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Programm und Abstracts der Sitzung HL 50

Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot

•Oliver Gywat¹, Hans-Andreas Engel¹, Daniel Loss¹, R.J. Epstein², F. Mendoza² und D.D. Awschalom^2
1Department of Physics and Astronomy, University of Basel, Switzerland
²Center for Spintronics and Quantum Computation, University of California, Santa Barbara, USA

We propose a method based on optically detected magnetic resonance (ODMR) to measure the decoherence time T₂ of a single electron spin in a semiconductor quantum dot. The electron spin resonance (ESR) of a single excess electron on a quantum dot is probed by circularly polarized laser excitation. Due to Pauli blocking, optical excitation is only possible for one of the electron spin states. The photoluminescence is modulated due to the ESR which enables the measurement of electron spin decoherence. We study different possible schemes for such an ODMR setup. (cond-mat/0307669)

Optical properties of localized excitons in InGaN quantum structures

•Til Bartel¹, Matthias Dworzak¹, Martin Strassburg², Axel Hoffmann¹, Andre Strittmatter¹ und Dieter Bimberg^1
1Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin
²Georgia State University, Dep. of Physics and Astronomy, Atlanta, GA-30303

Although InGaN structures already find application in optoelectronics, its optical transitions are still poorly understood and are subject to current investigation. Indium fluctuations in InGaN quantum-wells are centers of localization for excitons and show quantum dot-like behavior at low temperatures.

Wurzite InGaN/GaN single quantum wells were grown by metal organic chemical vapor deposition on Si(111) and their luminescence was investigated. High resolution m-photoluminescence (m-PL) spectra of masked samples showed lines as narrow as the resolution limit. This is interpreted as an indication for d-shaped density of states. Excitation density dependence allows the identification of excitons and biexcitones with positive and negative binding energies. Time resolved PL experiments show decay rates of less than 1 ns and investigation of temperature dependence of the photoluminescence reveal typical S-shape behavior. These results demonstrate the quantum dot-like character of the indium fluctuations.

Selective optical charging of self-assembled InGaAs quantum dots

•Miro Kroutvar, Yann Ducommun, Jon J Finley, Max Bichler und Gerhard Abstreiter
Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany

Charge and spin excitations in individual quantum dots (QDs) have been proposed as QBITS for implementation of quantum logic. One of the main challenges for these applications is the selective creation of electrons in such systems and control of their spin degree of freedom. We present a QD charging device which enables to optically generate single charges and potentially spins in sub-ensembles of self-assembled InGaAs QDs. The device consists of a single layer of InGaAs QDs embedded within the intrinsic region of a GaAs Schottky photodiode. The charge storage mechanism relies on selective exciton ionization following resonant optical generation. Our results demonstrate unambiguously selective charging with extremely long (>25 ms) charge storage lifetimes at low temperatures (10K). Analysis of the energy and temperature dependence of the charge storage signal permits investigation of the role of exciton-phonon coupling during the QD resonant absorption process. In addition, thermally-activated redistribution of resonantly stored charge among the QD ensemble is identified to be the cause of a time-dependent loss of spectral selectivity at elevated temperatures. Our storage device enables to probe directly resonant carrier excitation processes and potentially electron spin dynamics in micro-ensembles of semiconductor QDs.

TE- and TM-polarization resolved spectroscopy under normal incidence

•M. Schardt¹, C. Dotzler¹, S. Malzer¹, J. Müller², G. Rurimo², S. Quabis², G. Leuchs² und G.H. Döhler^1
1Institut für Technische Physik I
²Institut für Optik, Universität Erlangen-Nürnberg, Germany

Normally, optical experiments performed under normal incidence on semiconductor structures with broken symmetry regarding the direction perpendicular to the surface (quantum wells and quantum dots, e.g.) yield information only about transitions involving in-plane (p_x- and p_y-) components of the hole wave functions, because of the in-plane (TE) polarization of the light. Transitions sensitive to the p_z components are interacting only with TM-polarized light. Recently it has been demonstrated that a radially polarized laser beam focused through a microscope objective with a high numerical aperture (NA ³ 0.9) is perfectly polarized along the optical axis in its focus. The light is mostly TM polarized within the whole focus area. So far, experimental evidence of this feature has been limited to an indirect proof by comparing the resulting spot size and shape with the theoretical predictions [1]. We are now presenting an approach allowing for a direct proof of the TM polarization. It is based on photo current studies of heavy- and light-hole excitonic absorption in quantum wells and self-assembled dots embedded in pin diodes. At the same time, this approach represents a novel technique for polarization resolved spectroscopy.

