Direct Electromagnetic Generation of Transverse Acoustic Waves in Metals

Abstract

We present a theory to describe the direct generation of transverse acoustic waves in metals by electromagnetic radiation incident on the metal surface and to describe the converse effect, both in the presence and in the absence of an external magnetic field directed normally to the metal surface. This theory can be used to predict the fraction of the incident power which goes into the generated wave. The predictions of this theory are shown to be in substantial agreement with the experimental data reported by Houck et al. and others on the direct generation of transverse acoustic waves by radio-frequency radiation in aluminum in the presence of a magnetic field. However, when the predictions of this theory for the direct generation of transverse acoustic waves by microwave radiation in indium are compared with the experimental data of Abeles and others, the predicted insertion loss is found to be several orders of magnitude below that reported. The theory is developed from Maxwell's equations and the equation of motion of the lattice for a free-electron, semi-infinite metal with an electromagnetic field and/or a shear acoustic wave incident on the metal surface. The only forces acting on the ions in the metal are those forces present in the bulk metal or applied externally; that is, the theory is devoid of any forces on the ions resulting from electron scattering at the metal surface. The theory can be easily understood by imagining the unfilled half-space to be occupied by another piece of the same metal and introducing a current sheet and/or a shearing force in a plane containing the origin. The equations which describe the electric field and the ionic displacement field in the metal can then be derived in the context of an infinite metal in a manner which is analogous to that frequently used in deriving the expression for the surface impedance of a metal in the extreme anomalous limit.