A New Phonon-Focusing Phenomenon due to Elastic Mode Conversion on Silicon Surfaces

Abstract

In our experiment we measure the anisotropic transport of phonons by a time resolved phonon-imaging technique in reflection geometry (see Fig.l). The experimental technique is described by Northrop and Wolfe (1984). We observe phonon images showing a new stucture in addition to the well known “focusing”-patterns which can be explained as specular reflection at the back of the silicon crystal. Between the two ridges of the slow transverse (ST) phonons we observe an additional sharp line which extends from the <100>- to the <111>-directions of the crystal (Fig.2a). A time resolved analysis of the phonons forming the new ridge (Fig.2b) indicates that a conversion of longitudinal phonons into slow transverse ones causes the structure. This is most striking because the propagation of longitudinal phonons shows only a diffuse enhancement of phonon flux into the <111>-direction.