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Journal of the Physical Society of Japan

Nonlinear Luminescence Spectroscopy of Excitons in GaAs-AlAs Quantum Wells

Satoshi Ideshita, Yasuaki Masumoto

Published Jan 15, 1990• 6 citations
Publisher

Abstract

We present the detailed experimental results of the time-resolved nonlinear luminescence of excitons in undoped GaAs-AlAs quantum wells. The observed nonlinearity results from the saturation in luminescence originated from the phase space filling and exchange effects of carriers. In some temporal profiles of nonlinear luminescence, anomalous anti-correlation profile has been observed. The origin of anti-correlation is ascribed to be the finite rise time of the exciton population at the luminescent tail state in the exciton band.

DOI
10.1143/jpsj.59.331

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We present the detailed experimental results of the time-resolved nonlinear luminescence of excitons in undoped GaAs-AlAs quantum wells. The observed nonlinearity results from the saturation in luminescence originated from the phase space filling and exchange effects of carriers. In some temporal profiles of nonlinear luminescence, anomalous anti-correlation profile has been observed. The origin of anti-correlation is ascribed to be the finite rise time of the exciton population at the luminescent tail state in the exciton band.
Abstract : In this paper we describe a detailed study of the complete time evolution of the optical nonlinearities of type-11 GaAs/AlAs quantum wells using nanosecond and femtosecond pump/probe spectroscopy. We explain these nonlinearities by solving the generalized Wannier equation for the case of a type-II quantum-well structure. In contrast to the usual type-I quantum-wells, we observe different shifts and saturations of light- and heavy-hole excitons as well as the absence of gain. The dominant effects are a blue shift and saturation of the heavy-hole (HH) exciton.
The saturation behavior of the nonlinear index δnNL in the band tail of GaAs/AlGaAs multiple quantum wells is experimentally studied in quasi-steady state and in a low absorption region (below 300 cm−1), where dispersive optical bistability is observed. Measurements at different detunings from the exciton resonance allow us to study the absorption dependence of the saturation parameters. The saturation intensity IS is found inversely proportional to the linear absorption coefficient, while the saturating nonlinear index δnS is approximately constant. This evidences that the nonlinear index saturation is due to a saturation of the carrier density versus light intensity, due to the saturation of the band tail absorption itself.
Optical nonlinearities of Al(x)Ga(1-x)As/GaAs asymmetric coupled quantum wells within a p-i-n structure have been observed by using time-resolved spectroscopy. Under photoexcitation, exciton absorption lines exhibit spectral shifts of as much as approximately 1 meV. The recovery times of these spectral shifts are of the order of hundreds of picoseconds when the excitation photon is above the lowest exciton state but become less than 10 psec when excitation is below the lowest exciton state, indicating a virtual process. The behavior of these spectral shifts is consistent with the presence of a polarization induced by optical pumping. The polarization is believed to be due to the excitons that have nonvanishing electric dipole moments along the axis of the asymmetric coupled quantum wells.
I report on the effect of a moderate excess electron population on nonlinearities in modulation-doped CdTe quantum wells. I show that the electron population does not qualitatively affect the nature of correlations between excitons. In this respect, I bring strong evidence of the existence of unbound and bound (stable) two-exciton states in the presence of electrons and charged excitons (trions). In time-resolved pump and probe experiment, they lead to the observation of electromagnetically induced transparency and optical Stark shift of the exciton resonance. Rabi flopping of excitons within a sea of electrons is also clearly evidenced through ac Stark splitting and gain without inversion. The quantum coherence is more robust to electron induced dephasing than what would have been expected. I demonstrate third and higher-order exciton correlations in the presence of electrons, which manifest, for increasing exciton densities, through the blue-shift of counter-polarized exciton resonance, the red-shift of the biexciton resonance and the exciton to biexciton crossover. I also evidence correlated behavior of excitons and trions under excitation which manifests itself by crossed trion-exciton effects. I observe a wealth of phenomena encompassing bleaching, crossed bleaching, induced-absorption and energy shifts of the resonances. Significant differences are found between the nonlinear optical effects induced by an exciton and a trion population. Electron scattering with electron and exciton is shown to strongly broaden the high energy tail of exciton and trion resonance lineshapes. Variations of electron density result in a clear modification of both resonance lineshapes. The dynamics of the formation of trions is also studied. I propose a two-channel mechanism for their formation; they are formed through bi- and tri-molecular processes. This implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is applied to a time-resolved photoluminescence experiment performed on a very high quality InGaAs quantum well sample, in which the photoluminescence contributions at the energy of the trion, exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor is explained.
The pulse distortion in GaAs quantum wells was studied by means of a light-gating technique. In the experiment, we used the incoherent picosecond pulse to observe accurately the changes of the coherent spike and its surrounding envelope. The experimental results clearly showed that the pulse is not anomalously delayed but is strongly distorted around the exciton resonance. We also compared the results with a computational simulation based on the absorption saturation mechanism. The simulated results agreed well with experimental results. The simulation showed that the recovery time of the exciton saturation is 90 ps around the tail part of the exciton absorption. We also observed a reduction of the coherent spike around the exciton resonance. Theoretical calculations indicate that the coherent-propagation effect in the linear absorber causes the reduction of the coherent spike.