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Semiconductor Physics Tuesday HL 24.3 Tue 15:45 POT 151 Transient gain spectroscopy in III-V semiconductor structures — •Kapil Kohli 1 , Sebastian Borck 1 , Christoph Lange 1 , Sangam Chatterjee 1 , Kerstin Volz 1 , Wolfgang Stolz 1 , Lutz Geelhaar 2 , Henning Riechert 2 , Klaus Köhler 3 , and Wolfgang Rühle 1 — 1 Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, D-35032 Marburg, Germany — 2 Infineon Technologies AG, Corporate Research Photonics, D-81730 Munich, Germany — 3 Fraunhofer Institute for Applied Solid-State Physics, Tullastraße 72 D-79108 Freiburg, Germany The temporal dynamics of absorption and gain spectra after optical excitation with femtosecond pump pulses are measured on a femto- to picosecond time-scale by white-light probe pulses. The material systems under investigation are GaAs and (GaIn)(NAs). Both, multi quantum well structures and bulk material, are studied. HL 24.4 Tue 16:00 POT 151 Femtosecond formation of collective modes due to meanfield fluctuations — •K. Morawetz 1,2 , B. Schmidt 1 , M. Schreiber 1 , and P. Lipavsky 3 — 1 Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany — 2 Max-Planck-Institute for the Physics of Complex Systems, Nöthnitzer Str. 38, 01187 Dresden, Germany — 3 Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague 2 Starting from a quantum kinetic equation including the mean field and a conserving relaxation-time approximation we derive an analytic formula which describes the time dependence of the dielectric function in a plasma created by a short intense laser pulse. This formula reproduces universal features of the formation of collective modes seen in recent experimental data of femtosecond spectroscopy. The presented formula offers a tremendous simplification for the description of the formation of quasiparticle features in interacting systems. Numerical demanding treatments can now be focused on effects beyond these gross features found here to be describable analytically. [1] K. Morawetz, P. Lipavsk´y, M. Schreiber, Femtosecond formation of collective modes due to meanfield fluctuations, Phys. Rev. B in press, cond-mat/0506443 HL 24.5 Tue 16:15 POT 151 Observation of coherent acoustic phonons in semiconductors via asynchronous optical sampling — •Florian Hudert 1 , Albrecht Bartels 1 , Christof Janke 1 , Thomas Dekorsy 1 , and Klaus Köhler 2 — 1 Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz — 2 Fraunhofer- Institut für Angewandte Festkörperphysik, D-79108 Freiburg We report on the observation of coherent acoustic phonons in bulk semiconductors (GaAs, GaSb) as well as in semiconductor superlattices (GaAs/AlGaAs) via a recently developed femtosecond time resolved pump-probe technique without a mechanical delay line. The experimental setup constitutes of two modelocked femtosecond lasers with a repetition rate of 1 GHz that are coupled at a fixed difference of 11 kHz with one laser providing the pump and the other one providing the probe pulse. This setup allows scanning a measuring window of one nanosecond with 11 kHz and a resolution of about 200 femtoseconds. The measurement of the transient reflectivity reveals coherent longitudinal acoustic phonons over more than a few hundred picoseconds. The frequencies observed agree very well with Brillouin scattering theory. The decay of the coherent amplitude is dominated by propagation of the acoustic modes. HL 24.6 Tue 16:30 POT 151 Efficient terahertz radiation of a large-area photoconductive device. — •André Dreyhaupt 1 , Stephan Winnerl 1 , Thomas Dekorsy 2 , and Manfred Helm 1 — 1 Institute of Ion-Beam Physics and Materials Research, Forschungszentrum Rossendorf, 01314 Dresden, Germany — 2 Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany We present an approach of photoconductive terahertz (THz) generation providing a broad bandwidth and exceptional electric field amplitude. A large-area interdigitated two-electrode structure is applied to a GaAs substrate to offer high electric fields. Photocarriers excited by a Ti:Sapphire oscillator laser with MHz repetition rate are accelerated there, yielding an intense THz output. An appropriate binary mask covers every second electrode interval and carriers are excited in uniform