte Optik (4 courses)Research stay (scheduled):• Stay in the group of Prof. Dr. R. W. Eason, University of Southampton, Optoelectronicsresearch laboratory, Light-induced patterning of ferroelectric domains, fall2003 for 4 weeks.Duration of the dissertation: Start 01.10.2000, termination expected 31.03.2004Period of support in the College: 01.01.2001-31.12.2003Supervisor: Prof. Dr. Karsten Buse68

Dr. Axel PramannTopic: Frequency conversion,ResultsNonlinear optical processes in atomic and molecular clustersThe generation of line-tunable, coherent light of high resolution in the vacuum ultraviolet(VUV) and extreme ultraviolet (XUV) spectral range (>> 10 eV) in the laboratoryis an experimental task of high importance with respect to both applications such ashigh resolution spectroscopy in the vacuum ultraviolet and related subjects. Theavailability of a compact light source with such properties is of interest to fields suchas molecular spectroscopy, photochemistry, photoionization, and state selectivereaction dynamics. The lowest electronically excited states of small molecules (e. g.N 2 , H 2 O …) are found below 185 nm (7 – 10 eV). Therefore, it is important to haveaccess to these states by one photon excitations, which are induced by VUVradiation. The current project - and its results - are divided into several parts:A set up for the generation of high energetic VUV- and XUV-light was developed.The general experimental procedure makes use of nonlinear effects of frequency triplingof tunable laser radiation in the ultraviolet regime, incident on the gaseous triplingmedium. In the following step, the operation conditions and parameters of thefrequency tripling were tested and characterized using well-known systems such asN 2 and Kr.Moreover, these gases were also used to investigate so far unknown regions of frequencytripled radiation in the VUV regime.The experiment consists of at least three main components (Fig. 1). A pulsed dyelaser system (operated at 10 Hz repetition rate) is used for the generation of a fundamentalfrequency, which is tripled in a jet. The dye laser is pumped by an excimerlaser (308 nm, 300 mJ/pulse). The fundamental of the laser radiation was typically520-560 nm (20 – 25 mJ/pulse), which is subsequently frequency doubled by a BBO-1 crystal. With this standard method, laser light in the wavelength range of 260 – 280nm (3 – 5 mJ/pulse) is produced, which is used for the generation of the third harmoniclight in the VUV. The resolution of this fundamental light is better than 0.09cm -1 . The frequency doubled light is crossed by a pulsed molecular beam of the triplinggas in the center of a vacuum chamber.It is important to focus the laser light as near as possible to the orifice of a pulsedvalve (General Valve). Frequency tripling can only be achieved when the phasematching condition is fulfilled. The frequency tripled light is propagating collinear tothe initial direction of the ultraviolet laser light. Formation of a supersonic jet is accomplishedby expanding the gas at a high stagnation pressure p 0 (10 bar) through apulsed nozzle with a small orifice into a vacuum chamber. In the case of moleculargases, strong cooling of the internal degrees of freedom occurs, which leads to clusterformation. Subsequently, the frequency-tripled light (λ < 100 nm) enters a secondvacuum chamber, which is filled with a gas with high ionization potential (e. g. acetone:IP = 9.7 eV). It is important to note that the detection gas exhibits no multiplephoton processes, so that exclusively one photon processes lead to cation formation.74

Fig. 1. Experimental setupAs a result, one can easily distinguish between the fundamental and THG light. Theionized molecules are detected with an ion detector located in a third vacuum chamber.With this set up, time-of-flight (ToF) spectra of the ion bunches are measured.The integral of the respective ion signal is proportional to the intensity of the generatedfrequency tripled light. The power L 3 of the frequency tripled light is given by23π22 3 2L3 = N [ χ2 2 4 3(λ3)]L1Φ(1)ε 0cλ1Because of the quadratic relation between L 3 and the particle density N and the cubicrelation between L 3 and the power of the incident light L 1 , it is evident that for THGthe key parameters are both a high laser power of the frequency doubled light and ahigh density of the tripling medium.The experimental setup is tested and characterized using molecular nitrogen (N 2 ) asthe tripling medium, because it is known that N 2 has a high tripling conversion efficiencyin the energy range > 10 eV. 1 With this reference gas, the wavelength dependenceof THG is monitored in the range λ < 93 nm. In general, with the new setup75

the same two- and three-photon resonance-enhanced lines (highly populated rotationallines) similar to previous work of Lee and co-workers 1 are observed. For thecharacterization of the new machine it is useful to select an intense rotational line ofhigh intensity (Fig. 2).Fig. 2. Time-of-flight spectrum of C 3 H 6 O + generated at λ THG = 90.26 nm using N 2 asthe tripling medium (p 0 = 9.5 bar; P laser = 2.6 mJ/pulse).Here, the line at λ = 90.26 nm is used for characterization and optimization, becausethis line has been identified with a similar setup using a VUV-monochromator forwavelength detection in a previous work. 2 After the generation of THG light at thiswavelength, the operating conditions of the machine are optimized. First of all, thelaser power dependence of the intensity of the frequency tripled light is measured.Usually, an optimum conversion efficiency of 10 -6 between the UV-laser light and thefrequency tripled light is achieved. For N 2 , THG light is generated with laser powersbetween 2.4 and 3.5 mJ/pulse. As stated above (eq. 1), the THG intensity is proportionalto the cube of the UV- laser pulse. Another important parameter is the stagnationpressure dependence of the THG light intensity. The phase matching conditionfor THG is strongly dependent on the stagnation pressure p 0 of the gas prior to expansion.The stagnation pressure of N 2 is varied between 0 and 10 bar. THG signalsare detected in the range between p 0 = 4 and 10 bar. Maxima in THG are observedat p 0 = 7 – 8 and 9 – 10 bar corresponding to a high particle density at the point offrequency tripling. Because of the pulsed character of the experiment extensive caremust be taken not only for the geometrical adjustment of the laser and the molecularbeam. Additionally, a proper timing between the opening of a pulsed valve and thelaser pulse is of importance to fulfill the phase matching condition (Fig. 3).The highest signal intensity is found for a delay time of 500 – 600 µs between thenozzle opening and the laser shot.76

Fig. 3. THG signal intensity as a function of the time delay between the opening ofthe valve and the firing of the UV-laser (tripling gas: N 2 (p 0 = 9.5 bar, λ THG = 90.26nm).Fig. 4. ToF spectrum of C 3 H 6 O + measured at the Kr resonance line at 92.30 nm(P laser = 3.5 mJ/pulse).After the characterization of the experimental setup frequency tripling of gases at unknownwavelength ranges was investigated. For this purpose, a jet of krypton is77

used. In the work of Lee and co-workers 1 line tunable THG of Kr has been measuredfor the first time down to 90.4 nm. Some 4p – ns and 4p – nd Rydberg series in thatstudy are reproduced with our setup. As an example, we optimized the operatingconditions for Kr jets (as described for N 2 ) at the prominent line at 92.3 nm (Fig. 4).However, compared to the THG signals of N 2, the intensities of the Kr signals areabout one order of magnitude weaker. For beams of Kr, tunable THG is observed forthe first time in the VUV wavelength range down to 86 nm (14.4 eV). As an example,Fig. 5 shows a time-of-flight spectrum of C 3 H 6 O + molecules obtained at a triplingwavelength of 90.3 nm using a Kr beam.Kr90.3 nmFig. 5. ToF spectrum of C 3 H 6 O + obtained at a tripling wavelength of 90.3 nm using akrypton beam.The intensities and structures of the TOF signals in the wavelength range between90.3 and 89.1 nm exhibit all very similar structures and almost the same intensitieswithin the experimental error. Thus, a continuum of the THG signals in this spectralregion without prominent lines is observed. This behavior is in contrast to the sharpRydberg states in the wavelength range above 90.4 eV.This spectral pattern without detectable resonance structures is similar to that of xenonin the wavelength range between 90 and 92 nm.The second reason for using beams of Kr is the known ability of Kr to form clustersafter a supersonic expansion applying proper expansion parameters. This representsanother area of interest in the project, where size-dependent third-harmonic generationin clusters will be investigated. Currently, experiments on cluster production canbe performed with the new setup, so that new regions of frequency tripling are expectedto occur.78

References:1 R. H. Page, R. L. Larkin, A. H. Kung, Y. R. Shen, and Y. T. Lee, Rev. Sci. Instrum.58 (1987) 1616.2 J. Plenge, diploma thesis, University of Osnabrück, 1999.Attended lectures, conference visits, research stays- Seminars of the graduate college 695- Workshops of the graduate college 695 during 2002Duration of the dissertation: PostdocPeriod of support in the college: 15.11.2001 - 31.01.2003Supervisors: Prof. Dr. E. Rühl, Dr. R. FleschDr. Pramann left the Graduate College 31.01.03 to start an activity at the Physikalisch-TechnischeBundesanstalt (PTB), Braunschweig.79

