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Programm und Abstracts der Sitzung HL 4

Microscopic theory of carrier-wave Rabi flopping in semiconductors

Q. T. Vu¹, L. Bányai¹, H. Haug¹, O. D. Mücke², •T. Tritschler², M. Wegener², U. Morgner³ und F. X. Kärtner^4
1Institut für Theoretische Physik, Universität Frankfurt, 60054 Frankfurt am Main, Germany
²Institut für Angewandte Physik, Wolfgang-Gaede-Straße 1, Universität Karlsruhe (TH), 76131 Karlsruhe, Germany
³Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
⁴Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, MIT, Cambridge, MA 02139, U.S.A.

Carrier-wave Rabi flopping refers to a Rabi oscillation, the frequency of which becomes comparable to the carrier frequency of light. Recently, we have performed corresponding experiments on 50-100 nm thin GaAs films excited with intense 5 fs pulses. However, the semiconductor problem has only been discussed in terms of two-level systems [1]. Consequently, scepticism has been expressed that this simple although intuitive description is able to properly catch the underlying physics. Here, we will show that the experimental spectra are nicely reproduced by a microscopic theory based on the semiconductor Bloch equations, treating the GaAs band structure via a tight-binding approach and accounting for the wave-vector and carrier-density dependent relaxation and dephasing processes in a realistic manner. This shows that our intuitive interpretation in terms of carrier-wave Rabi flopping is

[correct indeed. 1] O. D. Mücke et al., Phys. Rev. Lett. 89, 127401 (2002)

Dephasing of excitonic and biexcitonic polarization: Intensity and temperature dependence

•Lars Wischmeier, Hans Georg Breunig, Tobias Voss, Ilja Rückmann und Jürgen Gutowski
Institut für Festkörperphysik, Universität Bremen, P.O. Box 330440, D-28334 Bremen

The decay of coherent excitonic and biexcitonic polarization was investigated in a high quality 10 nm ZnSe/ZnS_0,07Se single quantum well as a function of intensity, temperature (4 - 70 K), and polarization of the 110 fs excitation pulses. The spectral position of the pulses was tuned such that the resonances of the 1s heavy-hole exciton and the biexciton were exclusively excited. The dephasing rates were obtained by both four-wave-mixing (FWM) and pulse-transmission measurements. The experimental results were fitted with the temperature dependence of the homogeneous linewidth due to phonon scattering. From the temperature dependence of the decay of the FWM signal of the excitonic and biexcitonic polarizations the acoustic- and optical-phonon-scattering coefficients were determined.

Additionally the contrast ratio obtained from coherent-control measurements with two phase locked pulses was studied as a function of temperature and intensity.

Kohärente und inkohärente Ladungsträgerdynamik in Quantentopf-Infrarot-Photodetektoren

•Thomas Maier¹, Harald Schneider¹, Hui Chun Liu², Martin Walther¹ und Peter Koidl^1
1Fraunhofer-Institut für angewandte Festkörperphysik, Tullastr. 72, D-79108 Freiburg
²Institute for Microstructural Sciences, National Research Council, Ottawa K1A 0R6, Canada

Wir untersuchen Quantentopf-Infrarot-Photodetektoren (QWIPs), die so konzipiert sind, dass drei energetisch äquidistante Niveaus entstehen. Während die ersten beiden Zustände gebunden sind, stellt das dritte Niveau eine Kontinuumsresonanz dar. Da Elektronen, die zum Photostrom beitragen, über eine Zwei-Photon-Absorption in das Kontinuum gelangen, ergibt sich eine quadratische Abhängigkeit des Photostroms von der einfallenden Leistungsdichte. Führt man nun interferometrische Autokorrelationsmessungen zweiter Ordnung an ultrakurzen Pulsen mit einem solchen QWIP als Detektor durch, ist dessen Ansprechverhalten durch die Lebensdauer und die Dephasierungszeit des Zwischenzustands bestimmt. Eine numerische Analyse basierend auf den optischen Bloch-Gleichungen ergibt Lebensdauern im Bereich von 500 fs (750 fs) und Dephasierungszeiten von ca. 100 fs (250 fs) in topf-(modulations-)dotierten Strukturen.

