Brillouin Scattering in Opaque Semiconductors.
Surface Brillouin scattering involving bulk phonons at the sample surface has been extensively investigated. In previous work, Brillouin scattering at the surface was seen only by the surface ripple scattering mechanism. According to theory, surface scattering should also occur through an elasto-optic mechanism. This surface elasto-optic mechanism, which is very weak, was detected for the first time by deliberately isolating and observing the two surface scattering mechanisms separately on orthogonal faces of CdS samples. This was achieved by generating phonons in one branch only, by means of the acoustoelectric amplification process. The relative strengths of the two surface interaction mechanism were measured and the ratio of ripple to elasto-optic was found to vary as 1/f('3) in the range 0.2 (LESSTHEQ) f (LESSTHEQ) 0.9 GHz. The relative strength was also found to depend strongly on the photon energy. This was mainly due to the resonant dispersion of the elasto-optic mechanism. Furthermore, the surface elasto-optic mechanism was exploited to study resonant Brillouin scattering in the opaque regime of a semiconductor. In the opaque regime there has been no previous study of resonant Brillouin scattering. We have demonstrated that the technique of surface Brillouin scattering in conjunction with the use of acoustoelectrically amplified phonons can overcome the problems encountered in the conventional Brillouin scattering technique, to extend the study of resonant Brillouin scattering into the opaque regime. We have also extended our Brillouin scattering studies to thermal phonons. We have used the high contrast five-pass Fabry-Perot interferometer in our investigations. Both surface and bulk Brillouin scattering were investigated in opaque semiconductors. During the course of our study of surface Brillouin scattering with acoustoelectrically amplified phonons, we discovered that the acoustoelectric phonon domain generated both forward and backward propagating mechanical disturbances. These mechanical disturbances are unipolar stress pulses, always forming a ridge on one face and a trough on the corresponding opposite face. The ridge (trough) was measured to be (LESSTHEQ) 200 A in height and had the same width ((TURN) .5 mm) as the phonon domain. The generation mechanism was found to be the piezoelectric shock mechanism.
- Pub Date:
- Physics: Condensed Matter