Dipl.-Math. Dipl.-Phys. Florian RaheTopic: Space-charge waves in photorefractive crystalsResultsSpace-charge waves (SCW) are the eigenmodes of charge oscillation in a system oftraps and free carriers in semi-insulating solids, when carriers move in an electricfield. They were initially named trap recharging waves, because their nature is associatedwith trap charging and discharging by free carriers which are excited thermallyor by illumination. Their propagation is due to the influence of an applied electric field.These waves have very specific properties. For instance they are strongly attenuated,because their free path length is typically limited by the carrier drift length.Moreover, the propagation direction of the SCW is determined by the direction of theapplied field. Due to the fact that their wave vector is inversely proportional to theirfrequency, phase and group velocities are oppositely directed.Space-charge waves are of great interest in photorefractive crystals, especially forthe sillenite family Bi 12 MO 20 (where M = Ge, Ti or Si), because the dynamic propertiesof these crystals in the presence of an external electric field are very often determinedby these waves. This especially applies for the process of holographic recording,hologram relaxation and oscillations of holographic gratings. For example,the SCW excitation can provide an increase of the sensitivity of devices, which arebased on the principles of dynamic holography. SCW can also play an important rolein further semi-insulating semiconductors, i.e., GaAs, InP:Fe, CdTe:V and other materials.It can be supposed that some transient phenomena in photoreceivers are associatedwith SCW as well.There are several methods of SCW excitation, electrical or optical. The electricalmethods encounter serious experimental difficulties in the selective excitation ofSCW with a desired set of parameters. Much more flexible is the optical excitation ofSCW by illuminating the crystal with a periodic interference pattern. Optical methodsare pulse detection, a moving interference pattern or an oscillating interference pattern.A careful selection of the experimental method for the investigation of SCW isimportant, because the obtained information depends critically on the technique usedfor SCW excitation and detection.I investigated SCW in photorefractive crystals of the sillenite family (namelyB 12 GeO 20 , B 12 TiO 20 and B 12 SiO 20 ). An optical method for SCW excitation was used.The crystals were illuminated with an interference pattern, oscillating near a mid position.If the grating spacing and the oscillation frequency of the interference patterncoincide with the spatial period and temporal eigenfrequency of a space chargewave, resonance excitation occurs. The use of electro-optic crystals makes it easy todetect the SCW, because their space charge field can be detected via diffraction of atest laser.In the case of optical excitation, two main regimes can be considered. The first one isthe linear regime, when only effects proportional to the first power of the contrast ratiom of the interference pattern are taken into account. The second is the nonlinear regime,in which effects proportional to m 2 (or a higher power of m) become important.This situation can be compared with effects in nonlinear optics. In the case of effects80

proportional to m 2 , one can expect to observe phenomena similar to those knownfrom nonlinear optics, like second-harmonic generation and rectification.The main subject of my investigations is the nonlinear regime. The illumination of thesample with an oscillating interference pattern results in a simultaneous excitation ofSCW and the formation of a static space-charge grating, whose spacing is equal tothe spatial period of SCW. The interaction of the static space-charge grating and theSCW lead to new nonlinear effects, which don’t exist in nonlinear optics. They arespatial doubling, where doubling of the SCW wave vector occurs without frequencydoubling, and spatial rectification, where a spatially homogeneous electric field oscillatingwith frequency of the SCW arises. During my investigations I detected the effectsof second harmonic generation, where doubling of the SCW wave vector anddoubling of the SCW frequency occur, and rectification.Second harmonic generation was observed with the help of a test laser. The laserwas adjusted to read out the space charge grating with the doubled wave vector viathe electro optic effect. The diffracted beam was detected by a photodiode connectedto a lock in amplifier. The signal P 2f was detected at the second temporal harmonic toobserve second harmonic generation. In this case theory predicts three resonancepeaks, which arise due to different forced excitations of SCW. All three peaks weredetected (see figure 1) and their relative positions fit well to the theory. They also fulfilthe dispersion relation of SCW.P 2f [arb. units]3.53.02.52.01.51.00.50.010 1 10 2 10 3f [Hz]Figure 1. Frequency dependenceof the output signal P 2fat 2f for different appliedfields E 0 . For Bi 12 GeO 20 , m =0.43, W 0 = 130 mW/cm 2 , Θ =1 rad, Λ = 13 µm: ■: E 0 = 10kV/cm, ●: E 0 = 8 kV/cm, ▲: E 0= 6kV/cm. The lines areguides to the eye. Thechange of the resonance frequenciesfollows the dispersionrelation for SCW.For overall rectification the theory predicts a change in the current in the external circuit.This also implies a change of the static homogeneous field inside the crystal.Consequently, there are two possibilities to measure the rectification effect. First bymeasuring the DC current in the external circuit. This was realized by measuring theDC voltage over a loading resistance. A strong effect could be observed. A decreaseof the current up to 25% was observed. In figure 2 one can see the effect for differentoscillation amplitudes Θ of the interference pattern. The resonance frequencies fulfilthe dispersion relation of SCW. Again theory fits well with the measured data (figure3). To detect the change of the internal field of the crystal I used the electro-optic effect.The change of the internal field results in a change of one of the refractive indices.It results in a change of the polarisation of a test beam propagating through thecrystal. This can easily be detected with the help of a polarizer. The change of the81

internal field can be detected and the resonance frequencies coincide with the resonancefrequencies of the current measurements.I 0[arb. units]5.85.65.45.25.04.84.610 1 10 2 10 3f [Hz]Figure 2: DC current I 0 as afunction of the phasemodulation frequency f fordifferent oscillation amplitudesΘ: For Bi 12 GeO 20 , E 0= 8 kV/cm, Λ = 13.1 µm, m= 0.43 and W 0 = 130mW/cm 2 ; -■-: Θ = 0.2 π, -●-: Θ = 0.2 π, -▼-: Θ = 0.6 π,-♦-: Θ = 0.8 π, -○-: Θ = 1.0π . The lines are guides tothe eyes.5.85.6I0[arb. units]5.45.25.04.84.610 1 10 2 10 3f [Hz]Figure 3: Comparison betweentheory (lines) andexperiment (symbols) foroverall rectification inBi 12 GeO 20 . I 0 is the dc current,f is the phase modulationfrequency: Θ = 0.2 π(solid line, ●) Θ = 0.9 π(dashed line, ◄).In further experiments all nonlinear interactions have been investigated simultaneously.It turned out that not all resonance frequencies coincide, especially for highoscillation amplitudes Θ. The resonance frequencies for the nonlinear effects areshifted to lower frequencies. This seems to be in contradiction to the theory, howeverthe theory is only valid for small Θ. For high amplitudes terms of higher orders of Θhave to be taken into account, which results in a shift of the resonances to lower frequencies,especially in the case of nonlinear effects.With these measurements, especially by measuring the rectification effect, one candetermine crystal parameters like the product µτ of the charge carrier mobility andthe charge carrier lifetime or the “real” internal field, taking losses at the electrodesinto account.The experiments are not confined to photorefractive crystals. For measuring the rectificationeffect one doesn’t need the electro-optic effect. Therefore, this is a simple82

method for the characterisation of semi-insulating semiconductors. It is planned toinvestigate further semiconductors. Promising are also semi-insulating quantum-dotsemiconductors, because some interesting results can be expected.Publications• M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Optically InducedNonlinear Wave Processes in Photorefractive Crystals, Technical Digest IQEC2002, 375 (2002)• S. Schwalenberg, F. Rahe, E. Krätzig, Recording Mechanisms of AnisotropicHolographic Scattering Cones in Photorefractive Crystals, Optics Commun. 209,467 (2002)• M. P. Petrov, V. V. Bryksin, H. Vogt, F. Rahe, E. Krätzig, Overall Rectification andSecond Harmonic Generation of Space Charge Waves, Phys. Rev. B 66, 085107(2002)• M. P. Petrov, V. V. Bryksin, F. Rahe, C. E. Rüter, E. Krätzig, Space Charge RectificationEffects in Photorefractive Bi 12 TiO 20 Crystals, Optics Commun., submittedAttended lecturesLinear response theory (P. Hertel)The photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)Nonlinear wave equations (H.-J. Schmidt)Seminars and workshops of the Graduate CollegeConference visitsJune 22–27, 2002 IQEC/LAT 2002 in Moscow; contribution: Optically InducedNonlinear Wave Processes in Photorefractive CrystalsResearch stays• September 17 – December 02, 2001 National Academy of Sciences, Institute ofPhysics, Kiev, Ukraine (Prof. Dr. Serguey Odoulov).• June 14 – 21, 2002 IOFFE Physico-Technical Institute Russian Academy of Sciences,St. Petersburg, Russia (Prof. Dr. Mikhail P. Petrov).Duration of the dissertation: Start 01.01.01, termination expected end 2003Period of support in the college: 01.01.01 to 31.12.03Supervisor: Prof. Dr. E. Krätzig; in cooperation with Prof. Dr. M. P. Petrov, A. F. IoffePhysico-Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia.83

Prof. Dr. Marika SchlebergerTopic: Scanning force microscopy to image ferroelectric domainsResultsThe main goal of our project was to image ferroelectric domains in doped and undopedSBN single crystals by means of scanning force microscopy [1]. We wanted tofind out what the topography of the domains looks like and whether the size of thedomains depends significantly on the concentration of the dopant or not. The atomicforce microscope (AFM) is ideally suited for such investigations since the instrumentis capable of measuring the topography as well as electrostatic interactions with aspatial resolution of a few nanometers.Our first experiments with the AFM showed images which we interpreted as ferroelectricdomains. The measurements were done in the contact-mode with a Si 3 N 4 tipin air. The domain structure of the planes normal to the c-axis typically show a corallikepattern of troughs which are about 1.5 nm deep and roughly 100 x 100 nm 2 insize. The typical domains on the planes that are parallel to the c-axis are muchsmaller. They are elongated and exhibit the same depth of 1.5 nm.We had problems with a few of the crystals we looked at since we could not get thetip into contact with the surface. An effect that is most likely due to strong electrostaticinteractions. These crystals could only be measured in the non-contact modewhere the tip is oscillating at its resonance frequency some distance away from thesample. In this mode of measurement we make use of the frequency shift as thefeedback signal. The frequency shift is due to the charges present on the surface.We found basically the same domain pattern, however, the images appeared somewhatblurry.A clear improvement of the images could be achieved by using the damping of thecantilever instead. Since the damping - unlike the frequency shift - varies exponentiallywith the tip-sample distance heights can be measured even more exactly thenwith conventional AFM. However, this method was used for the first time, and therefore,we know little about the origin of the contrast in those images. The images mustthus be interpreted with some care and cannot be simply regarded as pure topographydata.We could not influence the domain structure by neither poling nor depoling the crystals.This can be easily explained if we assume the following: The domain structure isalready present after the growth of the crystals. The basic etch that is subsequentlyused for polishing is more aggressive in the domains with a corresponding polarization,i.e., there will be more material removed in these areas. The ferroelectric domainsare thus “written” into the crystal surface. This process is of course limited tothe immediate surface and has no influence on the ferrolectric properties of the bulk.With the AFM we see only the topography of the original ferroelectric domains. Newdomains or domain structures that are influenced by electric fields are obviously notformed on the surface or are to weak to be detected by the AFM. In order to test thistheory the crystals should be depoled before the polishing process. Unfortunately,these experiments could not be performed anymore in the frame of this short project.84

