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International Conference on Ultrafast Phenomena

Observation of Many-Body Effects in the Femtosecond Temporal Profile of Quasi-2D Exciton Free-Induction Decay

Shimon Weiss, Mary‐Ann Mycek, J.-Y. Bigot, Stefan Schmitt-Rink, D. S. Chemla

Published Jan 1, 1992• 1 citation
Publisher

Abstract

In time-resolved four-wave mixing (FWM) experiments, two ultrashort pulses delayed by Δt and with wavevectors k1 and k2 interfere in a sample to produce a diffracted signal (DS) in the direction ks = 2 k2 − k1. In the case of a homogeneously broadened system, the DS is emitted immediately after the second pulse corresponding to the "free- induction decay" of the nonlinear polarization. Because of the short dephasing times T2 in solids, the DS is usually not time-resolved, but is measured as a function of Δt using a slow detector. Valuable information contained in the temporal profile of the DS is thus lost Here, we report the first measurement of the temporal evolution of the DS on a fs time scale for the case of selective excitation of semiconductors.

DOI
10.1364/up.1992.mc4

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In time-resolved four-wave mixing (FWM) experiments, two ultrashort pulses delayed by Δt and with wavevectors k1 and k2 interfere in a sample to produce a diffracted signal (DS) in the direction ks = 2 k2 − k1. In the case of a homogeneously broadened system, the DS is emitted immediately after the second pulse corresponding to the "free- induction decay" of the nonlinear polarization. Because of the short dephasing times T2 in solids, the DS is usually not time-resolved, but is measured as a function of Δt using a slow detector. Valuable information contained in the temporal profile of the DS is thus lost Here, we report the first measurement of the temporal evolution of the DS on a fs time scale for the case of selective excitation of semiconductors.
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.
We present a unified microscopic approach to four-wave mixing (FWM) in semiconductors on an ultrashort time scale. The theory is valid for resonant excitation in the vicinity of the excitonic resonance and at low densities. The most important many-particle effects, i.e., static and dynamical exciton-exciton interaction as well as biexcitonic effects are incorporated. The internal fields resulting from these interaction processes give rise to pronounced many-particle effects in FWM signals. Our results explain the dependence of FWM signals on the polarization geometry, especially if biexcitons contribute. Time-resolved (TR) FWM experiments show that the diffraction of the interaction induced fields dominate the FWM signals completely. This dominance of the interaction induced field at low temperatures is true regardless of density, detuning, or polarization geometry. While spectrally resolved FWM (-FWM) shows biexcitonic or bound excitonic contributions under various experimental conditions, TR-FWM is always completely delayed, peaking roughly at the dephasing time after both beams passed through. \textcopyright{} 1996 The American Physical Society.