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Quantum Electronics and Laser Science Conference

Polarization studies of the free polarization decay on GaAs: evidence for excitonic coherence transfer

N. H. Bonadeo, Min Jiang, Vinod Subramaniam, Duncan G. Steel

Published May 22, 1995• 0 citations
OpenAlex

Abstract

When a system is excited by an ultrashort pulse of light, a macroscopic polarization is created. After the excitation, the coherently prepared dipoles radiate in the forward direction while losing coherence because of dephasing by scattering, diffusion, etc., resulting in an exponential decay in time of the macroscopic polarization. Thus, the signal in the forward direction, the free polarization decay (FPD), has a decay time related to this polarization decay time (T2), and in the limit of delta-function pulses it has no rise time.

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In recent years, transient four-wave mixing (FWM) techniques have been used to investigate the dephasing times of non-equilibrium carriers in semiconductor heterostructures by monitoring the decay of the macroscopic polarization. Initially, this decay was measured by temporally-integrating the scattered FWM signal as a function of time delay between two (or more) pump pulses. More recently, the macroscopic polarization decay has been measured by time-resolving the FWM signal by cross correlating it with an ultrashort laser pulse via frequency up-conversion in a nonlinear crystal. When excitons with slightly different energies are excited, oscillations (or beats) have been observed both in the time-integrated FWM signal and in the time-resolved FWM signal. Such beats have been observed, for example, between light- and heavy-hole excitons and between excitons in wells of different widths. Both the spectral behavior1 of the time-integrated FWM signal and the temporal behavior2 of the time-resolved signal have been shown to allow the distinction between the polarization interference associated with two independent oscillators and the quantum beats associated with two coupled oscillators which share a common level. In addition, it has been demonstrated3 that the quantum beats produced by incident pulses with parallel polarizations are exactly out of phase (by π) with the beats produced by orthogonally polarized pulses. In each case1-3, the period and phase of the beats and the polarization of FWM signal were explained by using a six-level model, which excluded many-body effects, to describe the J=1/2 conduction and J=3/2 valence states. Within the last few months, however, it has been shown that a complete description of the polarization selection rules for the FWM signal requires excitation-induced dephasing4 or disorder-induced coupling of the σ + and σ – excitonic transitions5.

International Conference on Ultrafast Phenomena•Jan 1, 1994•0 citations
When a system is excited by an ultrashort pulse of light, a macroscopic polarization is created. After the excitation, the coherently prepared dipoles radiate in the forward direction while losing coherence because of dephasing by scattering, diffusion, etc., resulting in an exponential decay in time of the macroscopic polarization. Thus, the signal in the forward direction, the free polarization decay (FPD), has a decay time related to this polarization decay time (T2), and in the limit of delta-function pulses it has no rise time.
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.
We have studied picosecond two-pulse degenerate four-wave mixing in GaAs/AlAs quantum wells under different polarization configurations of the two pump beams and the analyzed optical polarization. We observe two components, a prompt free polarization decay (FPD) and a delayed photon echo (PE), which show different polarization rules and dephasing times. The observed polarization rules are consistently explained in a theoretical model. Within this model, the two-component behavior is a direct consequence of short-range disorder. The PE dephasing rate is associated with the homogeneous broadening of the excitons while the FPD corresponds to the total dephasing rate, consisting of the homogeneous and inhomogeneous (short-range and long-range) broadenings.
Coherent Rayleigh scattering and incoherent luminescence are studied by femtosecond upconversion under resonant excitation as a function of exciton density in GaAs quantum wells. Comparing samples with different well widths allows to establish the relation between inhomogeneous broadening and the shape of resonant Rayleigh scattering. We find an excellent agreement between experiment and a classical model. In samples with moderate inhomogeneous broadening we provide evidence for the transition from coherent to incoherent emission. The comparison with excitonic dephasing times T2 obtained from four-wave-mixing experiments demonstrates that the incoherent exciton population rises within T2/2, at any exciton density when changed by more than an orders of magnitude. The experimental results confirm very clearly the concept predicting that the coherent polarization made up by localized excitons decays into an incoherent exciton population under conservation of the number of excited quanta.
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.
The exciton coherent signal decay rate in GaAs quantum wells, as measured in four-wave mixing experiments, depends on the polarization of the excitation pulses. Using polarization-dependent two-dimensional coherent spectroscopy, we show that this behavior is due to the bosonic character of excitons. Interference between two different quantum mechanical pathways results in a smaller decay rate for cocircular and colinear polarization of the optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a larger decay rate. Our result shows that the bosonic nature of excitons must be considered when interpreting ultrafast spectroscopic studies of exciton dephasing in semiconductors. This behavior should be considered while interpreting results of ultrafast spectroscopy experiments involving bosonlike excitations.
In recent years, transient four-wave mixing (FWM) techniques have been used to investigate the dephasing times of non-equilibrium carriers in semiconductor heterostructures by monitoring the decay of the macroscopic polarization. Initially, this decay was measured by temporally-integrating the scattered FWM signal as a function of time delay between two (or more) pump pulses. More recently, the macroscopic polarization decay has been measured by time-resolving the FWM signal by cross correlating it with an ultrashort laser pulse via frequency up-conversion in a nonlinear crystal. When excitons with slightly different energies are excited, oscillations (or beats) have been observed both in the time-integrated FWM signal and in the time-resolved FWM signal. Such beats have been observed, for example, between light- and heavy-hole excitons and between excitons in wells of different widths. Both the spectral behavior1 of the time-integrated FWM signal and the temporal behavior2 of the time-resolved signal have been shown to allow the distinction between the polarization interference associated with two independent oscillators and the quantum beats associated with two coupled oscillators which share a common level. In addition, it has been demonstrated3 that the quantum beats produced by incident pulses with parallel polarizations are exactly out of phase (by π) with the beats produced by orthogonally polarized pulses. In each case1-3, the period and phase of the beats and the polarization of FWM signal were explained by using a six-level model, which excluded many-body effects, to describe the J=1/2 conduction and J=3/2 valence states. Within the last few months, however, it has been shown that a complete description of the polarization selection rules for the FWM signal requires excitation-induced dephasing4 or disorder-induced coupling of the σ + and σ – excitonic transitions5.