All experiments were done in close collaboration with Martin Görlich and MonikaWesner.References[1] P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomains inthe uniaxial relaxor system Sr 0.61-x Ba 0.39 Nb 2 O 6 :Ce -x (3+) . Phys. Rev. B 6422, art. no.224109 (2001).Period of support in the college (postdoc): 01.01.01 to 30.04.01Supervisor: Prof. Dr. E. KrätzigDr. Schleberger left the graduate College 30.04.2001 to continue research within thescope of the Heisenberg-Programm of the DFG. Now she is working as a professorat the University of Essen.85

Attended lecturesThe photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)Nonlinear wave equations (H.-J. Schmidt)Seminars and workshops of the Graduate CollegeDuration of the dissertation: Start 01.12.01, termination expected end 2004Period of support in the college: 01.12.01 - 31.12.03Supervisor: PD Dr. J. Schnack, Prof. Dr. K. Bärwinkel, apl. Prof. Dr. H.-J. Schmidt89

Dipl.-Math. Elena D. SvetogorovaTopic: Reflection and transmission of a plane TE-wave at a lossless nonlineardielectric film with a permittivity depending on the transverse coordinate90

Attended lectures, conference visits, research staysWS 01/02: P. Hertel, Linear response theorySS 02: E. Krätzig/K. Ringhofer, The photorefractive nonlinearityWS 02/03: H.-J- Schmidt, Nonlinear wave equationsWorkshop "Photorefractive Nonlinearities" (October 2001, Osnabrück)Seminars of the Graduate College 695 (WS 01/02, SS 02, WS 02/03)Duration of the dissertation: Start 01.09.2001, termination expected 01.09.2004Period of support in the college: 01.09.2001 - 31.12.2003SupervisorsProf. Dr. H. W. Schürmann, Department of Physics, University of Osnabrück,Prof. Dr. V. S. Serov, Department of Mathematical Sciences, University of Oulu,Finland94

Dipl.-Phys. Arthur TunyagiTopic: Nonlinear optical and dielectric properties of undoped Strontium-Barium-Niobate near the phase transition.ResultsStrontium-Barium-Niobate (SBN), Sr x Ba 1-x Nb 2 O 6 , can be grown in a wide compositionrange of x=0.25…0.8 (for details of the crystal growth process see report of M. Ulex).The crystals undergo a structural phase transition from a ferroelectric lowtemperatureto a paraelectric high-temperature phase at temperatures above roomtemperature. The phase transition is of relaxor type – more or less broadened intemperature. Broadening and transition temperature depend on composition,dopants, and inhomogeneities. The aim of our project is the measurement of linearand nonlinear optical and of dielectric properties around the phase transition temperatureof SBN. For undoped crystals in the whole composition range, the influenceof the phase transition on these properties is studied . On the other hand, theseproperties – especially those which are very sensitive for the structural change at thephase transition – are used to investigate the phase transition itself.Construction and rebuilding of set-upsDuring the first period of the project it was necessary to design and build-up severalnew experimental set-ups. Furthermore, existing arrangements had to be updated orpartially renewed, redesigned computer control using C++ or Matlab had to beadded.Due to the main topic of the project – the temperature dependent study of nonlinearoptical properties – a set-up for the investigation of the second harmonic generation(SHG) of light was constructed. It offers now the possibility to measure the secondharmonic generated from a Nd:YAG laser (1064 nm) as a function of the temperatureof the sample. The control program consists of several C++ routines for heater control,temperature measurement, data acquisition using either single pulse detectionup to several kHz repetition rate or averaging, plotting, and more. The SHG measurementsdescribed were performed using this set-up.For measuring the permittivity a set-up using an LRC bridge (Hewlett-Packard4284A) and a commercial temperature controller (Profile PRO 800) has been built. Acontrol program in C++ was developed which allows to measure the permittivity as afunction of temperature and frequency. With this set-up the dielectric measurementson the crystals were performed.Furthermore, a data acquisition interface for a Fabry-Perot spectrometer has beenrenewed, which now consists of a photon counting card controlled by a C++ program.95

Refractive index measurements on Sr x Ba 1-x Nb 2 O 6The ordinary and the extraordinary refractive index for available compositions havebeen measured using a goniometer and the prism method. For the visible region amercury lamp was used, for the infrared region two laser-diodes of 790 nm and1550 nm. The infrared light was detected using an IR-sensitive video camera. Theexperimental points could be consistently described by Sellmeier relations. While theordinary refractive index is practically independent of the Sr/Ba ratio, the extraordinaryindex decreases with decreasing Sr content thus increasing the birefringence.The results are shown in Figure 1.Figure 1: Refractive index as a function of the wavelength for Sr x Ba 1-x Nb 2 O 6 with x =0.52 . . . 0.8.Second Harmonic Generation on Sr x Ba 1-x Nb 2 O 6The results of the refractive index measurements (Fig. 1) show that in SBN phasematchedsecond harmonic generation is not possible using a Nd:YAG laser as thefundamental light source. Yet, non-phase-matched SHG can be efficiently used tostudy the structural phase transition of SBN: From the crystal structure of SBN [1,2]with point symmetry 4 mm for the ferroelectric and 4 / mmm for the paraelectricphase one can derive that second harmonic light can be generated only in the ferroelectricphase. When the temperature is increased, the decay in the second harmonicintensity around the phase transition temperature reflects the transition from the noncentrosymmetriclow-temperature to the centrosymmetric high-temperature phase.96

A typical measurement of the second harmonic intensity as a function of temperatureis shown in Figure 2.Figure 2: The result of the SHG measurement for Sr x Ba 1-x Nb 2 O 6 with x=0.52 (uppercurve: SHG due to the tensor element d 33 , lower: tensor element d 31 ).Two different polarization geometries are chosen, polarization of the fundamentalbeam parallel or perpendicular to the polar axis of SBN (c-axis), respectively. In bothcases the second harmonic polarization was parallel to the c-axis. Thus, the tensorelements d 33 and d 31 can be derived from the measurements. As general trends for allcompositions investigated up to now we can derive:– d 33 generally is larger than d 31 throughout the whole composition range,– both d 33 and d 31 increase with increasing Ba content,– the difference between d 33 and d 31 increases with increasing Ba content.In the future we intend to make various measurements clarifying the correlation betweenpoling state and second harmonic intensity. The second harmonic intensitythen could be used as a sensitive measure for studying the poling dynamics.Very sensitive SHG measurements revealed a new, to date unknown, noncolinearSHG process which becomes visible when the laser beam is directed parallel to thec-axis of the crystal. The effect is closely connected to the domain geometry of thecrystals for which a needle-like structure had been postulated [3] and seems to bepresent in crystals of all compositions investigated up to now. Further, thoroughmeasurements are necessary to assure the features of this new effect and to deriveat least a simple physical model for it. Explanations developed for noncolinear SHGfound e. g. in lithium niobate [4,5] can not be adopted for SBN.Permittivity Measurements on Sr x Ba 1-x Nb 2 O 6More information about the phase transition characteristics can be derived from theelectric permittivity. At different frequencies, the capacitance of the sample wasmeasured as a function of temperature. From these measurements we were able to97

determine the relaxor-typical broadening of the phase transition in SBN [6,7] as afunction of the composition. A typical result is presented in Figure 3.Figure 3: A typical result for an electric permittivity measurement as a function oftemperature and frequency (Sr x Ba 1-x Nb 2 O 6 with x = 0.52 ).Analysing all results we can conclude that samples with higher strontium contentshow more expressed relaxor properties, whereas in the Ba-rich samples this featureis only weakly expressed. Because the relaxor features are more pronounced at lowfrequencies we intend to extend our measurements to that region in future.OH-stretching modes in Sr x Ba 1-x Nb 2 O 6In cooperation with C. David the behaviour of the OH-stretching modes in SBN wasmeasured. We observed a significant influence of the composition on the OHstretchingmode absorption spectra. With rising x, the absorption of the main band atabout 3495 cm -1 increases, the low energy shoulder decreases and an additionalbroad absorption is built up. This shows that hydrogen ions can occupy several differentpositions in the unfilled tungsten bronze structure of SBN which are energeticallynon-equivalent. A thorough evaluation is presently being developed. More detailsabout the sample treatment and the measurements are given in the report of C.David.References:[1] T.S. Chernya, B.A. Maksimov, I.V. Verin, L.I. Ivleva, V.I. Simonov ; Cryst. Reports,42, 375-380 (1997)[2] T.S. Chernya, B.A. Maksimov, I.V. Verin, L.I. Ivleva, V.I. Simonov ; Physics of theSolid State 42, 1716-1721 (2000)[3] S. Kawai, T. Ogawa, H. S. Lee, R. C. DeMattei, R. S. Feigelson : Appl. Phys. Lett.73, 6 (1998)[4] A. Reichert, K. Betzler : J. Appl. Phys. 79, 2209 (1996).[5] K.-U. Kasemir, K. Betzler: Appl. Phys. B 68, 763 (1999).[6] L.E. Cross Ferroelectrics 76, 241-267 (1987)[7] I.A. Santos, J.A. Eiras : J.Phys, Cond. Matter 13, 11733-11740 (2001)98

PublicationsCh. Bäumer, C. David, A. Tunyagi, K. Betzler, H. Hesse, E. Krätzig, M. Wöhlecke:Composition dependence of the ultraviolet absorption edge in lithium tantalate.J. Appl. Physics, in print (2003)C. David, A. Tunyagi et al.: OH stretching modes in Sr x Ba 1-x Nb 2 O 6 (in preparation)Attended lecturesWS 01/02 : P. Hertel: Linear response theorySS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearityWS 02/03: H.-J. Schmidt: Nonlinear wave equationsWS 01/02: V. Trepakov: Optics and Spectroscopy of semiconductors and insulatorsWorkshop “Photorefractive Nonlinearities” (October 2001, Osnabrueck)Workshop “SBN - a typical relaxor?” (May 2002, University of Osnabrueck)Workshop “SBN: Crystal Growth and Details of the Structure” (July 2002, Universityof Osnabrueck)Seminars of the Graduate College 695 (WS 01/02 SS 02 WS 02/03)Seminars of the Research Group “Optical Materials” (WS 01/02, SS 02, WS 02/03 )Contribution to the SeminarsSeminary Talk on 21.10.2002 “Second Harmonic Generation on SBN Crystals”Various short talks in the Research Group seminarExternal research stayIR-absorption Measurements performed on SZFKI institute in Budapest (08.07.2002– 19.07.2002)Duration of the dissertation: Start 01.09.2001, termination expected 31.08.2004Period of support in the College: 01.09.2001 – 31.12.2003Supervisor: Apl. Prof. Dr. Klaus Betzler99

