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Applied Physics Letters

Femtosecond all-optical polarization switching based on the virtual excitation of spin-polarized excitons in quantum wells

Eric J. Gansen, K. Jarašiūnas, Arthur L. Smirl

Published Feb 11, 2002• 39 citations
Publisher

Abstract

An optically addressed nonlinear polarization switch based on the near-resonant excitation of a spin-polarized population of virtual excitons is demonstrated in a multiple quantum well. Pulse-width-limited switching (∼400 fs full width at half maximum) and high-contrast performance (194:1) are achieved in a thin (40 well) sample at 100 K. Differential absorption measurements identify the dominant switching mechanisms.

DOI
10.1063/1.1447596

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Applied Physics Letters•Sep 1, 2005•35 citations
An optically addressed nonlinear polarization switch based on the near-resonant excitation of a spin-polarized population of virtual excitons is demonstrated in a multiple quantum well. Pulse-width-limited switching (∼400 fs full width at half maximum) and high-contrast performance (194:1) are achieved in a thin (40 well) sample at 100 K. Differential absorption measurements identify the dominant switching mechanisms.
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.
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.
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.
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.