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Plenarvorträge - DPG-Tagungen

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Halbleiterphysik Montag<br />

onal to the first one. On Si, slanted pores can be fabricated by using<br />

custom cut wafers[4]. These experiments are transferred to InP to investigate<br />

the orientation dependence of pore growth, in order to make a first<br />

step towards the still elusive large scale full 3D PBG.<br />

[1] A. Birner, R.B. Wehrspohn, U. Gösele,and K. Busch, Adv. Mater.<br />

13, 377 (2001).<br />

[2] S. Langa, I.M. Tiginyanu, M. Christophersen, J. Carstensen, and<br />

H.Föll, Appl- Phys. Lett., 82 (2003) 278.<br />

[3] A. Chelnokov, K. Wang, S. Rowson, P. Garoche, and J.-M. Lourtioz,<br />

Appl. Phys. Lett. 77, 2943 (2000).<br />

[4] CHRISTOPHERSEN, M., CARSTENSEN, J., RÖNNEBECK, S.,<br />

JÄGER, C., JÄGER, W., FÖLL, H., J. Electrochem. Soc. 148 (6), E267<br />

(2001).<br />

HL 12.35 Mo 16:30 Poster A<br />

Gas Sensors Based on 2D Photonic Crystals — •Torsten M.<br />

Geppert 1 , Stefan L. Schweizer 2 , Ralf. B. Wehrspohn 2 , and<br />

Armin Lambrecht 3 — 1 Max-Planck-Institut für Mikrostrukturphysik,<br />

Weinberg 2, 06120 Halle, Germany — 2 Universität Paderborn, Dept.<br />

Physik, Warburger Str. 100, 33098 Paderborn, Germany — 3 Fraunhofer-<br />

Institut für Physikalische Messtechnik, Heidenhofstr. 8, 79110 Freiburg,<br />

Germany<br />

We investigate a new kind of gas sensor devices utilizing 2D photonic<br />

crystals (PhCs) made up of straight air pores electrochemically etched<br />

into Si wafers. Based on the reduction of the group velocity of light<br />

in a PhC the interaction time between gas and light should be drastically<br />

increased allowing to minimize the size of the gas sensing device as<br />

compared to currently used gas sensor systems. To achieve this miniaturization,<br />

three requirements have to be met by the PhC. First, the<br />

photonic bandstructure should have only a very small slope for the frequency<br />

of interest to achieve a low group velocity. Second, the electric<br />

field has to be concentrated in the pores to allow interaction between<br />

the light and the gas. Third, the symmetry of the field distribution in<br />

the PhC has to allow the coupling of the incoming light to the photonic<br />

bandstructure. Unfortunately, due to the low group velocity, the reflection<br />

at the interface air-photonic crystal is very high resulting in a low<br />

coupling efficiency. This shortcoming could be circumvented by the use<br />

of appropriate tapers. In our study we investigate several geometries of<br />

the photonic crystal lattice and taper concepts to allow the use of 2D<br />

PhCs as gas sensor devices.<br />

HL 12.36 Mo 16:30 Poster A<br />

Controlling spontaneous emission from colloidal II-VI<br />

nanocrystals in microcavities — •Robert Kraus 1 , Dmitry<br />

Talapin 2 , Andrey L. Rogach 1 , John M. Lupton 1 , Jochen<br />

Feldmann 1 , and Horst Weller 2 — 1 Lehrstuhl für Photonik und<br />

Optoelektronik, Ludwig-Maximilians-Universität München — 2 Institut<br />

für Physikalische Chemie, Universität Hamburg<br />

Colloidal II-VI nanocrystals are an interesting class of materials for optoelectronic<br />

devices. The solution-based synthesis of these nanoparticles<br />

typically results in a certain degree of polydispersity, which in turn gives<br />

rise to substantial spectral broadening of the ensemble. We demonstrate<br />

here that we are able to deposit composite nanocrystal/polymer films of<br />

excellent optical quality, which can be used as the active layer in microcavities.<br />

These provide well-defined optical resonances, to which only a<br />

spectrally narrow ensemble of the nanocrystals couple, thereby resulting<br />

in an enhancement in spontaneous emission. We present data on time<br />

resolved fluorescence from these emitters embedded in microcavities and<br />

discuss the possibility of using the cavity induced field-enhancement to<br />

control energy transfer processes.<br />

HL 12.37 Mo 16:30 Poster A<br />

Lasertätigkeit in zweidimensionalen photonischen Kristallen —<br />

•Karen Forberich 1 , Andreas Gombert 2 , Marc Stroisch 3 , Uli<br />

Lemmer 3 , Marcus Diem 4 , Suresh Pereira 4 und Kurt Busch 4,5<br />

— 1 Freiburger Materialforschungszentrum, Universität Freiburg —<br />

2 Fraunhofer Institut für Solare Energiesysteme ISE, Freiburg —<br />

3 Lichttechnisches Institut, Universität Karlsruhe — 4 Institut für<br />

Theorie der Kondensierten Materie, Universität Karlsruhe — 5 School<br />

of Optics/CREOL & Department of Physics, University of Central<br />

Florida, Orlando, USA<br />

Organische Photonische Kristall-Laser lassen sich durch das Aufbringen<br />

eines organischen Farbstoffs auf ein mikrostrukturiertes Substrat herstellen.<br />

