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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

Coherent control of ground state excitons in the nonlinear regime within an ensemble of self‐assembled InAs quantum dots

Thomas Moldaschl, Thomas G. Muller, Sebastian Golka, Wolfgang Parz, G. Strasser, K. Unterrainer

Published Dec 19, 2008• 3 citations
Publisher

Abstract

Abstract In this work femtosecond spectral hole burning spectroscopy is used to resonantly excite ground state excitons in an ensemble of self‐assembled InAs/GaAs quantum dots with a strong pump pulse. Two fundamental coherent nonlinear effects are observed with the aid of the intrinsic time‐ and frequency resolution of the setup: The low temperature Rabi oscillation of the two‐level system associated with the excitonic ground state transition and the observation of two‐photon absorption in the surrounding GaAs crystal matrix. The emergence of the latter effect also infers the existence of charged excitons in the nominally undoped QD sample, backed up by the observation of additional spectral holes next to the excitonic transitions. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssc.200880637

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We investigate the dephasing mechanism of optically-induced exciton coherence in an ensemble of InAs self-assembled quantum dots (QDs) using a two-pulse photon-echo (PE) technique. The sample was fabricated on an InP (311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for enhancing the PE signal intensity. We show the measured decay time of PE signal intensity for resonant excitation of ground-state transition is extremely long, which corresponds to ultralong coherence times of excitons in our QDs. We confirmed that non-radiative population relaxation and pure dephasing are considerably smaller than the radiative relaxation at low temperature. Furthermore, we demonstrate the ultrafast coherent control of QD exciton polarization by changing the pulse areas (Rabi oscillations) in the PE experiment. We also propose a new scheme for implementing QD quantum memory by using the PE technique.
Abstract In a single self‐assembled InGaAs quantum dot, the one‐exciton ground‐state transition defines a two‐level system, which appears as an extremely narrow resonance of only a few μeV width. The resonant interaction of this two‐level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two‐level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two level system.
Femtosecond spectral hole burning experiments were performed on InAs/GaAs quantum dots to investigate the influence of high excitation intensities. The observation of additional spectral holes, as well as distinct absorption line broadening are both attributed to two-photon absorption processes. These spectral holes can be unambiguously assigned to degenerate two-photon biexciton creation, whereas the absorption line broadening is a direct result of Coulomb renormalization, initiated by carriers created through two-photon absorption in the surrounding GaAs crystal matrix and the InAs wetting layer.
Abstract We report the excitonic Rabi oscillations in self assembled quantum dots observed by photon echo spectroscopy. The amplitude of the observed Rabi oscillations increases with the pulse area, which is different from the results of previous reports. A numerical calculation of the Rabi oscillations including the local field effect in a quantum dot well reproduces the experimental oscillations, indicating that the local field effect is one of the important origins of the modification of the Rabi oscillations in quantum dots (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)