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Nonlinear optical properties and subpicosecond dynamics of excitons and electron-hole plasmas in multiple quantum well structures

Thomas F. Boggess, Arthur L. Smirl, R. A. Macfarlene, Jenny Lam

Published Dec 1, 1987• 0 citations
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Abstract

This report describes progress in measuring and modeling the nonlinear optical properties of and picosecond carrier dynamics in bulk ternary alloys and multiple quantum wells (MQWs). Studies of the bulk alloy Al(x)Ga(1-x)As have been produced the first experimental isolation of the electron exchange energy in a dense electron-hole plasma, and the observation of enhanced band-gap renormalization as a consequence of alloy disorder, stimulated emission from the indirect gap for x-values as large as 0.52, and large optical nonlinearities from band filling, band-gap renormalization, and screening of the excitonic continuum states. The latter appear promising for room temperature absorptive and dispersive optical bistability. An exact theory of the influence of an intense radiation field on the excitonic energy levels of a MQW has been developed and has led to the prediction of two-wave mixing gain. Keywords: Nonlinear optics; Ultrafast phenomena; Multiple quantum wells; Band gap renormalization; Alloy disorder; Optical bistability.

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This report describes progress in measuring and modeling the nonlinear optical properties of and picosecond carrier dynamics in bulk ternary alloys and multiple quantum wells (MQWs). Studies of the bulk alloy Al(x)Ga(1-x)As have been produced the first experimental isolation of the electron exchange energy in a dense electron-hole plasma, and the observation of enhanced band-gap renormalization as a consequence of alloy disorder, stimulated emission from the indirect gap for x-values as large as 0.52, and large optical nonlinearities from band filling, band-gap renormalization, and screening of the excitonic continuum states. The latter appear promising for room temperature absorptive and dispersive optical bistability. An exact theory of the influence of an intense radiation field on the excitonic energy levels of a MQW has been developed and has led to the prediction of two-wave mixing gain. Keywords: Nonlinear optics; Ultrafast phenomena; Multiple quantum wells; Band gap renormalization; Alloy disorder; Optical bistability.
We report on an experimental study of the origin of the nonlinearities and the size of the nonlinear index at various wavelengths close to the exciton resonance and the material band gap. It is concluded that for intensities required for optical bistability, the nonlinearity is mainly excitonic for MQWs with well sizes W of 50 and 152 Å which are either smaller or slightly larger than the exciton radius a. For 299-Å MQW, (W ≃ 2a), where the MQW behaves more like bulk but still has distinct excitonic features, the nonlinearity is due to the combination of exciton screening, band filling, and screening of the continuum states. For bulk GaAs, the effect of exciton is comparatively small and the latter two effects are responsible almost equally for the change in the refractive index. The measured nonlinear indices are rather similar for all these samples with the bulk GaAs and 299-Å MWQ exhibiting slightly larger Δn (change in index) for similar carrier densities (or for similar light intensities). The 50-Å MQW has a slightly smaller value of nonlinear index because the nonlinearity which in this sample is due to the saturation of the exciton resonance occurs at higher excitation intensities due to larger exciton binding energy.
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.
The bleaching of the n=1 and n=2 heavy-hole exciton absorption has been observed at room temperature in ternary alloy Al0.37Ga0.63As/AlAs multiple-quantum-well structures by nanoseconds pump-and-probe spectroscopy. The relative variation of the absorption coefficient [(−Δα)/α0] are about 15% and 8%, respectively. The nonlinear absorption coefficient (α2) and the nonlinear absorption cross section has been deduced. The value of the saturation density deduced from our spectra is in good agreement with the theoretical predictions. The influence of the staggered band alignment on the observed process is also discussed.
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 review the nonlinear-optical effects observed at room temperature in semiconductor quantum-well structures photoexcited near the band gap. A comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances. Experimental investigations using continuous-wave, picosecond-, and femtosecond-laser sources are presented. They show extremely efficient nonlinear processes. In the case of excitations that are long compared with the exciton-ionization time, the induced changes in absorption and refraction do not depend on the wavelength or on the duration of excitation. These changes depend only on the density of absorbed photons and are interpreted in terms of electron–hole plasma screening and band filling. In contrast, for ultrashort excitation, nonlinear processes depend critically on the excitation wavelength. The selective generation of excitons is found to produce effects larger than a plasma of the same density. This unexpected result is shown to arise from the low temperature of the exciton gas before it interacts with the lattice and from the decrease of screening that is the reduced dimensionality of quantum-well structures.