Dipl.-Phys. Michael UlexTopic : Growth and characterization of Sr x Ba 1-x Nb 2 O 6 crystals with x rangingfrom 0.2 to 0.8ResultsAbstractFifteen different compositions of Sr x Ba 1-x Nb 2 O 6 with crystal compositions from x =0.32 to 0.79 were grown and investigated. Their quality allows investigations of opticalproperties. A preliminary phase diagram has been determined. The lattice constantsand the densities were measured. In addition, using the lattice constants, thedensities were calculated; good agreement with the measured ones was found. Furtherproperties have been reported by C. David and A. Tunyagi.IntroductionSr x Ba 1-x Nb 2 O 6 solid solutions (SBN) with compositions ranging from x = 0.25 to 0.75are known since 1960 and studied by many groups.For the binary system SrNb 2 O 6 -BaNb 2 O 6 the phase diagram was determined by Carrutherset al. [1] in 1970, indicating a wide range of solid solution and a congruentlymelting composition at x = 0.5. Later Megumi et al. [2] found a value of the congruentlymelting composition of x = 0.61 (1977). However, Carruthers et al. did only determinethe liquidus curve, but not the solidus one. Therefore the exact compositionrange of the solid solution is unknown. Additionally, preliminary results of our crystalgrowth experiments showed, that the variation of the liquidus temperature with compositionis much smaller than determined by Carruthers et al.Studies of almost all important properties of SBN have been reported, but a systematicstudy of selected properties as a function of the composition is still missing and isbesides the crystal growth the subject of this project. These investigations will bedone in co-operation with the Ph.D.-students C. David und A. Tunyagi (see their reports).Crystal growth and first optical assessmentThe crystals are grown in a resistance-heated furnace with the Czochralskitechnique.Because of resistance-heating the temperature gradient ∆T within themelt is about 1 °C/cm while the temperature stability is better than 0.1 °C. The crystalsare grown in [001]-direction with a pulling-rate of 0.8 mm/h for compositions x cr ≥0.5 and with 0.4 mm/h for compositions x cr < 0.5. During the growth process the crystalrotates with 38 cycles per minute. The experiments are performed in the temperaturerange 1484 °C ... 1496 °C.Crystals with a composition x cr ranging from 0.32 to 0.79 have been grown. The crystalsof good optical quality are transparent and colourless and have a length up to 80mm and a diameter of about 5 mm.For the investigations of the physical properties the crystals were cut into eight differentobjects with shapes like plates, cubes and prisms and finally grinded and polished.100

Figure 1: SBN crystals grown within this study (x cr = 0,34: below, x cr = 0,61: above)The crystals have good quality suitable for optical measurements. However, testswith crossed polarizers have shown inhomogeneities for x cr ≠ 0.61, which increasewith compositions more off the congruently melting one.It is assumed, that these inhomogeneities arise from an accumulation or reduction ofSr or Ba at the phase boundary. Experiments to reduce this kind of inhomogeneitiesby variations of the rotation rate and the vertical temperature gradient in the crystalgrowth apparatus are in progress.Figure 2: Photo of an SBN-crystal (x cr = 0.79, c-cut) taken with crossed polarizers101

Determination of the phase diagramTo improve the phase diagram, the compositions of the crystals were determined byX-ray fluorescence analyses. For this purpose 500 mg of the crystal to be analyzedor of a standard with a well-known composition are solved in 5 g of Spectromelt A12(Merck) at 950 °C in a Pt/Au-crucible. The X-ray fluorescence analysis was poweredby a copper-source and analyzed by a LiF grating. The lines Lα1 (Ba) and Kα (Sr andNb) were measured with a statistical uncertainty of less then 0.13 %.With the help of the standard the composition of the crystal x cr can be determinedwith a reproducibility of ∆x = ± 0.005.Table 1: Composition of the melt (x m ) and of the crystals grown from this melt (x cr ):x cr 0.788 0.787 0.779 0.736 0.688 0.644 0.613 0.563x m 0.805 0.812 0.787 0.753 0.700 0.650 0.610 0.550x cr 0.511 0.477 0.446 0.404 0.382 0.341 0.322x m 0.492 0.431 0.373 0.319 0.300 0.243 0.194Figure 3 shows the measured compositions of the crystals and of the melt. The y-axisshows the growth-temperature at a crystal length of about 60 mm. The lines representa preliminary phase diagram. This phase diagram shows a small difference betweenthe solidus curve (solid line) and the liquidus curve (dotted line) at the Sr-richrange (∆x = 0.02 for x cr = 0.79) and a large difference at the Ba-rich range (∆x = 0.13for x cr = 0.32). The data include an error of about ± 3 °C relative to each other andhave an absolute uncertainty of ± 20 °C.15051500Temperature (°C)14951490148514801475x meltx crystal14700,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9Figure 3: Preliminary phase diagramCompositionDensity and lattice constantsThe lattice constants were measured in co-operation with Prof. Dr. Schmahl at theUniversity of Bochum. The measurements of the density were performed in cooperationwith Prof. Dr. Bohatý at the University of Cologne. The results of thesemeasurements are shown in figure 4 and 5.102

Lattice constants a (Å) .12,473,96a12,463,9512,453,9412,443,9312,433,9212,42c3,9112,413,900,50 0,55 0,60 0,65 0,70 0,75 0,80x crFigure 4: Lattice constants a (•) and c (■) as a function of composition x cr5,40Density (mg/mm 3 )5,355,305,255,200,35 0,45 0,55x cr0,65 0,75Figure 5: Density of SBN-crystals as a function of composition x crThe density was also calculated by using lattice constants. The result of the calculationagrees quite well, a constant difference of ∆ ρ = 0.039 mg/mm 3 ± 0.006 mg/mm 3or ∆ ρ/ρ = 0.8 % was found. The origin of this difference is still unclear and will besubject of further investigations.The following properties have been measured by my colleagues C. David and A.Tunyagi, who are members of the graduate school, too. They are concerned with:103

- Absorption measurements of the band edge and of the OH-stretching vibration (C.David)- Refractive indices (A. Tunyagi)- Second harmonic generation (A. Tunyagi)- Dielectric constants (C. David, A. Tunyagi).Future plansFor the construction of the phase diagram liquidus temperatures still have to bemeasured. For this purpose an existing furnace was modified [3].Further properties like thermal expansion of the crystals or the determination of thedistribution of Ba and Sr on the different lattice sites along the a- and b-direction willbe done in co-operation with other groups (Prof. Schmahl, University of Bochum,Prof. Bohatý, University of Cologne).References[1] J. R. Carruthers, M. Grasso: “Phase Equilibria Relations in the Ternary SystemBaO-SrO-Nb 2 O 5 “. Journal Electrochemical Society 117, 1426 (1970).[2] K. Megumi, N. Nagatsuma, Y. Kashiwada, Y. Furuhata: “The congruent meltingcomposition of SBN”. Journal of Materials Science 11, 1583 (1977).[3] Ch. Kuper, R. Pankrath, H. Hesse: “Growth and dielectric properties of congruentlymelting Ba 1-x Ca x TiO 3 crystals”. Applied Physics A, 65, 301 (1997).PublicationsC. David, A. Tunyagi, M. Ulex et al.: “OH stretching modes in Sr x Ba 1-x Nb 2 O 6 ” (inpreparation)Attended lectures, conference visits, research staysAttended lecturesWS 01/02: P. Hertel: Linear response theorySS 02: E. Krätzig, K. Ringhofer: The photorefractive nonlinearityWS 02/03: H.-J. Schmidt: Nonlinear wave equationsSeminarsSeminars of the Graduate College 695 (WS 01/02, SS 02, WS 02/03)Conference visitsWorking group „Kristalle für Laser und nichtlineare Optik“ of DGKK:- 27.-28.9.2001 in Köln- 26.-27.9.2002 in BonnAnnual conference of the DGKK:- 20.-21.3.2002 in Idar-ObersteinContribution to the seminarsSeminary talks on 11.11.2001 and 2.12.2002Various short talks in the seminar of the research groupExternal research stays30.11.2001 Institute of Crystal Growth Berlin-Adlershof, Dr. Reiche104

22.-26.4.2002 Institute of Physics of the Russian Academy of Science, Laboratoryfor Crystal growth, Dr. IvlevaInstitute of Crystallography of the Russian Academy of Science,Prof. Dr. Volk13.9.2002 University of Cologne, Institute of Crystallography, Prof. Dr. Bohatý5.11.2002 University of Bochum, Institute of Crystallography, Prof. Dr. Schmahl19.-20.12.2002 University of Cologne, Institute of Crystallography, Prof. Dr. BohatýDuration of the dissertationStart: 1.5.2001, determination expected 30.4.2004Period of support in the College1.5.2001 to 31.12.2003SupervisorsDr. Rainer Pankrath, apl. Prof. Dr. Klaus Betzler105

