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Le Journal de Physique Colloques

FEMTOSECOND SPECTRAL HOLE-BURNING IN GaAs/AlGaAs MULTIPLE QUANTUM WELLS

J. L. Oudar, J. Dubard, F. ALEXANDRE, Daniele Hulin, A. Migus, A. Antonetti

Published Nov 1, 1987• 3 citations
Publisher

Abstract

The dynamical spectral-hole-burning effect, due to selective saturation of the optically coupled states, has been observed in the femto-second regime in a GaAs/AlGaAs multiple quantum well structure. Comparison with previous results on bulk GaAs under similar excitation conditions shows that carrier-carrier collisions are about twice as efficient in the quantum wells, due to the reduction of screening efficiency in two-dimensional structures, as well as the increase of band-gap renormalization with respect to the bulk material.

DOI
10.1051/jphyscol:19875109

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The dynamical spectral-hole-burning effect, due to selective saturation of the optically coupled states, has been observed in the femto-second regime in a GaAs/AlGaAs multiple quantum well structure. Comparison with previous results on bulk GaAs under similar excitation conditions shows that carrier-carrier collisions are about twice as efficient in the quantum wells, due to the reduction of screening efficiency in two-dimensional structures, as well as the increase of band-gap renormalization with respect to the bulk material.
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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.
The spectra of spontaneous emission accompanying the generation of femtosecond superradiant pulses in GaAs/AlGaAs heterostructures are studied. It is clearly demonstrated that spontaneous emission spectra of electron–hole pairs, which have been left in the semiconductor after the formation of a coherent collective e–h state, correspond to the strong overheating of carriers. This phenomenon can be explained by the effect of dynamic cooling and nonequilibrium condensation of collectively paired carriers at the bottoms of the bands during the superradiant emission, which was observed earlier.
The lowest energy two-dimensional exciton in GaAs/GaAlAs multiquantum wells (MQW) is inhomogeneously broadened at low temperature by fluctuations in the layer width. The broadening permits probing of the excitons as a function of position within the inhomogeneous line[1]. At high densities interaction between the excitons becomes important and leads to saturation and to an increase of the homogeneous linewidth[2]. Using the picosecond pump/probe technique we have measured the saturation behavior as a function of exciton energy within the inhomogeneously broadened line. Figure 1 shows the transmission spectrum T of a weak narrowband probe (dotted) and the change in transmission ΔT of the probe induced by the presence of a pump for two different pump wavelength for 102 Å thick layers of GaAs separated by 200 Å layers of AlGaAs. An increase in transmission (holeburning) at the pump wavelength is observed showing that the interaction between resonant excitons is much stronger than between nonresonant excitons. The strength of the hole decreases rapidly on passing through the center of the line towards the high energy side. This is consistent with an increase in the homogeneous linewidth of the higher energy excitons[1,3]. By delaying the probe we measured the rate of decay of the hole which was found to depend strongly on pump intensity. At low intensity the hole lifetime approaches the luminescent lifetime but becomes shorter at exciton densities greater than ~109 excitons/cm2/layer. We attribute this to exciton-exciton scattering leading to spectral diffusion. The behavior was similar in 51 Å and 102 Å layers. In addition to holeburning, Figure 1 shows an increased absorption in the wings of the line whose shape is almost independent of pump wavelength and which is similar to that observed in bulk GaAs[2].
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.