[1] S. Quabis et al., Optics Communications 179 (2000) 1-7

Influence of nitrogen containing barrier layers on the emission wavelength of InAs/GaAs quantum dots

•Oliver Schumann, Lutz Geelhaar und Henning Riechert
Infineon Technologies AG, Corporate Research Photonics, 81730 München

We study the effect of varying the matrix material on the emission wavelength of InAs QDs. The addition of different combinations of nitrogen and indium into the matrix allows to tune the confining potential of the QDs and the local and global stress.

Samples have been grown by solid-source MBE assisted by a RF plasma source for nitrogen incorporation. They were examined by PL spectroscopy and TEM. We used a self-consistent Schroedinger-Poisson solver as a simple simulation tool for the changing confinement.

While growing the QDs on a GaAsN layer almost does not affect the PL wavelength, one can see a red shift of the emission wavelength greater than 100 nm after introducing nitrogen in the capping layer above the QDs. In this way an emission wavelength beyond 1.3 mm can easily be achieved. This tremendous red shift is mainly attributed to the change in the confining potential. By comparing QDs with quantum wells and samples with different concentrations of indium and/or nitrogen in the barrier layers, the role of stress with respect to the emission wavelength will also be discussed.

Blinking and Bleaching of Individual Silicon Nanocrystals and Nanocrystal Ensembles

•Jörg Martin^1,2, Frank Cichos¹ und Christian von Borczyskowski^2
1Institut für Physik 123705, TU Chemnitz, 09107 Chemnitz
²Institut für Physik 122501, TU Chemnitz, 09107 Chemnitz

We present for the first time detailed studies of individual silicon nanocrystals by confocal microscopy. Individual nanocrystals obey narrow emission spectra (150 meV) in the range between 500 nm and 600 nm. The emission of single nanocrystals shows a strong intermittency (so called blinking), which is the consequence of an electron tunneling to surrounding trap states. The blinking process itself obeys a power law statistics, which is equivalent to a non-stationary behavior. Indeed we can show, that characteristic times of the blinking, especially the mean lifetime of dark periods during the blinking are a function of the observation time. This non-stationarity shows up as a bleaching of the ensemble, which we can fully explain by the blinking statistics. In addition, an excitation intensity dependence of the blinking statistics is observed. This intensity dependence is the result of different tunneling mechanisms, such as Auger assisted tunneling, which becomes active or inactive at different excitation intensities. A further consequence of the charge tunneling to trap states in the environment is a delayed luminescence of nanocrystal ensembles, which results from the return of charges from the traps to the nanocrystals conduction band. This delayed luminescence fits well to the observed blinking behavior of individual nanocrystals.

How individual are individual CdSe nanocrystals? A single nanocrystal study.

•Frank Cichos¹, Abey Issac², Thomas Blaudeck¹ und Christian von Borczyskowski^2
1Institut für Physik 123705, TU Chemnitz, 09107 Chemnitz
²Institut für Physik 121501, TU Chemnitz, 09107 Chemnitz

The control of the emission intermittency (blinking) of single semiconductor nanocrystals is one of the challenging tasks to create nano-units i.e. for optoelectronic applications. Such a control has to be based on the understanding of the blinking process itself, which is currently only poorly understood. For this purpose we study the blinking process of individual ZnS capped CdSe nanocrystals over long time periods to compare the behavior of different nanocrystals. While fluorescent dye molecules commonly show a very specific blinking statistics, which differs from molecule to molecule due to the local environment of the molecule, all studied nanocrystals obey the same blinking statistics within the measurement accuracy. Further, a change of the surrounding matrix does not influence the statistics of the blinking process. Therefore, the processes behind the blinking have to be hierarchical processes. We propose that one of these processes, which is responsible for the dark periods in the emission time traces of individual nanocrystals is a charge diffusion through trap states in the direct vicinity of the nanocrystal. A more detailed analysis of the blinking further reveals that the processes which limit the emitting (on) and non-emitting (off) periods of the nanocrystal are partly decoupled, since on and off periods follow different statistical laws.