THz emission from GaSb samples with modified surfaces

•Stephan Winnerl, Thomas Dekorsy und Manfred Helm
Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, PF 510119, 01314 Dresden

The optimization of emitters for THz radiation pulses has gained a lot of interest in recent years. They can be based on accelerating charge carriers, which are photoexcited by fs-laser pulses in the surface field of a semiconductor. We studied the THz emission from GaSb samples that were annealed at different temperatures in the range from 300 C to 700 C. Without annealing, no THz emission was observed under fs excitation at 800 nm. This is consistent with the fact, that for GaSb no surface states in the band gap have been found [1]. The THz emission was strongest for the annealing temperature of 450 C with amplitudes comparable to that of an InGaAs emitter. We attribute the THz emission to a surface field caused by decomposition of the surface of the annealed samples. The decomposition is confirmed by measuring the surface stoichiometry using Auger electron spectroscopy. We suggest that the decrease of THz intensity for the higher annealing temperatures is due to a lowering of the carrier mobility. Our experiment demonstrates the possibility to modify materials for THz emission in a very simple way.

[1] P.W. Chey, I.A. Babalado, T. Sukegawa, and W.E. Spicer, Phys. Rev. Lett. 35, 1602 (1975).

Coherent switching of polariton modes and their quantum beats in four-wave mixing and real-time resolved pulse-transmission experiments on ZnSe SQWs

•I. Kudyk, T. Voss, H. G. Breunig, I. Rückmann und J. Gutowski
Institute of Solid-State Physics, University of Bremen, POB 330440, D-28334 Bremen, Germany

For layer thicknesses above 20 nm the optical properties of ZnSe nanostructures are usually dominated by the existence of different polariton modes and their interactions.

Optical coherent control of these polariton modes is investigated by use of a pair of 100 fs long phase-locked pulses in both pulse-transmission experiments with real-time resolution and four-wave-mixing (FWM) experiments. It is shown that for low excitation densities the contributions of the different resonances can be selectively enhanced or diminished. When continuously changing the inter-pulse delay time a succession of constructive and destructive interference is observed separately for each resonance. The coherent control of polariton modes also allows for a directed manipulation of the beat structures on the real-time transients as well as in the FWM signals.

Furthermore, the dephasing time of the polariton modes is analyzed in spectrally resolved, two-pulse degenerate FWM experiments and studied as a function of the excitation density and temperature. By additional measurements of the polarization states of the different spectral components the relative phase of the polariton modes is investigated.

Ladungsträger-Dynamik in Stickstoff-implantiertem GaAs

•S. Sinning, T. Dekorsy und M. Helm
Forschungszentrum Rossendorf, Postfach 51 01 19, 01314 Dresden

III-V-Halbleitern mit geringen Stickstoffkonzentrationen gelten sowohl als Materialsystem als auch für Anwendungen seit einiger Zeit größeres Interesse. Neben epitaktischen Methoden bietet die Ionen-Implantation einen effektiven Weg zur Einbringung des Stickstoffs in das Substrat. Nachteil dieses Verfahrens ist der dem Gitter durch die Ionen-Implantation zugefügte Schaden.

Wir untersuchen die Effektivität des Einbaus von aktivem Stickstoff nach Implantation und thermischer Ausheilung (RTA). Bei RTA-Bedingungen von 650 \mathsurround = 0pt^°C/30s ist der Einbau des Stickstoffs in die Matrix optimal. Eine Verbesserung der Gitterqualität kann durch Implantation bei erhöhten Temperaturen (T > 200 \mathsurround = 0pt^°C) erreicht werden. Die Ladungsträger-Dynamik im sub-Pikosekunden-Bereich für Implantationen bei Raumtemperatur und bei erhöhten Temperaturen wird mit der von nicht-implantiertem GaAs verglichen. Es werden signifikante Unterschiede beobachtet, die auf eine starke Modifizierung der Bandstruktur des stickstoffhaltigen GaAs zurückzuführen sind.