Dr. Monika WesnerTopic: Nonlinear optical properties of photorefractive strontium-barium niobatecrystalsResultsDuring the period of support in the graduate college I was able to finish the researchon nonlinear optical properties of photorefractive strontium-barium niobate crystals(SBN), and to write the dissertation “Nichtlineare optische Effekte im FerroelektrikumStrontiumbariumniobat”. Oxide ferroelectric strontium-barium niobate crystals are thebasis of my investigations. They are favorably suited for experiments in nonlinearoptics. Their special features are large nonlinear, for example pyro- and piezoelectric,electro- and thermooptic coefficients, robustness and a high optical quality. Moreover,excellent barrier-waveguides can be produced by ion-implantation. The ionimplantationis performed in collaboration with Dr. P. Moretti from the University ofLyon, France. During my dissertation, several other oxide crystals have beenchecked as an alternative for SBN, too [D, I ]. However, for the performed nonlinearopticalexperiments in the field of thermooptic beam self-focusing, photorefractivemodulational instability, pattern and soliton formation, other crystals rarely prove tobe a comparable alternative to SBN volume crystals or SBN waveguides.SBN crystals of the congruently melting composition, grown by Dr. R. Pankrath atthe University of Osnabrück, possess an internationally acknowledged quality. Thedominating electrooptic coefficient r 33 could be shown to be larger than 200 pm/Veven for near infrared wavelengths up to λ = 1.5 µm [E]. Due to the large electroopticcoefficients, photorefractive effects are possible even in the infrared, though ferroelectricinsulator crystals are seldom investigated in this interesting (telecommunication)wavelengths range. After the polishing of the crystals, the surfaces are opticallyflat. Checks of the surface quality were performed with an atomic force microscope(AFM) in collaboration with Prof. Dr. M. Schleberger, now University of Essen. Shecould prove the existence of worm- or leavelike structures on the (001)-surfaces ofthe SBN-crystals. The depth of these structures does not exceed 2 nm. We haveclear hints, that the structures are indeed ferroelectric domain patterns, which areconserved during the polishing process. Up to now, there exist only few figures of thedomain structure of SBN throughout the literature [1].By thermooptic self-focusing effects, so-called thermal lenses could be induced inSBN-waveguides for the first time. In contrast to usual observations in volume oxidecrystals, the induced thermal lenses maintain their spherical properties over a largerange of laser powers. For thermooptic effects, the laser power determines the magnitudeof the nonlinearity. An example is demonstrated in Fig. 1. Here, a focusedbeam of an Ar + -laser is coupled into the SBN-waveguide. Shown is the laser beamprofile at the crystal’s endface. In the figure, the changes of the beam profile withincreasing input laser power P in , i. e., with increasing nonlinearity, are shown in acontour plot. Due to thermooptic refractive index changes, the beam self-focuses upto laser powers of about 50 mW. By this way, spherical lenses with focal lengthsaround 1 mm can be induced. Thermooptic beam filamentation occurs, if the focalpoint of the lens reaches the inner part of the waveguide. This can also be seen inFig. 1 for laser powers larger than P in = 50 mW. The filamentation can be shown tobe as well explainable with the model of spherical aberration of the thermal lens [2] orwith modulational instabilities [3]. Thermooptic refractive index changes build up106

comparatively fast (100 µs-magnitude) [A] and are shown to be useful to switch andfocus light beams and divide them into different channels.Fig. 1: Example of thermoopticnonlinear effects inSBN-waveguides. Shown isa contour plot of the beamprofile of an incoupled Ar + -laser beam at the crystal’sendface for increasing laserpower P in .A further part of my doctoral thesis is devoted to modulational instabilities, whichmanifest in the filamentation of an initially homogeneous beam profile. Modulationalinstabilities are induced by perturbations of the beam profile which grow exponentially.The instabilities occur, sometimes inevitable, as side effect during experimentsof other nonlinear phenomena, especially at the photorefractive soliton formation. Adetailed knowledge of them is therefore of importance. However, this subject is seldomtreated experimentally for the photorefractive nonlinearity. Just as rare are theoreticalconsiderations concerning this nonlinearity which can be approximated as anonlinear Schödinger equation with saturable nonlinearity (see report F. Homann).Experimentally, input beam configurations are found, which have a high resistanceagainst modulational instabilities. Following, this finding proves helpful in experimentswith photorefractive solitons. If, instead of a single laser beam, two counterpropagatingbeams are used, the irregular filaments of the modulational instability order, forinstance to hexagonal patterns. The additionally observed more complicated structurespoint to the fact that in photorefractive crystals even patterns with higher thanhexagonal symmetry can be induced, provided that the saturation grade of thenonlinearity is large enough.An important part of my dissertation is devoted to photorefractive spatial solitons.Spatial solitons are stable light beams propagating with a constant beam shape. Dueto their energy-conserving properties they are promising for applications, for examplein telecommunications. Besides the usefulness for applications, the special solitons’features make them interesting for theoretical physicists and mathematicians sincethe first investigations in the 1870 th . Since 1993 it is known that solitons can be inducedin photorefractive crystals [4]. Following, a research boom set in, because nowphysicists had found an easily accessibly optical system, where diverse aspects ofsoliton formation could be studied experimentally. The experiments done in Osnabrückwere the first measurements in SBN-waveguides [ B, C, E, G - I]. Part of thework has been done in collaboration with Prof. Dr. V. Shandarov from the Universityof Tomsk, Russia, and Prof. Dr. J. Xu, Nankai University, China. The experimentalproof of the existence of a soliton is difficult as a matter of principle. We could verify107

for the first time that photorefractive solitons exist in SBN-waveguides up to wavelengthsof 1.5 µm, i. e., up to the telecommunication wavelengths region. One of themost convincing experiments is demonstrated in Fig. 2. Shown are the intensity profilesat the endfaces of samples of the same SBN-crystal, but with different propagationlengths. The input conditions, especially the input beam width of 44 µm, are keptconstant. In Fig. 2 obviously the output beam profiles are almost equal despite thedifferent propagation lengths – which only can be explained by soliton formation. Thesoliton-like behavior is in striking contrast to the usual one, which is a “normal”nonlinear lens as demonstrated in Fig. 1.Fig. 2 Experiment to verifythe existence of photorefractivesolitons in SBNwaveguidesat a wavelengthλ = 1310 nm. Shownare beam profiles at theendfaces of three samplesof the same SBN-crystalwith different propagationlengths z = 1.7, 5.2, and7.8 mm. The input beamwidth is kept constant at 44µm.We further found that the input beam configuration mostly used in experiments withphotorefractive solitons is not ideal. With a changed beam geometry we were able toinduce photorefractive solitons over an intensity range of five orders of magnitude,which has to be compared with two orders of magnitude demonstrated in the literatureso far. Moreover, the easily inducible solitons also prove to be well suited as ameasurement method, for instance to determine the so-called dark intensity (a ratioof photo- and dark conductivity) or to investigate the poling state of a ferroelectriccrystal spatially-resolved.In investigations of the temporal development of the solitons we were able to provethe experimental value of a theoretical model of Fressengeas et al. [5]. This modelallows far reaching predictions, however, at the expense of rough approximations.Because of that, at the beginning, the experimental applicability of the model wasdoubtful. This work is, to our knowledge, the only one where the complicated temporaldevelopment of solitons is extensively classified. One of the most remarkable resultsof the theoretical and experimental investigations is that the build-up of photorefractivesolitons is not markedly influenced by the wavelength. This is in striking contrastto other photorefractive phenomena, e. g., the common photorefractive twobeamcoupling [6], which has a distinctly slower development time in the infrared thanat visible wavelengths.A large section of my thesis covers problems of the switching of the polarization inSBN. Despite the long time of research in ferroelectrics (since the 1940 th ), theswitching behavior of ferroelectrics still pose questions. This applies the more for relaxor-ferroelectricslike SBN, which are characterized by a broadened phase transi-108

tion. Important information about the poling state of the crystal can be gained withself-focusing methods developed during the dissertation (see above), as well as byfrequency-doubling microscopy. The frequency-doubling measurements have beenperformed in collaboration with A. Rosenfeldt, University of Münster, and PD Dr. M.Flörsheimer, INE Forschungszentrum Karlsruhe GmbH. In frequency-doubling microscopy,ferroelectric domain walls are made visible. In our investigations we foundthat if a SBN-crystal is poled in opposite directions by application of external electricfields at room temperature, the switchable polarization decreases (“ages”) from thepositive towards the negative electrode. In the literature, “aging” is mostly measuredin the volume of the crystal. For this reason there are only few hints, that “aging” canindeed be a spatially inhomogeneous effect.Moreover, for the first time systematically depolarized stripes with widths in the micronrange have been produced by intense illumination and application of electricfields [F]. The depolarized stripes could be shown to possess an enlarged refractiveindex compared to the poled material [7]. Because of that the depolarized stripesserve as waveguides for light. This can be seen in Fig. 3. Here, each image showsan about 80 x 80 µm² wide area of the crystal’s endface. The lateral position of thecrystal is changed relatively to the illuminating stripe laser beam. The relative positionis mentioned below each picture. Row a) shows the initial intensity distribution, whichis the same for all positions. After the production of the stripes, in row b), obviouslytwo waveguiding channels appear at the positions +/-0.03 mm. The channels arestable for at least a month if the crystal is kept in the dark, and for at least a week ifthe crystal is illuminated with intense read-out light.Fig. 3: Each picture shows an about 80 x 80 mm² wide area of the crystals endface.The crystal position is changed relative to an illuminating stripe laser beam. The positionis mentioned below each picture. (a) inititial intensity distribution (b) after fabricationof two waveguides at x = +/- 0.03 mm.More complicated structures can be formed with this new method of electrical fixing,too. Promise have produced patterns of poled and unpoled stripes, with stripe widthsbelow 1 micron, which can be used for quasi-phase-matched frequency doubling.References:[1] P. Lehnen, W. Kleemann, T. Woike, R. Pankrath: Ferroelectric nanodomains inthe uniaxial relaxor system Sr 0.61-x Ba 0.39 Nb 2 O 6 :Ce -x (3+) . Phys. Rev. B 6422, art. no.224109 (2001).[2] S. A. Akhmanov, D. P. Krindach, A. V. Migulin, A. P. Sukhorukov, R. V. Khokhlov.IEEE J. Quantum Electron. QE-4, 568 (1968).109