Femtosecond nonlinear spectroscopy of two quantum dots coupled by dipole-dipole interaction

•Kerstin Müller¹, Thomas Unold¹, Christoph Lienau¹, Thomas Elsaesser¹ und Andreas D. Wieck^2
1Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin
²Ruhr-Universität Bochum, D-44870 Bochum

The transient optical nonlinearities of single semiconductor quantum dots (QDs) are intensely investigated as QDs receive attention for implementations of quantum logic. For realizing potentially scalable quantum gates it is essential to demonstrate coherent interactions between different quantum dots, e.g. via dipole-dipole coupling[1]. Here, we report the first study of the transient optical nonlinearities of two coupled interface quantum dots. With a 2-ps pump laser, we excite the excitonic resonance of a first QD (QD 1) and monitor the pump-induced biexcitonic nonlinearity of this QD with an ultrafast probe laser in a near-field spectrometer[2]. Varying the intensity of the pump laser, pronounced Rabi oscillations are observed. We simultaneously monitor also the excitonic nonlinearities of one neighboring QD within the 200 nm spot size of the near-field experiment. We find a clear pump-induced spectral shift of the exciton resonance of QD 2 due to the dipole-dipole interaction between both QDs. The observation of Rabi oscillations on the second quantum dot after resonant excitation of QD 1 unambiguously establishes dipole-dipole coupling

[between two individual quantum dots. 1] E. Biolatti et al.,

[Phys. Rev. Lett. 85, 5647 (2000). 2] T. Guenther et al., Phys. Rev. Lett. 89 057401 (2002).

Optische Spektroskopie an selbstorganisierten InAs-Quantenpunkten

•Stephan Lüttjohann¹, Cedrik Meier¹, Axel Lorke¹ und Dirk Reuter^2
1Laboratorium für Festkörperphysik, Universität Duisburg-Essen, Lotharstraße 1, 47048 Duisburg
²Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum

Die Photolumineszenz (PL) von selbstorganisierten InAs-Quantenpunkten, eingebettet in eine Feldeffekttransistor-Struktur (MISFET), wird in Abhängigkeit der angelegten Spannung und der Anregungsleistung untersucht. Bei moderaten Anregungsleistungen (P £ 2mW/cm²) können dabei gleichzeitig Kapazitäts-Spannungs-Spektren (CV) aufgenommen und so die Anzahl der Elektronen pro Quantenpunkt bestimmt werden. Es zeigt sich, dass mit zunehmender Anregungsleistung die CV-Spektren zu kleineren Gatespannungen verschieben. Dies lässt sich auf eine Löcherakkumulation an der GaAs/AlGaAs-Grenzfläche der MISFET-Struktur zurückführen. Die bei mittlerer Anregungsleistung auftauchende Rekombination aus p-Zuständen ist damit nicht auf Pauli-Blocking zurückzuführen, sondern hängt mit der Verschiebung der Gatespannungsskala zusammen, die durch die zunehmende Grenzflächenladung verursacht wird. Darüber hinaus kann aus der gatespannungsabhängigen PL die Energiedifferenz zwischen X^2-- und X^3--Rekombination zu 2,6 meV bestimmt werden.

Wavelength selective carrier storage in self-organized quantum dots

•Till Warming¹, F. Guffarth¹, R. Heitz¹, M. Geller¹, P. Brunkov², V.M. Ustinov² und D. Bimberg^1
2A.F.Ioffe Physico-Technical Institute RAS, 194021 St-Petersburg, Russia
¹Institut für Festkörperphysik, Technische Universität Berlin,

Wavelength selective carrier storage in self-organized InAs/GaAs quantum dots (QDs) is investigated. The small homogeneous broadening of the exciton transition of a single quantum dot is in high contrast to the large inhomogeneous broadening of the ground state transition energy of an ensemble of self-organized QDs. This benefit enables in addition to the spatially addressing wavelength parallel data storage in future memory devices. Here we analyze the data storage process in two-color photocurrent experiments and investigate the dependencies of the electric field, the temperature and the laser power. This work was funded by the Nanomat project of the European Commission Growth Programme, contract number G5RD-CT-2001- 00545, Intas project 2001-774, and SFB 296 of DFG.

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