Stimulated bosonic scattering in the exciton-biexciton system of a ZnSe single quantum well

Daniel Hägele, •Stefan Pfalz und Michael Oestreich
Universität Hannover, Institut für Festkörperphysik, Abteilung Nanostrukturen, Appelstraß e 2, 30167 Hannover

Although it has been long known that semiconductor excitons and biexcitons are quasiparticles of bosonic nature, all experiments claiming bosonic effects are discussed controversially. Here, we give direct experimental evidence that the decay of a spin-zero biexciton into an exciton and a photon with opposite spins (±1) is subject to bosonic enhancement caused by the presence of other excitons. In an optically created spin polarized gas of excitons, biexcitons decay preferentially into that final state whose resulting exciton has the same spin orientation as the majority of excitons. We measure in a 10 nm ZnSe quantum well a photoluminescence polarization degree of up to 8 % at the biexciton emission line due to stimulated bosonic scattering. The biexcitonic PL polarization is opposite to that of the excitons which exhibit a polarization degree of -50 %. While the polarization of the exciton line can be fully understood by the usual optical spin selection rules, the biexcitonic polarization can only be explained by the bosonic nature of excitons.

How fast is the insulator-to-metal transition in VO₂?

•T. Dekorsy¹, A. Cavalleri², H.H. Chong², J.C. Kiefer³ und R.W. Schoenlein^2
1Forschungszentrum Rossendorf, 01314 Dresden
²Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
³Universite du Quebec, INRS energie et materiaux, Varennes, Quebec

We investigate the photo-induced insulator-to-metal transition in VO₂, a strongly correlated semiconductor with monoclinic structure. The insulator-to-metal transition (T_c= 341 K) is initiated by photo-doping of holes into the correlated valence band with high-power femtosecond laser pulses. Experiments with time-resolutions ranging from the 100-fs regime down to 10-fs reveal a minimum time constant for the phase transition to be accomplished of approximately 75 fs. This observation gives evidence for a temporal bottleneck of the phase transition since an atomic rearrangement and symmetry increase for the formation of the high-temperature metal phase has to take place. Femtosecond excitation of VO₂ with low power laser pulses reveals the excitation of low-energy coherent phonons which have displacements along the coordinates relevant for the phase transition. Interestingly the observed temporal bottleneck coincides with half the period of these phonons. Our findings demonstrate a new approach for the investigation of the interplay between atomic and electronic structure on ultrashort time scales.

Optically induced realspace-transfer between mesoscopic quantum dots

•Claus Metzner und Dominik Stehr
Technische Physik I, Universität Erlangen Nürnberg, Erwin-Rommel-Str. 1, 91052 Erlangen

We theoretically investigate a physical scenario in which packets of electrons (less than 100) are moving coherently over mesoscopic distances in a two-dimensional potential landscape, created by laterally selective doping of a quantum well. The motion of the charge packets is controlled and monitored in a purely optical way by using ultrashort light (or THz) pulses, acting as time-dependent electric driving forces. In the strong field regime, the interacting few-particle system shows a complex, nonlinear, collective dynamics, partially corrupted by stochastic phonon scattering events. Nevertheless, a controlled transfer between metastable states (corresponding to spatially separated potential minima, or meso-dots) can be achieved with proper light pulses. The pulse parameters can be found automatically by evolutionary optimization.

THz photomixer based on quasi-ballistic transport in AlGaAs nipnip-superlattices

•Frank H. Renner¹, O. Klar¹, S. Malzer¹, M. Eckardt¹, A. Schwanhäußer¹, G. Loata², T. Löffler², H. Roskos², D. Driscoll³, M. Hanson³, A.C. Gossard³ und G.H. Döhler^1
1Insitut für Technische Physik I, Universität Erlangen-Nürnberg , Germany
²Lehrstuhl für Ultrakurzzeitphysik, Universität Frankfurt a.M., Germany
³Materials Departement, UC Santa Barbara, U.S.A.

