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EXCITONIC STIMULATED RECOMBINATION IN LOW TEMPERATURE GaAs/AlGaAs MULTIPLE QUANTUM WELLS

J. L. Oudar, Juan Ariel Levenson

Published Jul 18, 1988• 0 citations
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Abstract

Recent studies of the transient behavior of absorption saturation in low temperature Multiple Quantum Wells (MQW) samples have revealed a surprisingly fast recovery of absorption (within 10 ps at 15 K) after excitation by intense ultrashort light pulses1. This has been attributed to the effect of the large stimulated recombination rate caused by amplified spontaneous luminescence guided along the MQW structure, as confirmed by streak camera observations of the edge-emitted luminescence1, and by measurements of time-resolved luminescence spectra with subpicosecond resolution2. This effect could be used to increase the speed of optical gates based on absorption saturation or carrier-induced nonlinear refraction, but a more complete understanding of the mechanisms involved is clearly needed in order to evaluate the interest of this approach.

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Recent studies of the transient behavior of absorption saturation in low temperature Multiple Quantum Wells (MQW) samples have revealed a surprisingly fast recovery of absorption (within 10 ps at 15 K) after excitation by intense ultrashort light pulses1. This has been attributed to the effect of the large stimulated recombination rate caused by amplified spontaneous luminescence guided along the MQW structure, as confirmed by streak camera observations of the edge-emitted luminescence1, and by measurements of time-resolved luminescence spectra with subpicosecond resolution2. This effect could be used to increase the speed of optical gates based on absorption saturation or carrier-induced nonlinear refraction, but a more complete understanding of the mechanisms involved is clearly needed in order to evaluate the interest of this approach.
An examination of the contribution of excitons, compared to the continuum states, to the optical properties of GaAs/AlGaAs multiple quantum wells illuminates the mechanisms underlying nonlinear absorption and nonlinear refraction and yields information which is useful for optimizing devices which incorporate these materials. We have measured the absorption as a function of intensity at three different wavelengths—above the band edge, on the excitonic resonance, and far below the band edge—to separate the effects. We find that different well thicknesses have different saturation intensities both for excitonic and continuum absorption.
Semiconductor multiple quantum well structures exhibit very large, room temperature, nonlinear refractive effects associated with resolvable excitonic features [1]. These nonlinearities may find use in all-optical devices if sufficient refractive index change can be achieved before the exciton completely saturates [2]. A unique aspect of band gap resonant optical, nonlinearities in quantum wells is the spatial confinement of the generated carriers within the layers. We have exploited the excitonic refractive nonlinearity in GaAs/AlGaAs multiple quantum wells in two separate degenerate four wave mixing configurations. These compare the intra-well and cross-well contributions to highly anisotropic carrier diffusion and also allow measurement of carrier relaxation rates and the saturation of the excitonic nonlinear refraction with increasing intensity.
Optical bistable devices containing GaAs/AlGaAs multiple quantum well (MQW) structures have shown great potential for performing fast low-power optical switching at room temperature.1 The presence of a much stronger exciton absorption feature in the MQW structures compared to bulk GaAs suggests far superior performance in the former. We report the observation of room-temperature bistability in bulk GaAs with input power <10 mW and intensities of a few kW/cm2. The intensities are consistent with the data of Miller et al.2 which show a saturation intensity of nearly 4 kW/cm2 in a MQW sample with a detuning of less than a halfwidth from the peak of the exciton resonance. Our detunings are always several halfwidths.
The spatial confinement of carriers in semiconductor quantum well structures leads to resolvable excitonic features at room temperature. Saturation of these due to phase space filling gives rise to a very large non-linear refractive index [1,2]. We have used this non-linearity in GaAs/AlGaAs multiple quantum wells in two separate degenerate four wave mixing configurations. The two arrangements, forward travelling and counter-propagating (see fig.1), produce transient phase gratings across and parallel to the plane of the wells respectively. Time resolved measurements of the decay of these gratings allow comparison of intra-well and cross-well contributions to carrier diffusion, which is highly anisotropic, and reveal decay rates dependent upon the angle of the sample to the normal. In addition such experiments give carrier relaxation times and the intensity at which the excitonic non-linear refraction starts to saturate.
We have investigated intersubband absorptions between the conduction ground state and the first excited state of two AlxGa1−xAs/GaAs/AlxGa1−xAs multiple quantum well structures with x=0.3 and 85 Å well width, and with x=0.57 and 96 Å well width. Small-signal measurements show absorption peaks at 10.45 and 10.15 μm, respectively. Under an intense resonant excitation from a pulsed CO2 laser, saturation of the intersubband absorption occurred. The saturation intensity is estimated to be 340±120 kW/cm2 for the first sample and 375±120 kW/cm2 for the second. From these values, we have deduced subband decay times of the order of 10.6±3.5 ps for the first sample and 15.5±5 ps for the second.