[3] V. I. Bespalov and V. I. Talanov. JETP Lett. 3, 307 (1966).[4] G. C. Duree, J. L. Shultz, G. J. Salamo, M. Segev , A. Yariv, B. Crosignani, P.DiPorto, E. J. Sharp, R. R. Neurgaonkar. Phys. Rev. Lett. 71, 533 (1993).[5] N. Fressengeas, J. Maufoy, G. Kugel. Phys. Rev. E 54, 6866 (1996).[6] D. L. Staebler, J. J. Amodei. J. Appl. Phys. 43, 1042 (1972).[7] Th. Woike, T. Granzow, U. Dörfler, C. Poetsch, M. Wöhlecke, R. Pankrath.phys. stat. sol. (a) 186, R13 (2001).Publications[A] D. Kip, M. Wesner, E. Krätzig, V. Shandarov, P. Moretti, "All-optical beamdeflectionand switching in planar strontium-barium niobate waveguides“. Appl. Phys.Lett. 72, 1960 (1998).[B] D. Kip, M. Wesner, V. Shandarov, P. Moretti, “Observation of bright spatialphotorefractive solitons in a planar strontium-barium niobate waveguide”. Opt. Lett.23, 821 (1998).[C] D. Kip, M. Wesner, C. Herden, V. Shandarov, "Interaction of spatial photorefractivesolitons in a planar waveguide”. Appl. Phys. B 68, 971 (1999).[D] V. Shandarov, M. Wesner, J. Hukriede, D. Kip, "Observation of dark spatialphotovoltaic solitons in planar waveguides in lithium niobate”. J. Opt. A: Pure Appl.Opt. 2, 500 (2000).[E] M. Wesner, C. Herden, D. Kip, E. Krätzig, P. Moretti, "Photorefractive steadystatesolitons up to telecommunication wavelengths in planar SBN waveguides”. Opt.Commun. 188, 69 (2001).[F] M. Wesner, C. Herden, D. Kip, "Electrical fixing of waveguide channels in strontium-bariumniobate crystals”. Appl. Phys. B 72, 733 (2001).[G] M. Wesner, C. Herden, R. Pankrath, D. Kip, P. Moretti, "Temporal developmentof photorefractive solitons up to telecommunication wavelengths in SBN”. Phys. Rev.E 64, 36613 (2001).[H] D. Kip, C. Herden, M. Wesner, "All-optical signal routing using interaction of mutuallyincoherent spatial solitons”. Ferroelectrics 274, 135 (2002).[I] J. Xu, V. Shandarov, M. Wesner, D. Kip, "Observation of two-dimensional spatialsolitons in iron-doped barium-calcium titanate crystals”. phys. stat. sol. (a)189, R4 (2002).[J] D. Kip, M. Wesner, E. Krätzig, V. Shandarov, P. Moretti, "Bright photorefractivespatial solitons in optical waveguides on SBN“. Proc. SPIE 3733, 155 – 162 (1998).[K] D. Kip, M. Wesner, C. Herden, V. Shandarov, P. Moretti, “Spatial photorefractivesolitons in planar strontium-barium niobate waveguides”. OSA TOPS 27, 479 – 482(1999).[L] M. Wesner, D. Kip, V. Shandarov, P. Moretti, “Thermally-induced all-optical beamsteering and switching properties of SBN waveguides”. OSA TOPS 27, 441 – 446(1999).[M] D. Kip, J. Hukriede, M. Wesner, E. Krätzig. "Photorefractive waveguides“. Proc.SPIE 3801, 9 – 23 (1999).[N] V. Shandarov, D. Kip, M. Wesner, J. Hukriede, "Development and collapse ofdark spatial optical solitons in planar waveguides in lithium niobate”. Technical DigestCLEO Europe, CMG5 (2000).110

[O] D. Kip, C. Herden, M. Wesner, "Electrical fixing of waveguide channels usingdynamic self-focusing in strontium-barium niobate crystals”. Technical Digest CLEOEurope, CFF1 (2000).[P] M. Wesner, D. Kip, P. Moretti, "Infrared photorefractive effects in ion-implantedSBN waveguides”. Technical Digest CLEO Europe, CFF6 (2000).[Q] M. Wesner, C. Herden, D. Kip, "A new method of electrical fixing in strontiumbariumniobate crystals”. OSA TOPS 62, 152 - 157 (2001).[R] D. Kip, C. Herden, M. Wesner, "All-optical signal router based on the interactionof mutually incoherent solitons”. OSA TOPS 62, 685 - 689 (2001).[S] V. Shandarov, D. Kip, M. Wesner, “Distinctions of the characteristics of brightspatial solitons in SBN crystals form existence curve predictions”. OSA TOPS 62,690 - 695 (2001).Attended lectures• P. Hertel, Linear response theory, WS 01/02• E. Krätzig, K. Ringhofer, The photorefractive nonlinearity, SS02• Seminars of the Graduate College SS 01, WS 01/02, SS 02, and WS 02/03• Workshop on Photorefractive Nonlinearities, Oct. 4 – 5, 2001, Contribution: talk “IRphotorefractive solitons in SBN”.• Workshop “SBN: Crystal Growth and Details of the Structure” July 1, 2002• Workshop “Strontium-Barium-Niobate (SBN) – a typical relaxor?” May 6, 2002.Conference visits• Topical Meeting on Photorefractive Materials, Effects, and Devices, Elsinore,Denmark, June 25 – 27, 1999; contribution: poster “Thermally-induced all-opticalbeam steering and switching properties of SBN waveguides”.• Spring Conference of the German Physical Society, Bonn, Germany, April 3 – 7,2000; contribution: talk "Infrarote photorefraktive Solitonen in SBN”.• CLEO Europe 2000, Nice, France, Sept. 10 – 15, 2000; contributions: talk “Electricalfixing of waveguide channels using dynamic self-focusing in strontium-bariumniobate crystals” and talk “Infrared photorefractive effects in ion-implanted SBNWaveguides”.• Spring Conference of the German Physical Society, Hamburg, Germany, March26 – 30, 2001; contribution: talk “Zeitliche Entwicklung photorefraktiver Solitonenin planaren SBN-Wellenleitern”.• Topical Meeting on Photorefractive Materials, Effects, and Devices, Delavan,USA, July 8 - 12, 2001; contribution: talk “A new method of electrical fixing instrontium-barium niobate crystals”.Duration of the dissertation: 01.12.1998 – 07.03.2003Period of support in the College: --Supervisors: Prof. Dr. E. Krätzig, Prof. Dr. D. Kip111

Dipl.-Phys. Albert WirpTopic: Optical Frequency Conversion in Oxide WaveguidesResults1. IntroductionGreen and blue laser sources are very interesting for many applications like datastorage, medical applications, printing, laser TV and material treatment.Semiconductor lasers of these wavelengths are rarely available. One solution isfrequency conversion, especially Second Harmonic Generation (SHG). Near infraredlaser light is frequency doubled to blue and green light, respectively.Lithium niobate (LiNbO 3 ) and lithium tantalate (LiTaO 3 ) crystals are promisingcandidates, because these materials are commercially available and they possesslarge nonlinear coefficients. The transmission ranges are up to 320 nm for LiNbO 3and up to 270 nm for LiTaO 3 . To achieve SHG the phase matching condition for thefundamental and the frequency doubled waves must be fulfilled. Due to the smallbirefringence, conventional phase matching is not possible. Thus, a Quasi PhaseMatching (QPM) condition is needed. This is realised by periodic inversion of theferroeletric domains. The period length of the domain inversion depends on thewavelength of the fundamental light. This means all wavelengths the crystal istransparent for are useable. Here, LiTaO 3 is favourably suited due to its broadtransparency range.The efficiency of the SHG depends on the square of the intensity of the fundamentallight. In channel waveguides light is guided in a small area, which implies a high lightintensity over the whole conversion or waveguide length.Under illumination LiNbO 3 and LiTaO 3 show photorefractive index changes, whichhave to be minimized for efficient SHG. This effect can be reduced by doping thecrystal with, e.g. Mg, Zn or by using stoichiometric crystals fabricated for example bya vapour transport equilibration technique.2. Simulations2.1 The WaveguidesThe number of modes guided in a waveguide fabricated by diffusion of metal ionsdepends on the refractive index profile and the wavelength. Important parameters arediffusion temperature and the time of indiffusion. Furthermore, the effective refractiveindex depends on temperature. To optimize these values the profiles of thewaveguides are simulated for several sets of parameters. Useful parameters are a50 nm-thick titanium layer in a 4 µm-wide stripe diffused into LiNbO 3 for 20 hours at1000 °C. This results in an index profile shown in Fig. 1. The waveguide issinlgemode for infrared light, but multimode for visible light. This is the best set ofparameters found so far.112

Fig 1:The index profile of a typicalwaveguide: a 4 µm-wide and 50 nmthicklayer of titanium is diffused intoLiNbO 3 for 20 hours at 1000 ° C.2.2 Temperature Dependence of the Refractive IndexThe refractive index varies with temperature due to the thermooptic effect. Thetemperature dependence can be described by a Sellmeier equation. Furthermore, anincrease of the crystal length due to an increased temperature has to be taken intoaccount. The thermooptic effect proves to be the major effect, but the thermalexpansion is also measured.3. Experimental Methods3.1 Preparation of the WaveguidesThe crystals are cut from a 0.5 mm-thick wafer (z-cut orientation) to pieces ofapproximately 8 x 10 mm 2 . The crystals are carefully cleaned and evapored withlayers of 50 nm titanium. The whole samples are covered with photoresist and stripesare formed using lithographic techniques. The uncovered titanium is etched with anappropriate acid and the photoresist is removed afterwords, too. Then the titanium isdiffused into the crystal for 20 hours at 1000 °C and 1250 °C for LiNbO 3 and LiTaO 3 ,respectively. Finally, the front and rear faces of the waveguides are polished tooptical quality. For this step a glass plate is clambed onto the crystal to get a sharpand rectangular edge, to allow endface-coupling into the 10 µm-thick waveguides.3.2 Fabry-Perot InterferometerThe temperature dependence of the refractive index of our waveguide samples ismeasured with a fabry-perot interferometer and the results are compared with theory.This is necessary to distinguish between thermal refractive-index changes and lightinducedrefractive index changes due to the photorefractive effect. The experimentalsetup is shown in Fig. 2. Different wavelengths are used: 1064 nm of a Nd:YAGlaser,633 nm of a He-Ne laser and several lines of an argon ion laser (different linesfrom 514 nm to 456 nm). The light is coupled into the waveguide by a microscopeobjective. The endface of the waveguide is imaged onto a photodetector. The crystalis mounted on a peltier element, which allows to vary the temperature between roomtemperature and 50 °C. Lower temperatures are not possible because of watercondensation on the crystal surface. An interference pattern is formed by the113

transmitted beam and the beam which is reflected at the rear and at the front face ofthe waveguide. On both sides only a few percent of the incident light is reflected, andincluding the effect of light damping in the waveguide only small modulation on alarge background intensity is detected.Fig 2: Experimental setup. Thedetection of fabry perot interferencesallows to determine thethermal expansion and thermaland light induced refractive indexchanges at different wavelength.4. Results and OutlookFigure 3 shows the fabry-perot interferences at 1064 nm. The thermal expansion inthis measurement is calculated to α = (1.1 ± 0.4) x 10 -5 K -1 . This is in good agreementwith the published value of α = 1.5 x 10 -5 K -1 [1]. The growing period length withincreasing temperature is caused by an increasing temperature gradient inside thecrystal, because the temperature is measured directly at the peltier element.If the temperature is kept constant, one period of the oscillation corresponds to arefractive-index change of 4 x 10 -5 . This means that light-induced refractive-indexchanges as small as 2 x 10 -6 should be detectable with this fabry-perotinterferometer.In the future, light-induced refractive-index changes will be investigated inwaveguides with different lithium concentrations and dopings. Furthermore, thepoling behaviour of these waveguides will be examined.Fig 3: Transmitted intensity vs. temperaturefor a wavelength of 1064nm. The fabry perot interferencesare modulations on an offset.114

