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Semiconductor Science and Technology

Nonlinear absorption and refraction in strained InGaAs/InP multiple quantum wells for all-optical switching

P J Bradley, Riccardo A. Calvani, Domenico Campi, Renato Caponi, F Genova

Published Apr 1, 1992• 10 citations
Publisher

Abstract

Absorption spectra of strained InGaAs/InP quantum wells exhibit a single, unusually large exciton feature. 60% quenching is obtained for 4.3 mW Nd:YAG effective pump power at a wavelength of 1.064 mu m. A simple two-level formula fits well the behaviour at the exciton peak, and absorption spectra are reasonably approximated by a standard two-dimensional exciton model. Saturation intensities calculated with these techniques range around 200 W cm-2. Finally the Kramers-Kronig relations are used to calculate the associated refractive index change.

DOI
10.1088/0268-1242/7/4/019

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Absorption spectra of strained InGaAs/InP quantum wells exhibit a single, unusually large exciton feature. 60% quenching is obtained for 4.3 mW Nd:YAG effective pump power at a wavelength of 1.064 mu m. A simple two-level formula fits well the behaviour at the exciton peak, and absorption spectra are reasonably approximated by a standard two-dimensional exciton model. Saturation intensities calculated with these techniques range around 200 W cm-2. Finally the Kramers-Kronig relations are used to calculate the associated refractive index change.
A realistic model of the electromagnetic response in excited, type-I semiconductor quantum wells is presented. A multisubband analysis of the room temperature band edge spectra, in which the plasma effects are determined through an iterative, numerical solution of a generalized Bethe-Salpeter equation, is introduced. The method allows the treatment of situations in which transitions higher than the first give sizeable contributions to the nonlinear optical properties, as in coupled quantum wells or in the propagation of TM modes within waveguides based on quantum confined structures. Further, the method accounts for the effects of finite quantum-well thickness and incorporates a phenomenological description of the width of the resonance lines. The behavior is compared to that exhibited by more idealized models introduced in earlier works, where single-subband, purely 2-D systems were treated. Computer generated spectra are compared with absorption spectra experimentally obtained for moderately strained InGaAs-InP QWs.>
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 theoretical studies1,2 and experimental measurements3,4,5 have shown that there is a large inherent anisotropy in the absorption coefficient of excitons in biaxially strained quantum wells (QWs) grown on (110) surfaces. Due to the symmetry properties of the excitonic wavefunctions, there is no such anisotropy in the absorption spectra of unstrained or biaxially strained QWs grown on (100) or (111) surfaces. Such an anisotropy in the absorption coefficient can be exploited to construct optical switches or modulators. For example, a contrast ratio of 330:1 between on and off states has been achieved in uniaxially strained [100]-oriented QW structures using 100 wells.6 In this work, we present the first measurements of optically-induced and intrinsic anisotropies in the nonlinear absorption of a biaxially strained, [110]-oriented MQW structure using picosecond pump-probe and differential transmission spectroscopy. We also demonstrate how the polarization rotation associated with these anisotropies can be used to construct an all optical on/off switch or modulator with a contrast ratio of 25:1 using only 50 wells.
Ultrafast absorption saturation in an exciton Bragg reflector, consisting of InGaAs quantum wells embedded in InP/InGaAsP Bragg layers, is characterized by pump-induced transient reflectance spectroscopy at room temperature. Spectral modulation of the reflected probe pulses is caused by ultrafast absorption saturation of excitons in the quantum wells. The spectral modulation leads to an optical phase shift of 1.6 π. The exciton-induced phase shift is more than four times as large as that induced by free carriers in a Bragg reflector.
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.