electric field areas only. Hence contrary carrier acceleration and destructive interference is avoided. The maximum THz field amplitude, detected by electro-optic sampling, is 1.5 kV/cm, which is almost one order of magnitude more of what is achieved with other photoconductive oscillator-excited emitters. For an excitation spot diameter of 300 µm, which corresponds to the central wavelength of the THz pulses, the THz generation is most efficient. An average THz power of 145 µW is generated with a NIR-to-THz power-conversion efficiency of 2 × 10 −4 . The THz power can be improved by a sufficient cooling system. The use of LT GaAs instead of semi insulating GaAs can result in larger THz bandwidth. HL 24.7 Tue 16:45 POT 151 Ultrafast Dynamics at the Quantum Hall Ferromagnet — •Bertram Su 1 , Detlef Heitmann 1 , Werner Wegscheider 2 , and Christian Schüller 2 — 1 Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg — 2 Institut für Experimentelle und Angewandte Physik, Universität Regensburg We report ultrafast spectroscopy experiments at the v = 1 quantum Hall state of a high-mobility two-dimensional electron system (2DES) in a GaAs single quantum well. Via a semitransparent metallic gate, the density of the 2DES can be tuned in a broad range. By spectrallyresolved four-wave-mixing (SR-FWM) experiments we have investigated the coherent dynamics of photo-excited electron-hole pairs. We observe a strong maximum of the SR-FWM signal around v = 1 in the rightcircularly polarized spectra, i.e., if we selectively excite into the empty upper spin level. In this situation, the dephasing time T2 is as long as 14 ps. At v < 1 and v > 1, the dephasing time decreases dramatically, where the decreasing is faster for v < 1. We assume that around v = 1 the formation dynamics of spin textures is dominating the ultrafast dynamics. HL 24.8 Tue 17:00 POT 151 Generation of high-frequency transverse polarized monochromatic acoustic phonons by ultrafast optical excitation in gallium arsenide — •D. Lehmann 1 , A.J. Kent 2 , R.N. Kini 2 , N.M. Stanton 2 , Cz. Jasiukiewicz 3 , and M. Henini 2 — 1 Institute of Theoretical Physics, TU Dresden, D-01062 Dresden, Germany — 2 School of Physics and Astronomy, University of Nottingham, NG7 2RD, Nottingham, UK — 3 Department of Physics, University of Technology, ul. W. Pola 2, PL-35-359 Rzeszow, Poland There is interest in developing useable sources of THz monochromatic acoustic phonon beams for applications in phonon spectroscopy, acoustic microscopy, probing of nanostructures etc. In recent years, the generation of high intensity pulses of acoustic phonons by ultrafast optical techniques has been demonstrated, but the work has concentrated mainly on longitudinally polarized phonons [1]. We have generated coherent transverse polarized acoustic phonons by ultrafast optical excitation of GaAs/AlGaAs superlattices grown on lowindex planes of GaAs [2]. The frequency of the generated phonons is determined by the superlattice period. The phonons can leak out of the superlattice and propagate over macroscopic distances as a monochromatic pulse. We show that theoretical estimations and pump probe measurements suggest a Raman scattering process is responsible for the coherent phonon generation. [1] N.M. Stanton, R.N. Kini, A.J. Kent, M.Henini and D. Lehmann, Phys. Rev. B 68, 113302 (2003) [2] R.N. Kini et al., submitted to Appl. Phys. Lett. HL 24.9 Tue 17:15 POT 151 Ultrafast dynamics of the mid-infrared response of carbon nanotubes and graphite — •Christian Frischkorn, Tobias Kampfrath, Luca Perfetti, and Martin Wolf — Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin We report on time-resolved measurements of low-energy excitations in 1- and 2-dimensional solids like carbon nanotubes and thin graphite films. The mid-infrared response of these systems has been obtained from time-resolved THz spectroscopy data in the 10 - 30 THz spectral range. For carbon nanotubes, we find essentially two processes governing an electronic current dynamics [1]. First, strongly bound excitons are the main photoproduct in large-band gap tubes and thus prevent a typical free-carrier response, while in small-gap and metallic tubes carrier localization due to defects is observed as manifested in a substantial dichroism. In these measurements, the reduced polarizability perpendicular to the tube axis is exploited. In the case of graphite, a 2-dimensional semimetal, our results show that strongly coupled optical phonons in the