Conventional THz-photomixers are antenna-structures on a LT-GaAs-layer and are based on the photoconductivity of LT-GaAs, characterized by an extremely short carrier-lifetime . We have developed a novel concept for photomixing based on the quasi-ballisitic transport of electrons in AlGaAs-i-layers. In this concept, the emitter is not limited by the lifetime of the photogenerated carriers and its efficiency proves to be higher than in conventional LT-photomixers.

The emitter consists of a stack of nano-pin-diodes. The lengths and Al-contents of the i-layers in this nipnip-superlattice are optimized for the transport of the carriers. Recombination of the photogenerated carriers takes place inside the recombination-enhanced np-diodes between the nano-pin-diodes. The THz-modulated current is fed into an attached planar antenna, which is either a resonant dipole-antenna or a non-resonant spiral- or log. periodic-antenna.

In this contribution we present the concept of the nipnip-emitter and its realization in the AlGaAs material system, including experimental results on the THz-output and frequency response of the emitter.

Unipolar impact ionization in GaAs/AlGaAs heterostructures

•O. Schmidt¹, M. Eckardt¹, A. Schwanhäusser¹, G.H. Döhler¹, S. Trumm², M. Betz², F. Sotier², M. Hanson³ und A.C. Gossard^3
1Technische Physik I, Universität Erlangen-Nürnberg
²Physik-Department E11, Technische Universität München
³Materials Department, UCSB Santa Barbara, USA

Impact ionization and the resulting avalanche multiplication in pin-diodes represent one of the technically most feasible methods for detecting and amplifying small optical signals. The ratio of the electron and hole ionization coefficient a/b determines the ultimate multiplication and noise performance of Avalanche Photodiodes. This study indicated, that the ratio a/b is much higher in a specially designed heterostructure than in a comparable homogeneous AlGaAs-diode.
Our bandgap engineered AlGaAs pin heterostructure allows for spatially selective carrier injection by a wavelength adapted pump pulse. This makes it possible to measure multiplication factors for unipolar initiated transport and deduce the ionization coefficient for electrons and holes, respectively. From steady state experiments we found that in graded band structures the electron ionization coefficient is similar to that of homogeneous structures, whereas the hole coefficient is much smaller.
Our Monte-Carlo calculations are supporting these findings. To gain more information about differences of electron and hole impact ionization with a resolution of fs in time and nm in space, in addition, we are performing two color fs pump and probe experiments.

Phasenaufgelöste Pulsreflexion an gepumpten ZnSe-Schichten

•Matthias Seemann, Frank Kieseling, Heinrich Stolz, Günter Manzke und Klaus Henneberger
Universität Rostock, Fachbereich Physik, D-18051 Rostock

Wir berichten über Reflexionsexperimente eines schwachen 150 fs Testpulses an einer ZnSe-Epitaxieschicht. Die Reflexion kann durch die komplexe dielektrische Funktion (DF) beschrieben werden. Diese kann im linearen Fall mit Hilfe der spektralen Interferometrie ohne Kenntnis der Phase des Testpulses bestimmt werden. Der Halbleiter wird durch einen nichtkollinearen resonanten Anregungspuls optisch gepumpt. Dieser generiert Ladungsträger (Exzitonengas oder ein Elektron-Loch-Plasma), die die optischen Eigenschaften verändern. In Abhängigkeit von der Verzögerungszeit und der Leistung des Anregungspulses kann das Umklappen eines Pi-Phasensprunges am Schwerlochexziton gegenüber dem ungepumpten Halbleiter beobachtet werden. Das Verhalten widerspiegelt sich in der theoretischen Beschreibung des Systems, die den Einfluß der Vielteilcheneffekte auf die DF im Rahmen der Halbleiter-Bloch-Gleichungen berücksichtigt.

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