Optical detection and imaging of nonequilibrium phonons in GaAs using excitonic photoluminescence

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Abstract
We have developed a time- and space-resolved optical detector of nonequilibrium phonons utilizing excitonic photoluminescence at low temperatures. An epilayer of GaAs is ‘‘sensitized’’ by excitation with a low-power, focused ‘‘probe’’ beam, which creates free excitons (FE’s) and bound excitons (BE’s) in the layer. The photoluminescence intensity from these species is reduced by a flux of nonequilibrium phonons, created on the opposite side of the GaAs substrate by optical excitation of a metal film with a ‘‘pump’’ beam. A simple model is proposed for the phonon-induced change in photoluminescence intensity of the FE’s; previous work (e.g., by Blank et al. [Sov. Phys. Semicond. 25, 39 (1991)]) has treated the case of BE’s. Both time-resolved heat pulses and space-resolved heat flux associated with phonon focusing are observed. The heat pulses are characterized by a broad temporal distribution, the exact origin of which is not determined. If the sensitivity of this detection technique could be raised sufficiently, space and time resolutions of 1 μm and 1 ns, respectively, would be feasible.