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Applied Physics Letters

A femtosecond, polarization-sensitive optically addressed modulator based on virtual exciton effects in an anisotropically strained multiple quantum well

Michael Wraback, H. Shen

Published Mar 6, 2000• 20 citations
Publisher

Abstract

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.

DOI
10.1063/1.126011

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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.
Summary form only given. We describe a novel optically-addressed coherent polarization switch that is based on the non-resonant excitation of spin-polarized "virtual excitons" in unstrained MQWs. By operating near (but below) resonance, we achieve nonlinearities that approach the magnitudes of those that can be obtained with resonant excitation, yet we are able to avoid the restrictions that accompany carrier generation, and we take advantage of speeds that are characteristic of non-resonant excitation. Consequently, our device exhibits a pulse-width-limited response that is determined solely by the length of the control and signal pulses and produces large contrast ratios in thin MQW samples.
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.
By using a circularly polarized pulse that is tuned well below resonance, we create a virtual population of spin-polarized coherent excitons in a GaAs/AlGaAs multiple quantum well, and we study its effects on the polarization state of a weak signal pulse using a combination of time-resolved and spectrally resolved ellipsometric techniques. From these data the polarization state contributions to an all-optical polarization switch, which exhibits a ~415 fs switching time and large contrast ratios, are identified and the performance of the switch is demonstrated.
Summary from only given. Recently we have demonstrated an optically-addressed coherent polarization switch that is based on the near-resonant excitation of a spin-polarized population of virtual excitons in MQWs. By taking advantage of the nonlinearities associated with a virtual carrier population, this near resonant device exhibits a pulse-width-limited response; by operating near resonance it is capable of producing relatively large contrast ratios in thin samples; and finally, by generating a spin-polarized carrier population, the device produces a rotation of the signal polarization in unstrained MQWs. Here, we experimentally analyze the switching mechanisms by systematically performing spectrally and temporally resolved differential transmission measurements and by fully determining the polarization state of the transmitted signal as a function of time delay.
Excitonic resonances in semiconductors are very sensitive to applied optical fields. We present here a comparison between the effects of resonant and non resonant optical excitation, creating respectively real or virtual excitons. A blue shift of the exciton line and a reduction of the oscillator strength are simultaneously observed in multiple-quantum-well-structures for the two following cases : presence of a population of real excitons in very thin wells (two-dimensional systems), or presence of an intense offresonance non absorbed optical field (virtual excitons) independently of the well thickness. The latter configuration (optical Stark effect) has been studied as a function of pump intensity, determining in particular how the saturation of the blue shift is related to the exciton bleaching.