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Physical Review B

Rapid dephasing related to intersubband transitions induced by exciton quantum beats observed by a pump-probe technique in a GaAs/AlAs multiple quantum well

Osamu Kojima, Kenta Kojima, Takashi Kita, Kouichi Akahane

Published Mar 20, 2015• 4 citations
Publisher

Abstract

We discuss the dephasing of quantum beat oscillation related to the intersubband transition between heavy-hole and light-hole excitons in a GaAs/AlAs multiple quantum well as measured by a pump-probe technique. We investigate the dependence of dephasing on temperature and pump energy. The analysis of the time-domain signals reveals that a strong quantum beat induces the rapid dephasing related to intersubband transition; the dependencies of the dephasing rate correspond to those of the amplitude. This implies that a quantum beat is useful in applications utilizing an ultrafast optical response.

DOI
10.1103/physrevb.91.125307

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We report pulse modulation caused by the exciton quantum beat in a GaAs/AlAs multiple quantum well.The modulation was evaluated by measuring the cross-correlation signal which is the second harmonic light generated by the probe pulse reflected from the sample and the gate pulse.The intensity of the correlationsignal decreases owing to the generation of the exciton quantum beat, and recovers with dephasing of the quantum beat oscillation.Moreover, we found that the decrease caused by the quantum beat is larger than that by changing the refractive index due to the exciton generation.These results indicate that quantum beats can be a potential mechanism to enable low power operation of ultrafast optical switches.
Shifts in the relative phases of quantum beats in the time-resolved four-wave-mixing signals from a GaAs/Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum well are observed. The relative phases are shown to depend strongly on the intensities of the incident pulses as well as their detuning from the heavy-hole exciton. A simple coupled six-level model shows that the phases of the quantum beats are directly determined by many-body effects such as density-dependent dephasing and the local-field correction.
In this study, we report the effect of the excitation of non-exciton components caused by broadband pulses on quantum beat oscillation. Using a spectrally controlled pump pulse, a long-lived oscillation is clearly observed, and the pump-power dependence shows the suppression of the dephasing rate of the oscillation. Our results from incoherent carrier generation using a continuous wave laser demonstrate that the non-exciton components behaving as free carriers increase the oscillation dephasing rate.
We investigate the spectral properties of the excitonic nonlinear response in a GaAs quantum-well sample containing wells of different thicknesses. We observe both the heavy-hole biexciton and the light-hole biexciton in the transient four-wave mixing spectrum and estimate their binding energy dependence on well width. For moderate excitation densities the signal for negative delay at the heavy-hole exciton and biexciton resonances is due to two-photon transitions to the biexciton states through the exciton states. The temporal behavior of the transient four-wave mixing signal from the heavy-hole exciton resonance is strongly dependent on the spectral position of the laser. We show that this effect is due to interference phenomena and to excitation-induced dephasing.
We present an experimental and theoretical study of the dephasing of heavy-hole excitons in a 70-\AA{} GaAs/Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-well stucture. We measure the time-integrated three-pulse four-wave-mixing signal of heavy-hole excitons in the quantum-well sample. We model the third-order nonlinear optical response incorporating the effects of a stochastic modulation of the transition frequency. We find that the dephasing of the nonlinear polarization is mainly the result of a spectral-migration process of the excitons and, to a lesser extent, due to a homogeneous dephasing process. The time dependence of the four-wave-mixing signal is very well described by theory if the spectral-migration process is assumed to be energy dependent.
Three-beam time-integrated transient four-wave mixing (TFWM) is used to probe both light-hole (LH) and heavy-hole (HH) exciton coherences in weakly disordered GaAs quantum wells. The period of the quantum beats between the LH and the HH exciton coherences changes as a function of first-second pulse delay. A theoretical model for the TFWM signal, including exciton dephasing rates that vary with frequency across the inhomogeneous linewidth, reproduces the observations. The inclusion of inhomogeneous dephasing also improves the correspondence between theory and experiment when data as a function of both first-second and second-third pulse delays are simultaneously examined.