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Precision Nonlinear Laser Spectroscopy Of Exciton Dynamics In GaAs/AlGaAs Multiple Quantum Well Structures

Frequency domain four wave mixing provides a unique method for studies of dynamics of excitations in solids. It can eliminate contributions from inhomogeneous broadening and provides a direct measurement of the homogeneous line shape of the excitationl. The narrow optical band width (3 MHz) used in the measurement allows the measurement of excitation spectral diffusion as well as spatial diffusion at a given energy. In this paper, we discuss the use of these new nonlinear laser spectroscopy methods to studies of the lowest energy heavy hole exciton (HH1) in a GaAs/AlGaAs multiple quantum well structure (MQWS) at low temperature. In the MQWS, interface roughness of one monolayer can result in a change in the exciton energy on the order of several meV. Consequently, heavy hole excitons at the low energy region of the HH1 absorption spectrum can be localized, while excitons at higher energy remain delocalized2. Localization significantly changes exciton dynamics, and gives rise to such spectral diffusion behaviors as phonon assisted migration and thermal activation of localized excitons to delocalized states3. The experimental configuration is based on backward four wave mixing, in which two counterpropagating pump beams interact in the sample with the probe beam through the third order nonlinear polarization producing a coherent signal beam1 . Information on the exciton homogeneous line shape is obtained by measuring the nonlinear response as a function of the backward pump frequency while fixing the frequencies of the forward pump and probe (designated the FWMb response). For excitons whose energy remains unchanged after creation, it follows from a simple hole burning picture that the FWMb response should be a symmetric function of forward pump and backward pump detuning. Our measurements at exciton energies below the absorption line center, however, show a strikingly asymmetric line shape, clearly indicating the presence of exciton spectral diffusion. Exciton spectral diffusion is further examined by measuring the nonlinear response as a function of forward pump and probe detuning while fixing the frequencies of the backward pump and probe (designated the FWMf response). The FWMf response provides a measurement of the exciton relaxation rate at a given exciton energy. The dependence of this rate on the angle between the forward pump and probe also provides a measurement of exciton spatial diffusion coefficient. FWMf responses obtained below the exciton absorption line center show the absence of exciton spatial diffusion indicating excitons are localized in that energy region. Further measurements of the dependence of the exciton relaxation rate on the temperature shows that exciton dynamics is determined by phonon assisted migration up to a temperature of 15 K.