Semiconductor Physics Tuesday graphite layer dominate the ultrafast energy and transport relaxation dynamics after optical excitation [2]. These phonon modes heat up on a femtosecond time scale and cool down with a time constant of several picoseconds. Moreover, the observed pronounced increase in the Drude relaxation rate significantly originates from these few active lattice vibrations. Our findings for both carbon nanotubes and graphite are of fundamental importance for technological applications in nanoelectronics. - [1] PRL (submitted); [2] PRL 95, 187403 (2005). HL 24.10 Tue 17:30 POT 151 Intersubband relaxation dynamics in narrow InGaAs/AlAsSb and InGaAs/AlAs quantum well structures using pump-probe spectroscopy — •C V-B Tribuzy 1 , S Ohser 1 , J Neuhaus 2 , T Dekorsy 2 , S Winnerl 1 , H Schneider 1 , M Helm 1 , K Biermann 3 , H Künzel 3 , M.P Semtsiv 4 , and W.T Masselink 4 — 1 Inst of Ion Beam Phys and Mat Res, Fz-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany — 2 Dep Phys, Univ Konstanz, 78457 Konstanz, Germany — 3 FI für Nachrichtentechnik-HHI, 10587 Berlin, Germany — 4 Dep Phys, Humboldt-Univ. of Berlin, 12489 Berlin, Germany Intersubband (ISB) transitions in semiconductor quantum wells (QWs) can be employed for various mid-infrared optoelectronic devices. Presently there is strong interest to extend the available wavelength range into the near infrared, by using materials with a large conduction band offset. To achieve such short wavelengths thin QWs are required, where the first excited state inside the QW may lie higher than some state related to indirect valleys. Examples for such material systems are strained InGaAs/AlAs or lattice matched InGaAs/AlAsSb, both grown on InP. HL 25 C/diamond We have studied the ISB relaxation dynamics in multi QWs of both material systems by femtosecond pump-probe measurements. The transient transmission as a function of the pump-probe delay does not show a single-exponential decay, indicating a more complicated relaxation dynamics. This can be caused by transfer of electrons to X- or L- states in the QWs or the barriers. We will show results on samples with different QW thicknesses and compare them to simulations based on rate equations. HL 24.11 Tue 17:45 POT 151 Ultrafast near resonant nonlinear propagation in GaAs - a theoretical model study — •Martin Schaarschmidt, Jan Kloppenburg, and Andreas Knorr — Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany Pulse propagation close but well below the semiconductor excitonic resonance is investigated. In this spectral region the excitonic resonance often dominates over most off-resonant material resonances. We discuss the suitability of different model systems for the description of occuring material nonlinearities ranging from Kerr nonlinearity, excitonic Pauli- Blocking up to semiconductor Bloch equations (SBE) including many particle effects. The differences between the nonlinear Schrödinger equation with Kerr-type nonlinearity usually applied for propagation far from the resonance, the optical Bloch equations known from propagation in atomic gases and the Maxwell semiconductor Bloch equations are studied and compared with recent experiments. Time: Tuesday 18:00–19:00 Room: POT 151 HL 25.1 Tue 18:00 POT 151 Electronic and optical properties of boron-doped nanocrystalline diamond — •Wojciech Gajewski 1 , Jose Garrido 1 , Ken Haenen 2 , Oliver Williams 2 , and Martin Stutzmann 1 — 1 Walter Schottky Institute, - TU München, Am Coulombwall 3, 85748 Garching — 2 Institute for Materials Research, University of Hasselt, Wetenschapspark 1, 3590, Belgium We report the optical and electronic properties of boron-doped nanocrystalline diamond (B-NCD) thin films, grown on quartz substrates by CH4/H2 plasma CVD. Diamond thin films with a thickness below 200 nm and with boron concentractions rangeing from 0 to 5000 ppm have been investigated. Hall effect measurements confirmed the expected ptype conductivity. The conductivity of B-doped NCD samples with low boron concentrationin the gas phase strongly