Phonon imaging with frequency-selective detectors was used to investigate scattering from residual defects in semi-insulating material. As in the case of previous heat-pulse studies, it was found that the transmission of non-equilibrium phonons with longitudinal and transverse polarizations could be modified by near-infrared excitation of the crystal at low temperatures. These effects were attributed to metastable states of the ubiquitous EL2 defect in liquid-encapsulated Czochralski-grown material. Here, dispersive shifts in the phonon-focusing pattern were used to obtain information concerning the frequency dependence of these optically induced changes. It was shown that, for particular polarizations and propagation directions, near-infrared excitation caused a huge decrease in the scattering of phonons with frequencies above 700GHz. The present results showed that, unlike the elastic scattering of simple mass defects, the residual defects were effective inelastic scatterers of phonons. An analysis of this data, on the basis of the theory of Nowick and Berry, indicated that a change in the transmission of phonons following photo-excitation could not be attributed to an unique symmetry change of a single defect.

J.A.Shields, J.P.Wolfe: Physical Review B, 1994, 50[12], 8297-304