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AIP conference proceedings

Superradiance of excitons in multi-quantum-well structures based on InAs-GaSb coupled quantum wells

Boris Laikhtman

Published Jan 1, 2005• 2 citations
Publisher

Abstract

We suggest the system of excitons in multiple InAs‐GaSb coupled quantum well as a new source of superradiance. Coupled InAs‐GaSb quantum well can be tuned to a very large wavelength that makes possible to fabricate a sample with a large number of quantum wells and in this way to increase the superradiant effect.
DOI
10.1063/1.1994438

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We suggest a system of ground state excitons in multiple InAs-GaSb coupled quantum wells as a new source of superradiance. Coupled InAs-GaSb quantum wells can be tuned to a very large wavelength that makes it possible to fabricate a sample with a large number of quantum wells radiating in phase, and in this way to increase the superradiant effect. Depending on the parameters of the system, this superradiance may be a direct analogy of the known Dicke case, or a realization of new type NNN-level superradiance. In the last case, the time dependence of the luminescence presents NNN decaying peaks. This dependence may be considered as a manifestation of transition to the Bose condensate phase.
Rapid superradiant decay of coherently excited excitons in thin quantum wells (QWs) on a time scale of approximately 10 ps has been theoretically predicted by several groups [1], however, the experimental observation of this ultrafast radiative recombination is still the subject of controversial discussions. A recent theoretical paper [2] concludes, that this radiative decay rate should be strongly enhanced in multiple QW (MQW) Bragg structures due to radiative coupling of excitons excited in different wells via the exchange of transverse photons.
Time-integrated four-wave mixing experiments reveal strong influence of the cladding layer thickness d1, its refractive index and the reflectivity of the sample–air interface on the radiative dephasing time Trad of excitons in single quantum wells and multiple quantum well structures. A cladding layer thickness of λ/2 and λ/4 decreases and increases, respectively, Trad. Furthermore, we show that the superradiance is not limited to the heavy-hole exciton but can be observed even on excited and light-hole–exciton states as well as on correlated electron–hole-pair states. All findings agree fairly well with solutions of the semiconductor Maxwell-Bloch equations.
The theoretically predicted rapid superradiant decay of coherent 2D excitons on a time scale of approximately 10 ps [1] is expected to be strongly enhanced in multiple QW (MQW) Bragg structures consisting of N identical QWs separated by (Ga,Al)As barriers the thickness of which was adjusted so that the QWs had a distance d equal to one half of the heavy-hole exciton resonance wavelength λ in the medium [2]. This enhancement originates from efficient radiative coupling of excitons excited in different wells via the exchange of transverse photons. Recently we have published [3] degenerate-four-wave-mixing experiments with fs optical pulses on GaAs QW Bragg structures which demonstrate that the radiative lifetime in high-quality GaAs single QWs (SQW) is approximately 5 ps and decreases to less than 2 ps in Bragg samples with N=10. These studies revealed that radiative recombination provides the dominant contribution to the homogenous linewidth of the 2D exciton in high-quality samples and exceeds nonradiative contributions caused by scattering with sample imperfections in contrast to previous assumptions.
It is shown that the transition process to BEC phase of excitons can be manifested as non‐monotonic kinetics of photoluminescence decay. This decay looks as a series of subsequent peaks resulting from superradiance phenomenon. We show that decay of PL of certain InAs‐GaSb coupled quantum wells having, for instance, in average 5 ground state excitons per island may demonstrate up to 5 consecutive maximums followed by exponential decay. This dependence may be considered as a manifestation of transition to the Bose condensate phase. The radiation kinetics of such a system is a generalization of the known two‐level Dicke superradiance to an ensemble of multilevel systems.