Literature:[1] Y. S. Kim and R. T. Smith, “Thermal expansion of lithium niobate single crystals,”J. Appl. Phys. 40, 4637-4641 (1969).PublicationsJ. Imbrock, A. Wirp, D. Kip, E. Krätzig, and D. Berben, “Photorefractive properties oflithium and copper in-diffused lithium niobate crystals,” J. Opt. Soc. Am. B 19, 1822-1829 (2002)Attended lectures, conference visitsLinear response theory (P. Hertel)The photorefractive nonlinearity (E. Krätzig and K. H. Ringhofer)Nonlinear wave equations (H.-J. Schmidt)Seminar of the Graduate CollegeQuantum Optic School, Universities of Bonn and Potsdam, 02.04.- 12.04.2002Research staysUniversity of Bonn, Group of Prof. Buse, 03.08.2001University of Colone, “Institut für Mineralogie und Geochemie”, Group of PD Dr.Woike, 27.11.2001Duration of the dissertation: Start August 2001, termination expected July 2004Period of support in the College: 01.08.2001 – 31.12.2003Supervisors: Prof. Dr. D. Kip, Prof. Dr. E. Krätzig115

2. Auflistung aller Kollegiat(inn)enDoktorand(inn)enNameZeitpunkt bzw.voraussichtlicherZeitpunktder PromotionAlterbeiEintrittin dasKollegZeitpunkt und Ortdes ersten berufsqualifizierendenAbschlussesDavid, Calin September 04 25 Juni 99 in Klausenburg(Rumänien)Filippov, Oleg Oktober 04 23 Januar 99 in Moskau(Russland)FörderzeitraumimGraduiertenkolleg01.10.01 -31.12.0315.11.01 -31.12.03BetreuerWöhleckeRinghofer,Gorkounov,KrätzigKapphanGoubaev, Airat Dezember 05 23 Juni 02 in Kazan 01.12.02 -31.12.03Geisler, Oktober 03 32 Juli 94 in Hannover -- SchürmannAndreasHomann, Felix März 04 29 August 94 in OsnabrückKislova, InnaRückkehr nachTwer (Krankheitdes Vaters)25 August 99 in Twer(Russland)Lapine, Mikhail Oktober 04 25 Juni 97 in Moskau(Russland)Müller, Manfred Dezember 03 25 März 00 in OsnabrückPlenge, Jürgen Dezember 02 28 Juli 99 in OsnabrückRahe, Florian Dezember 03 25 Oktober 98 in OsnabrückShelokovskyy,PavloSvetogorova,ElenaNovember 04 24 Dezember 00 inKharkiv (Ukraine)August 04 21 Juni 01 in Moskau(Russland)Tunyagi, Arthur August 04 24 Juni 00 in Klausenburg(Rumänien)02.04.01 -31.12.0301.08.01 -31.10.0201.11.01 -31.12.0301.01.01 -31.12.0301.01.01 -31.12.0301.12.01 -31.12.0311.09.01 -31.12.0301.09.01 -31.12.03Ulex, Michael April 04 36 April 99 in Berlin 02.05.01 -31.12.03Wesner, Monika März 03 32 November 98 inOsnabrückWirp, Albert Juli 04 26 Juli 01 in Osnabrück01.08.01 -31.12.03Schmidt,Bärwinkel,SchnackKapphanRinghofer,Gorkounov,BetzlerBuse-- Rühl, FleschKrätzigSchnack,Bärwinkel,SchmidtSchürmannBetzlerBetzler,Pankrath-- Krätzig,KipKip,Krätzig116

Postdoktorand(inn)enNameDr. Kamenov,VladimirProf. Dr. Schleberger,MarikaDr. Pramann,AxelZeitpunkt derPromotionAlter beiEintrittin dasKollegZeitpunkt und Ortdes ersten berufsqualifizierendenAbschlussesOktober 00 28 Juli 95 in Rousse(Bulgarien)FörderzeitraumimGraduiertenkolleg01.01.01 -31.08.01Juni 93 35 Oktober 90 in Osnabrück01.01.01 -30.04.01Juli 00 34 Oktober 94 inBraunschweig15.11.01 -14.11.03BetreuerRinghofer,KrätzigKrätzigRühl,Flesch3. Auswahl der Kollegiat(inn)enAlle 13 Stipendien wurden national und international ausgeschrieben. Es gingen insgesamt82 Bewerbungen ein, darunter waren 8 interne Bewerbungen und 15 Bewerbungenvon Frauen. Die Auswahl der Stipendiat(inn)en wurden von den einzelnenProjektleitern unter Beachtung der Regeln der DFG vorgenommen, wobei häufig Kollegenzu Rate gezogen wurden. Zusätzlich wurden von der Mitgliederversammlung 3weitere Kollegiat(inn)en aufgenommen.Die insgesamt 19 Kollegiat(inn)en (das Postdoktorandenstipendium wurde nacheinander3-mal vergeben, ein Doktorandenstipendium 2-mal) teilen sich wie folgt auf: 8intern, 11 von außerhalb; 4 Frauen, 15 Männer; 10 Deutsche, 9 Ausländer.Weiter ist noch anzumerken, dass sich unter den internen Kollegiaten die beidenKandidaten (Plenge, Müller) befinden, die in den Jahren 1999 und 2000 für die bestenStudienleistungen im Fachbereich Physik der Universität Osnabrück ausgezeichnetwurden.4. Durchführung des StudienprogrammsIm Antrag waren als Studienprogramm (zusätzlich zu den üblichen Veranstaltungendes Fachbereichs) eine integrierte Ringvorlesung 'Nichtlinearitäten optischer Materialien',ein regelmäßig stattfindendes Seminar und mindestens einmal im Jahr einWorkshop vorgesehen. Diese Veranstaltungen sind alle in der vorgesehenen Weisein englischer Sprache durchgeführt worden.Die Ringvorlesung erstreckt sich über 4 Semester und begann im WS 01/02, alsnahezu alle Stipendien vergeben waren. P. Hertel begann mit der Einführung 'Linearresponse theory'. Hier wurden zunächst die Grundlagen linearer Theorien besprochen,ein Schwerpunkt lag jedoch auch auf der Erweiterung in Bezug auf nichtlineareEffekte. Das Vorlesungsskriptum liegt als Anhang 1 im Teil 2 dieses Arbeits- undErgebnisberichtes bei. Im SS 02 folgte die Vorlesung 'The photorefractive nonlinearity'von E. Krätzig und K. Ringhofer. Die Grundlagen der Photorefraktion wurden voneinem experimentell und einem theoretisch arbeitendem Physiker beleuchtet. Wichti-117

ge Experimente wurden von Hilfskräften vorbereitet und in den Forschungslaborsden Kollegiat(inn)en vorgeführt. Das Skriptum ist in Anlage 2 im Teil 2 dieses Berichtszu finden. Im WS 02/03 behandelte H.-J. Schmidt 'Nonlinear wave equations'.Nach einem Überblick standen solitäre Lösungen in verschiedenen Bereichen im Mittelpunkt,z. B. in der Hydrodynamik, der nichtlinearen Optik oder der Plasmaphysik.In Anlage 3 ist das Skriptum aufgeführt. Im SS 03 werden K. Betzler, M. Imlau undM. Wöhlecke über ’Frequency conversion and wave mixing’ vortragen.Weiter haben wir den Kollegiat(inn)en noch folgende spezielle Veranstaltungen angeboten:Writer's Workshop (P. Hertel) und Graphic Workshop (K. Betzler).Das Seminar des Graduiertenkollegs startete im SS 01 und wurde regelmäßig weitergeführt.Kollegiat(inn)en, Gäste und Betreuer stellten ihre Ergebnisse vor. Die Seminarprogrammeliegen als Anhang 4 bei. Neben diesem Hauptseminar gab es nochzahlreiche Seminare kleinerer Gruppen und Arbeitsbesprechungen.Bisher wurden 3 Workshops zu Kernfragen des Graduiertenkollegs durchgeführt.Zahlreiche auswärtige Gäste nahmen teil. Die Themen der Workshops lauteten:'Photorefractive Nonlinearities' (04. – 05.10.01), 'Strontium-Barium-Niobate (SBN) – atypical relaxor?' (06.05.02) und 'SBN: Crystal Growth and Details of the Structure'(01.07.02). Einzelheiten sind aus Anhang 5 zu ersehen. – Im SS 03 ist ein Sonderkolloquiumin Memoriam Klaus Ringhofer geplant.5. Angaben zur Vergabe der KoordinationsmittelDie Koordinationsmittel wurden einmal für Hilfskräfte eingesetzt, die folgende Aufgabenfür das Kolleg durchführten: Hilfe beim Schreiben der Skripten, vor allem bei derErstellung der Bilder; Einrichtung der Rechner; Vorbereitung von Laborexperimentenfür die Vorlesung; Durchführung numerischer Rechnungen. Dann wurde die halbeStelle der Sekretärin auf eine dreiviertel Stelle aufgestockt. Kopierkosten des Kollegswurden bezahlt und Programme von allgemeinem Interesse beschafft.6. Interne Erfolgskontrolle des KollegsZunächst übernahm jeder Betreuer die Aufgabe, den Stand der Arbeiten seiner Kollegiat(inn)enständig zu verfolgen und gegebenenfalls steuernd einzugreifen. Dazudienten auch regelmäßige Arbeitsbesprechungen. Weiter haben alle Kollegiat(inn)enmindestens einmal im Jahr im Seminar des Kollegs über die Fortschritte vorgetragen.Diesen Vorträgen folgte stets eine Diskussion, die weitere Schlüsse auf den Erfolgder Arbeiten zuließ. Der Gesamtstand des Kollegs wurde auch bei 8 Mitgliederversammlungenerörtert und überprüft.118

