Acoustical Activity and Other First-Order Spatial Dispersion Effects in Crystals

Abstract

It is demonstrated phenomenologically that as a result of first-order spatial dispersion an "acoustical activity" of transverse acoustic waves, the analog of optical activity, arises in certain crystal classes. Crystals which are optically active are also found to be acoustically active. Furthermore, in the case of propagation along high-symmetry axes, the effect manifests itself, as in the case of optical activity, as a simple rotation of the plane of polarization of transverse acoustic waves. Acoustic effects of first-order spatial dispersion which have no optical analog are also treated. Acoustical activity can also be inferred from the transformation properties of the acoustic phonons, and the acoustical activity of crystals may thereby be derived from phonon-dispersion curves. An estimate of the magnitude of the acoustical activity of tellurium is so obtained. The effects of first-order spatial dispersion on transverse-optical phonons and the optical properties which arise from them are also considered. Practical problems involved in the observation of acoustical activity are discussed.