depends on temperature. The carrier concentration in the temperature range from 400K to 700K is thermally activated. At higher boron doping, the conductivity and the carrier concentration are no longer temperature dependent, and the samples exhibit quasimetallic properties. The Hall mobility shows no clear temperature dependence. As expected, the higher the carrier concentration, the lower the mobility. Spectrally resolved photocurrent measurements have revealed a strong dependence of the photosignal on the surface termination. Major differences occur in the energy range 3.5 - 5.4 eV. The influence of grain boundaries and surface states on the electronic transport and optical properties will be discussed. HL 25.2 Tue 18:15 POT 151 Thin films of metallic carbon nanotubes and their optical spectra — •Sabine Blatt 1 , Frank Hennrich 1 , Hilbert v. Löhneysen 2,3 , and Ralph Krupke 1 — 1 Forschungszentrum Karlsruhe, Institut für Nanotechnologie, 76021 Karlsruhe — 2 Physikalisches Institut, Universität Karlsruhe, 76128 Karlsruhe — 3 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe The exceptional electrical and structural properties of single-walled carbon nanotubes (SWNTs) open up a wide range of future applications provided that metallic and semiconducting nanotubes can be separated on a large scale. The separation through dielectrophoresis makes use of the different electric field-induced polarizabilities of the two types leading to a separation in ac electric fields [1, 2]. This method allows for the first time to produce thin films of only metallic SWNTs and to measure their optical absorption spectra [3]. Recent experiments address the dependence of the separation yield on the type and the concentration of the surfactant used to disperse the nanotubes and on the conductivity of the dispersion. For a better understanding of the dielectrophoretic forces finite elements simulations are performed additionally. [1] Krupke,R.; Hennrich, F.; v. Löhneysen, H.; Kappes, M. M. Science 2003, 301, 344-347 [2] Krupke R.; Hennrich F.; Kappes M. M.; v. Löhneysen H. Nano Lett. 2004, 4, 1395-1399 [3] Krupke, R.; Linden, S.; Rapp, M.; Hennrich, F. cond-mat/0508418 HL 25.3 Tue 18:30 POT 151 Chemical functionalization of ultrananocrystalline diamond thin films with aromatic molecules — •S.Q. Lud 1 , M. Dankerl 1 , J. Hernando 1 , M. Steenackers 2 , R. Jordan 2 , P. Bruno 3 , D.M. Gruen 3 , P. Feulner 4 , J.A. Garrido 1 , and M. Stutzmann 1 — 1 Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany — 2 Lehrstuhl für Makromolekulare Stoffe, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany — 3 Materials Science Department, Argonne National Laboratory, Argonne, Illinois 60439, USA — 4 Physics Department E20, Technische Universität München, James-Franck-Strasse, 85748, Garching, Germany We present a novel approach for the surface functionalization of ultrananocrystalline diamond films via grafting of high-reactive 4-nitrobiphenyl-4-diazonium cations. The grafting process is associated with the cleavage of dinitrogen followed by the binding of the remaining radicals to the surface, forming a stable and covalent C-C bond. X-ray photoelectron spectroscopy, ac impedance spectroscopy, atomic force microscopy and cyclic voltammetry have been used to investigate the stucture and surface coverage of the organic overlayer. We have further explored the electrochemically controlled conversion of the surface nitro group to an amino group. The resulting surface offers a reliable surface chemistry for subsequent modification in contrast to the non-reactive nitro group. With this approach, an adressable coupling of biological molecules to an inert and inorganic solid substrate, with considerable importance for biosensing tasks, can be explored. HL 25.4 Tue 18:45 POT 151 Simulations of Novel Nanoporous Carbon Materials — •Johan M. Carlsson and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin Carbon is one of the most versatile elements in nature and Nanoporous carbon (NPC) constitutes yet another class of carbon materials that exhibit unusual properties. NPC has the ability to catalyze dehydrogenation reactions, but the actuating chemical reaction steps are still unclear.
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