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Physical Review Letters

Quantum beats versus polarization interference: An experimental distinction

Martín Koch, J. Feldmann, G. von Plessen, Ernst Otto Göbel, P. Thomas, Klaus Köhler

Published Dec 21, 1992• 195 citations
Publisher

Abstract

We demonstrate that transient four-wave mixing with time-resolved detection of the coherent nonlinear signal allows unambiguous experimental distinction between quantum beats and beats due to interfering polarizations of independent quantum-mechanical systems. The experimental demonstration is performed employing especially designed semicondutor quantum-well structures.

DOI
10.1103/physrevlett.69.3631

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We demonstrate that transient four-wave mixing with time-resolved detection of the coherent nonlinear signal allows unambiguous experimental distinction between quantum beats and beats due to interfering polarizations of independent quantum-mechanical systems. The experimental demonstration is performed employing especially designed semicondutor quantum-well structures.
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 analyze theoretically the quantum beat and the polarization interference phenomena observed as oscillations in pulsed four-wave mixing. We show that resolution of the signal with respect to the delay between the incoming pulses as well as the detection frequency provides valuable information on the nature of the oscillations. Most importantly, it is very simple to distinguish between polarization interference from two independent optical transitions and quantum beats from a three-level system. In case of polarization interference, the phase of the oscillations shifts \ensuremath{\pi} when the detected frequency passes any of the resonances. In case of quantum beat, the phase has little dependence on the detected frequency near the resonance.
The nature of the beat modulations observed in nonlinear four-wave mixing in semiconductors is determined by spectral resolution of the time-integrated four-wave-mixing signals. Beats between close-lying bound-exciton resonances in CdSe are found to be described by macroscopic polarization interferences between two noninteracting two-level systems. Beats between heavy-hole excitons and light-hole excitons in GaAs quantum wells, on the other hand, are described by a three-level system, i.e., as a true quantum interference phenomenon.
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In degenerate four-wave-mixing (DFWM) experiments the polarization of the incident laser pulses can have a strong influence on the measured signal. For GaAs quantum wells a number of DFWM experiments were performed by using either parallel or perpendicular polarization of the incident laser pulses. Experimentally, it turned out that some characteristic features of the signal differ appreciably for the two configurations. In particular, different dephasing times, phase shifts of the quantum beats, different time-resolved behavior (echoes versus free-induction decay), and intensity dependence of the measured signal have been reported.1–3
We have performed spectrally resolved four-wave mixing measurements on inhomogeneously broadened GaAs/AlAs quantum-well structures. Under crosslinear polarization we observed a beating structure as a function of the delay time at higher energy region of the emission band with period of 0.95 ps. The beating was constructive at delaytime τ=0 ps. This behavior results from the interference between two biexciton intermediate states with different energies coupled to a common one-exciton state, as we show by a comparison with analytical model calculation of the double Feynman diagrams of the same type.