Phonon Reflection Imaging: A Determination of Specular versus Diffuse Boundary Scattering

An important unsettled question exists whether high-frequency (gigahertz to terahertz) acoustic phonons undergo specular or diffuse scattering from a crystal boundary. In this Letter, we demonstrate a phonon-imaging method which clearly differentiates between these two processes. For optically polished sapphire (${\mathrm{Al}}_2$ ${\mathrm{O}}_3$) at 1.6 K, we observe sharp singularity patterns which are unambiguously identified with specular reflection. For a degraded surface, a broad spatial pattern associated with diffuse scattering is observed. A theoretical basis for phonon reflection imaging is presented.