Optical phonon modulation in semiconductors by surface acoustic waves

Abstract

We investigate the modulation of optical phonons in semiconductor crystals by a surface acoustic wave (SAW) propagating on the crystal surface. The SAW fields induce changes on the order of 10-3 in the time-averaged Raman peak intensity by optical phonons in Si and GaN crystals. The SAW-induced modifications in the intensity of the Raman lines are dominated by the modulation of the longitudinal optical (LO) phonon energy by the SAW strain field. We show that while the strain field of the excited Rayleigh SAWs changes the LO phonon energy, it does not mix it with the transversal optical modes. In addition to the previous contribution, which is of a local character, the experiments give evidence for a weaker and nonlocal contribution attributed to the spatial variation of the SAW strain field. The latter activates optical modes with large wave vectors and, therefore, lower energies. The experimental results, which are well described by theoretical models for the two contributions, prove that optical phonons can be manipulated by SAWs with μ m wavelengths.