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Journal of Applied Physics

Observation of population transfer to dark exciton states by using spin-diffracted four-wave mixing

Osamu Ikeuchi, S. Adachi, H. Sasakura, Shunichi Muto

Published Jun 6, 2003• 11 citations
Publisher

Abstract

We observed the exciton spin dynamical processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this method as compared with the pump–probe method, we discuss the excitation power dependence of exciton spin relaxation and the population transfer to the dark states.

DOI
10.1063/1.1575920

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We observed the exciton spin dynamical processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this method as compared with the pump–probe method, we discuss the excitation power dependence of exciton spin relaxation and the population transfer to the dark states.
We have observed the exciton spin relaxation processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this new method as compared with the pump-probe technique, we discuss the population transfer to the dark states through the measurements of the excitation-power dependence of exciton spin relaxation. Spin-diffracted four-wave mixing method is a powerful tool to investigate exciton spin dynamical processes where the pump-probe and time-resolved photoluminescence measurements have been used so far, since the method can be used in reflection geometry and be applicable to thin films and quantum dots.
Summary form only given. Ultrafast dephasing of excitons in semiconductor quantum wells is important because it presents insight into fundamental interactions and possible coherent control of optical properties. Standard measurement techniques include time- and frequency-domain four-wave mixing, resonant Rayleigh scattering, and Faraday rotation. We report on a new time-resolved technique based on reflective probing of pump-induced transient linear birefringence. This has various advantages, including sensitivity, applicability to opaque as-grown samples, ease of implementation, and freedom from superimposed signals.
We use reflective probing of transient linear birefringence to study coherent exciton dynamics in GaAs quantum wells as a function of applied magnetic field at low temperatures. The results show the significance of exchange-enhanced exciton spin relaxation for dephasing at zero field. In applied field the signals show background-free quantum beating which gives the exciton g factor and field-dependent dephasing time. The latter is dominated by spin relaxation which shows for the first time the dramatic slowdown predicted in earlier work.
We study the coherent response of an optically dense exciton system in GaAs quantum wells (QWs). For this, we resonantly excite 1s1s1s heavy-hole excitons in the QWs at 8 K with subpicosecond laser pulses. The excitonic polarization is probed using time integrated (TI) degenerate four wave mixing (DFWM) measurements in self-diffraction geometry. The average power density (I1){(I}_{1})(I1​) of the leading laser pulse is systematically increased to large values, keeping that of the delayed pulse small and fixed. The dephasing rate of the excitonic coherent polarization (determined by the rate of decrease of the TI-DFWM signal with delay between the exciting pulses) attains saturation at large I1{I}_{1}I1​ after an initial rise at small I1.{I}_{1}.I1​. In comparison, the coherent polarization (as given by the DFWM signal at a fixed delay) increases initially with I1,{I}_{1},I1​, reaches a peak and then decreases at large I1.{I}_{1}.I1​.
We perform two beam time integrated (TI) degenerate four wave mixing (DFWM) measurements on 8~nm GaAs quantum wells (QW). Pulses from a Ti-Sapphire laser resonantly excite only 1s heavy hole excitons. The power density of the leading pulse (I1) is increased, keeping that of the delayed pulse fixed and relatively small. We measure the TI-DFWM signal at different delays as well as the DFWM spectra at a fixed delay, as I1 is increased. We find that the dephasing rate, after an initial increase with I1, becomes independent of I1 for large I1. The spectrally integrated DFWM signal, which probes the coherent polarization of the resonantly excited QWs, initially increases with I1, reaches a peak and then decreases as 1/(root)I1 at large I1. The results are compared with theoretical models of exciton coherence.