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Le Journal de Physique Colloques

GEOMETRICAL DEPENDENCE OF TRANSIENT NONLINEARITIES IN MULTIPLE QUANTUM WELL STRUCTURES

R. J. Manning, A. MILLER, D. W. CRUST, K. Woodbridge

Published Jun 1, 1988• 0 citations
Publisher

Abstract

The transient grating technique has been used with sub-picosecond pulses to study the magnitude, dynamics and saturation of the excitonic non-linear refraction in room temperature GaAs/AlGaAs multiple quantum wells. The dependence of grating decay time with sample orientation in the phase conjugate geometry, was measured and interpreted in terms of intra-well and cross-well contributions to the highly anisotropic carrier diffusion.

DOI
10.1051/jphyscol:1988256

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The transient grating technique has been used with sub-picosecond pulses to study the magnitude, dynamics and saturation of the excitonic non-linear refraction in room temperature GaAs/AlGaAs multiple quantum wells. The dependence of grating decay time with sample orientation in the phase conjugate geometry, was measured and interpreted in terms of intra-well and cross-well contributions to the highly anisotropic carrier diffusion.
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 report time-resolved studies of transient gratings in room-temperature GaAs/AlGaAs multiple-quantum-well structures made using a subpicosecond laser system. Forward-traveling and counterpropagating four-wave mixing geometries produced carrier modulation either parallel or perpendicular to the quantum wells such that the highly anisotropic carrier diffusion could be monitored. Carrier heating and cross-well transport was monitored and geometrical control of the diffusion and resulting four-wave mixing time constant demonstrated. Measured diffraction efficiencies gave the refractive-index change of the order of Δn ~ 0.02 at the onset of saturation of the heavy-hole exciton nonlinearity. Implications for refractive optical bistability are discussed through a determination of the limiting conditions for switching.
We report degenerate four-wave mixing experiments at room temperature, using a three-beam geometry and sub-picosecond laser to study GaAs/GaAlAs multiple quantum well material. Diffraction efficiencies and grating decay times for two samples of different carrier lifetimes have been measured in the spectral region corresponding to excitonic resonances. The measurements provide values of the nonlinear refractive index per carrier pair (n eh) deriving from the excitonic saturation, and also the recombination times and diffusion coefficients for the samples.
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.
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.