Total internal reflection of high-frequency phonons: A test of specular refraction

The transmission of phonons with frequencies above 140 GHz across a solid-solid interface is examined by phonon imaging. Heat pulses transmitted through varying thicknesses of MgO films on Ge clearly display total internal reflection beyond a critical angle and frustrated total internal refraction when the film thickness is comparable to the phonon wavelength. These manifestations of Snell's law demonstrate the specular (i.e., not diffuse) refraction expected from acoustic mismatch theory, in contrast to most earlier measurements.