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International Conference on Ultrafast Phenomena

Ultrafast polarization dynamics induced by the “virtual excitation” of spin-polarized carriers in semiconductors

Arthur L. Smirl, Eric J. Gansen

Published Jan 1, 2002• 0 citations
Publisher

Abstract

The ultrafast changes in the polarization state of a weak probe pulse induced by the creation of spin-polarized virtual carriers in a multiple quantum well are measured, discussed in terms of saturation and exciton-exciton interactions, and used to explain the operation of an all-optical switch.
DOI
10.1364/up.2002.tud2

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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.
Within the framework of light-semiconductor interaction and the Lutting-Kohn model for the semiconductor band structure, we theoretically studied the mechanism of the polarization switch with coherent transient features. The result shows, in the coherent limitation, when the pump pulse Ep and signal pulse Es are orthogonal polarized, the third order polarization P(3), with first order of Es and second order of |Ep|, approach to zero, due to the destructive quantum interference, which results in an ultrafast pump-signal relative polarization dependent P(3), therewith a polarization switch. The study also shows that, when the light interacts with semiconductors coherently, the polarization state of the light is transferred to the relative phase of the electron spin, rather than the electron spin itself. The population of the polarization eigenstate excited by the pump pulse, do not origin from the Coulomb force of electrons, and should not thought phenomenologically as the exciton characterized by light polarization. This population will dephase to real carriers population, i.e. electron with exact spin |↑> or |↓>, by losing phase coefficients of the polarization eigenstates, in an ultrashort quantum dephase time.
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.
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.