Transport of superradiant excitons in GaAs single quantum wells

Abstract

The dynamics of resonantly excited 1s-HH excitons in GaAs single quantum wells is studied by picosecond transient reflection and photon echo. The energy relaxation and transport properties of the excitons are probed, making use of the surface electric field. At low exciton densities and 5 K, transport appears to be absent, and superradiant decay takes the place of free excitons with characteristic times ranging from 5 to 13 ps, depending on the well width. The polarization decay is governed by energy relaxation, and a coherence length is found for these free excitons of ϳ200 nm. Low in the exciton resonance, we measure a much slower radiative decay and smaller coherence lengths, due to localization. Here both radiative decay and pure dephasing contribute to the polarization dephasing rate. At higher exciton densities and temperature, transport is observed, induced by exciton-exciton and exciton-phonon collisions. For free excitons it is demonstrated that the increased polarization dephasing rate is exclusively due to exciton transport, and not to pure dephasing. For localized excitons, pure dephasing processes prevail in T 2 , and thermally activated transport is found. ͓S0163-1829͑97͒05847-5͔

Ambipolar diffusion of photoexcited carriers in bulk GaAs

“…However, most experimental studies have been performed on GaAs quantum-well samples. [8][9][10][11]15,16,[24][25][26][27][28] Since the scattering rates of carriers in quantum-well samples are different from bulk crystals due to the change of the density of states caused by the quantum confinement, the diffusion coefficients in quantum-well samples are different from the bulk values. Furthermore, in quantum-well samples the interface between the quantum well and the barrier causes additional scattering mechanisms.…”

Dynamics and control of nonradiative excitons - free carriers mixture in GaAs/AlGaAs quantum wells

“…The model described in the previous section predicts also the time dependence of the PL signal. Indeed, the PL profile is described by the last term in the last equation of systems (4,6,7),…”

“…The radiative excitons with small wave vectors that live within the light cone, K X < K c , are characterized by a very short radiative lifetimes of the order of 10 ps [2][3][4][5][6] . Therefore the areal density of the nonradiative excitons can exceed that of the radiative excitons by orders of magnitude in the steady-state experiments.…”

Temporal evolution of the excitonic distribution function inGaAs/Al0.33Ga0.67Shtrichman1, Ron2, Gershoni3 et al. 2002Phys. Rev. B

“…Therefore, our photon probe is not subjected to the exclusion principle as in the case of interband pump-probe experiments. [12][13][14][15] In addition, due to the difference in the two subband dispersions, the energy of the photon provides a sensitive probe to the momentum state of the electron whose optical transition is being induced. 16 We study three SL samples grown by molecular beam epitaxy on a ͑100͒-oriented GaAs substrate.…”

“…3,[9][10][11][12][13] We circumvent this obstacle by using instead an intersubband probe, which induces an optical transition between the first (E1, occupied͒ and second (E2, empty͒ conduction subbands. Therefore, our photon probe is not subjected to the exclusion principle as in the case of interband pump-probe experiments.…”