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Quantum Optoelectronics

Intrinsic and Induced Anisotropy of the Nonlinear Absorption in Strained [110]-Oriented Multiple Quantum Wells.

X.R. Huang, David S. McCallum, Arthur L. Smirl, Dongming Sun, E. Towe

Published Jan 1, 1995• 0 citations
Publisher

Abstract

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.

DOI
10.1364/qo.1995.qthe9

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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.
The quantum confined Stark effect has found wide application in devices for optical modulation and bistable switching. Whilst normally-incident strained layer modulators and self electro-optic effect devices (SEEDs) have been demonstrated [1-3], the introduction of strain has provided little performance advantage. Recently there has been a growing interest in the properties of strained piezoelectric structures resulting from growth on orientations other than the (100) [4-6]. In this communication we present the results of an investigation of the electro-optical absorption modulation properties of a strained InGaAs/GaAs multiple quantum well p-i(MQW)-n grown on a (111)B GaAs substrate. Subsequently we demonstrate optical bistability using the self electro-optic effect without the application of an external bias.
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.
High contrast and femtosecond speed have been achieved in an optically addressed anisotropically strained multiple quantum well modulator by exploiting polarization-sensitive optical nonlinearities associated with the interaction of resonantly excited excitons with a population of nonresonantly created excitons that adiabatically follows the temporal intensity profile of the excitation pulse. The resultant modulation is characterized by a contrast ratio of 380:1, corresponding to a dynamic polarization rotation of 18°, and a pulse-width-limited full width at half maximum of 165 fs.
Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20>20>20 K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.