7. Gastwissenschaftlerprogramm7.1 GastvorträgeProf. Dr. V. Serov, Moskau State Univ., Russland“Some mathematical aspects of soliton stability” 23.07.01Prof. Dr. V. Vikhnin, Ioffe Inst. St. Petersburg, Russland“Report about IMF-10 Meeting results and recent theoreticaldevelopments in the field of defects in oxidic crystals” 10.09.01Dr. V. Matusevich, Univ. Jena“Application of wave mixing processes in BCT” 04.10.01Prof. Dr. C. Denz, TU Darmstadt“Storage of volume holograms in photorefractive materials” 04.10.01Prof. Dr. W. Lange, Univ. Münster“What can we learn from spontaneous opticalpatterns and from localized in atomic vapors?” 04.10.01Dr. A. Kießling, Univ. Jena“Soliton-like structures in BTO” 04.10.01Prof. Dr. R. Kowarschik, Univ. Jena“Application of wave mixing processes in BCT” 04.10.01Dr. M. Goulkov, Academy of Sciences, Kiev, Ukraine“Nature and applications of light scattering inphotorefractive crystals” 04.10.01Prof. Dr. R. Rupp, Univ. Wien“Holographic scattering: angular and spectral properties” 04.10.01Prof. Dr. M. P. Petrov, Ioffe Institute, St. Petersburg, RussiaNonlinear interactions and scattering of space charge waves 05.10.01Prof. Dr. T.Tschudi, Univ. Darmstadt“Novelty filters in photorefractive crystals” 05.10.01Prof. Dr. T. Volk, Institute of Crystallography, Moscow, RussiaFerroelectricity-driven holographic properties of RE-doped SBN 05.10.01Dr. U. Dörfler, Univ. Köln“Holographic studies of SBN doped with Ce and Cr” 19.10.01Dr. T. Granzow, Univ. Köln“Relaxor ferroelectrics: Ce-doped SBN as an example” 19.10.01PD Dr. T. Woike, Univ. Köln“Holographic scattering in SBN, LiNbO3:Fe and LiTaO3:Fe” 26.10.01Dr. A. Pramann, FU Berlin“Anion photoelectron spectroscopy of size-selectedbimetallic clusters in molecular beams” 29.10.01Prof. Dr. V. Serov, Moskau State Univ., Russland119

“Particular solutions of nonlinear wave equations”and “Lie groups and the sine-Gordon equation” 06.11.01Prof. Dr. Jingjun Xu, Nankai University Tianjin, P. R. China“Research at the Photonics Research Center at theNankai University Tianjin” 09.11.01Prof. Dr. B. Wellegehausen, Univ. Hannover“Generation of coherent VUV- and XUV-radiationby high intensity laser-matter interaction” 19.11.01Dr. M. Fally, Universität Wien“10 years neutron diffraction from light-induced gratings“ 23.11.01Dr. E. Chamonina, Oxford University, UK“Photorefractive scattering” 03.12.01Dr. M. Ellaban, Universität Wien„Holographic scattering in LiNbO 3 “ 14.12.01Prof. Dr. L. Wöste, Freie Universität Berlin“Perspectives of femtosecond spectroscopy:from clusters to clouds” 28.01.02Prof. Dr. V. Serov, Moskau, Russland“Some mathematical results of nonlinear wave guiding structures”and ”Nonlinear evolution equations: elliptic solutions” 28./29.1.02Prof. Dr. M. P. Petrov, Ioffe Institute St. Petersburg, Russland“Overall rectification and second harmonic generationof space charge waves” 08.04.02Prof. Dr. W. Schmahl, Univ. Bochum“Elements of the Landau theory of phase transitions” 06.05.02Dr. T. Granzow, Univ. Köln“Experimental overview of relaxor-type properties of SBN” 06.05.02Prof. Dr. W. Kleemann, Univ. Duisburg“Disordered polar systems-an overview of concepts” 06.05.02PD Dr. M. Flörsheimer, Univ. Karlsruhe“Second-harmonic and sum frequency imaging ofinterfaces in materials science, biophysics andenvironmental geochemistry” 13.05.02Prof. Dr. N. Hansen, Univ. Henri Poivcare, Nancy, Frankreich“Crystal structure and electron density distributionfrom Bragg scattering-What can be learned ingeneral and what about SBN” 01.07.02Dr. V. Petricek, Academy of Sciences, Prag, Tschechien“Determination of the Modulated Structure of SBN” 01.07.02Dr. J. Schefer, Lab. for Neutron Scattering, ETHZ & PSI, Villingen“Structure measurements of SBN by neutrons” 01.07.02120

Prof. Dr. V. Serov, Moskau State Univ., Russland„Appoximative solutions of nonlinear integral equationsvia iteration procedure" 22.07.02Prof. Dr. Smirnov Moskau State Univ., Russland"Propagation of Electromagnetic Waves in OpenCylindrical Waveguides with Nonlinear Media" 22.07.02Dr. M. Goulkov, Academy of Sciences, Kiev, Ukraine“Insight into the nature of light-induced scattering inphotorefractive crystals with dominating local response” 26.07.02Prof. Dr. S. Odoulov, Academy of Sciences, Kiev, Ukraine“Photorefraction in periodically poled lithium niobate” 02.08.02Dr. Boris Sturmann, Academy of Sciences, Novosibirsk, Russland"Singular nonlinear response and soliton-like beam propagation infast photorefractive crystals" 25.10.02PD Dr. T. Woike und Dipl.-Phys. P. Herth, Univ. Köln,“Holographische Streuung und Polaronen“ 13.11.02Prof. Dr. T. Volk, Academy of Sciences, Moskau, Russland“Polarization kinetics and the domain structure of SBN crystals” 26.11.02PD Dr. M. Flörsheimer, Univ. Karlsruhe„Beobachtung der Domänenstruktur ferroelektrischer Kristalledurch nichtlineare Mikroskopie“ 29.11.02Prof. Dr. V. Vikhnin, Ioffe Inst. St. Petersburg, Russland“Two types of CTVEs and two types of recombinationluminescence in ferroelectric oxides” 17.12.02Prof. Dr. M. P. Petrov, Ioffe Institute, St. PetersburgNonlinear Interactions of Space Charge Waves 17.01.03Dr. B. Briat, Laboratoire d’Optique Physique, ESPCI, ParisA combined Optical/ Magneto-Optical / ODMR approach tothe identification of defects and charge transfer processes 24.01.03Dipl.-Phys. F. Meier, Universität Basel“Magnetische Moleküle und MQC“ 04.02.03121

7.2 GastaufenthalteDr. Alexey Kutsenko St. Petersburg, Russland 01.04.01 – 30.04.01Dr. Vladimir Shandarov Tomsk, Russland 07.04.01 – 27.05.01Prof. Dr. Michael Petrov St. Petersburg, Russland 23.09.01 – 15.12.01Prof. Dr. Michael Petrov St. Petersburg, Russland 01.04.02 – 14.05.02Dr. Boris Sturmann Novosibirsk, Russland 27.04.02 – 14.07.02Prof. Dr. Tatjana Volk Moskau, Russland 07.11.02 – 15.12.02Prof. Dr. Valeri S. Serov Moskau, Russland 04.12.02 – 11.12.02Prof. Dr. Vladimir Trepakov St. Petersburg, Russland 27.11.02 – 05.12.028. Zwischenbilanz des KollegsAus unserer Sicht sind die im Antrag formulierten Ziele bisher voll erreicht worden. Inder Forschung haben wir bisher drei Themenkreise behandelt: Photorefraktive Nichtlinearitäten,Frequenzkonversion und Wellenmischen sowie Nichtlinearitäten bei derWellenleitung. Auf diesen Gebieten sind in Osnabrück bereits umfangreiche Vorarbeitengeleistet worden. Die Grundausstattung, die vor allem im Rahmen der Programmedes Sonderforschungsbereichs 225 'Oxidische Kristalle für elektro- undmagnetooptische Anwendungen' beschafft wurde, konnten wir effizient nutzen.Ein kohärentes, auf Integration ausgerichtetes Studienprogramm wird durchgeführt,um einerseits einer allzu einseitigen Spezialisierung vorbeugen und den Blick für umfassendeZusammenhänge zu schärfen und um andererseits zu helfen, unnötig langePromotionszeiten zu verringern. Durch Zusammenarbeit mit Gastwissenschaftlern,durch Forschungsaufenthalte an anderen wissenschaftlichen Einrichtungen unddurch die Teilnahme an Fachtagungen ist die Ausbildung der Kollegiat(inn)en vertieftsowie die Mobilität, die Diskussions- und die Präsentationsfähigkeit gefördert worden.Wir gehen davon aus, dass der überwiegende Teil der Kollegiat(inn)en die Promotionin drei Jahren abschließen wird.Der Fachbereich Physik hat eine Graduiertenschule eingerichtet. Dabei fällt demGraduiertenkolleg 'Nichtlinearitäten optischer Materialien' eine wesentliche Rolle zu.Die gewonnenen Erfahrungen sollen auch auf andere Bereiche übertragen werden.Die neue Promotionsordnung (gültig seit dem 26.11.2002) ist bereits stark von denGraduiertenkollegs beeinflusst worden.122