Die so erzeugten Strukturen werden mit einer semiklassischen<br />

Theorie der Lasertätigkeit in photonischen Kristallen beschrieben [1]. Im<br />

Rahmen dieser Theorie lassen sich effektive Parameter herleiten, mit denen<br />

die Lasereigenschaften charakterisiert werden können.<br />

Die Berechnung der Verstärkung erfolgt durch zweidimensionale<br />

Bandstrukturrechnungen, wobei die tatsächliche Oberflächenstruktur<br />

durch einen planaren Wellenleiter mit ortsabhängiger Dicke angenähert<br />

wird. Die Verluste werden durch Reflexionsrechnungen mit Hilfe<br />

einer Streu-Matrix-Methode bestimmt. Durch den Vergleich von<br />

Emissionsmessungen mit diesen Bandstruktur- und Verlustrechnungen<br />

können die tatsächlichen Lasermoden bestimmt werden.<br />

[1] L. Florescu, K. Busch, and S. John, J. Opt. Soc. Am. B 19, 2215–2223<br />

(2002)<br />

HL 12.38 Mo 16:30 Poster A<br />

Structure of Spin-Flip Excitations in Interacting Electron Systems<br />

at High Magnetic Fields — •U. Schmitt and G. Meissner<br />

— Theoretische Physik, Universität des Saarlandes, Postfach 15 11 50,<br />

D-66041 Saarbrücken<br />

Similarities found in the structure of certain inter-Landau level and<br />

intra-Landau level collective excitations of systems exhibiting the integer<br />

and the fractional quantum Hall effect, respectively, are examined. Therefore,<br />

we compare results obtained, e.g., for the dispersion relation of the<br />

spin-flip mode between the filled lowest Landau level and the next and<br />

for the collective excitations of Laughlin’s incompressible quantum liquid<br />

at the corresponding filling factor one third according to the composite<br />

Fermion picture. Modifications due to finite thickness effects in these interacting<br />

two-dimensional electron systems at high magnetic fields are<br />

discussed in view of a comparison of the calculated dispersion relations<br />

with inelastic light-scattering experiments.<br />

HL 12.39 Mo 16:30 Poster A<br />

Strukturierungsmethoden überwachsener Heterostruktur-<br />

Spaltkanten — •Markus Lermer, Dieter Weiss und Werner<br />

Wegscheider — Institut für Experimentelle und Angewandte Physik,<br />

Universität Regensburg, 93040 Regensburg<br />

Wir präsentieren neue Methoden zur Strukturierung der mittels MBE<br />

überwachsenen Spaltfläche von AlxGa(1−x)As/GaAs Heterostrukturen<br />

und deren elektrische Charakterisierung im Magnetotransportexperiment.<br />

Eine spezielle Sandwichtechnik erlaubt die Definition von Mesastrukturen<br />

bis an den Rand der weniger als 100µm breiten Spaltfläche.<br />

Kleinere Strukturen im 10nm Bereich werden elektronenstrahllithographisch<br />

in an der Oberfläche adsorbierte, dünne selbstorganisierte Molekülmonolagen<br />

geschrieben und anschließend nasschemisch übertragen.<br />

Mit letztgenannter Methode periodisch dichtemodulierte 2-dimensionale<br />

Elektronensysteme zeigen deutliche Kommensurabilitätseffekte in ihrem<br />

Magnetotransportverhalten. Im Zusammenhang mit den Transportuntersuchungen<br />

an cleaved-edge overgrowth Strukturen wurde auch der Transport<br />

durch das darunterliegende Übergitter kleinster Periode im äußeren<br />

Magnetfeld untersucht und eine Spannungspositionsabhängigkeit der Bereiche<br />

negativer differentieller Leitfähigkeit festgestellt.<br />

HL 12.40 Mo 16:30 Poster A<br />

Density of States Tails in Random Magnetic Fields —<br />

•Riccardo Mazzarello, Stefan Kettemann, and Bernhard<br />

Kramer — I Institut fuer Theoretische Physik, Universitaet Hamburg,<br />

20355 Hamburg, Germany<br />

Recently, the problem of a quantum particle moving in a random magnetic<br />

field (RMF) has been met in a number of contexts in the physics<br />

of 2D-systems. For instance, in the Composite Fermion (CF) theory of<br />

the Fractional Quantum Hall Effect, electrons are transformed into particles<br />

with statistical flux attached to them: CFs experience a fictitious,<br />

statistical magnetic field proportional to their density in addition to the<br />

external one. The presence of a random, scalar potential induces fluctuations<br />

in the CFs density which give rise to a static RMF. We studied<br />

the tails of the density of states (DOS) of fermions subject to a constant<br />

magnetic field B plus a weak RMF with zero mean in the framework of<br />

the Optimum Fluctuation Method. We show that, near the centres of<br />

the broadened Landau levels, the DOS is a Gaussian, whereas the energy<br />

dependence of the DOS is non-analytic near the band edge (E=0). The<br />

B-dependence of the DOS in both regions is also estimated.

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