Direct Measurement of Phonon Specularity in Silicon membranes using Transient Grating

Boundary scattering of thermal phonons is an important physical process that remains poorly understood. While a quantitative understanding of the process has long been established for many types of waves, the precise manner in which thermal phonons reflect from atomically rough surfaces remains unclear owing to limited control over THz frequency phonon generation and detection. Here, we report measurements of a key property for these efforts, the specularity parameter, which describes the probability of specular reflection of thermal phonons at a surface, by performing transient grating thermal conductivity measurements on free standing silicon membranes. Our observations demonstrate that thermal phonons coherently reflect from surfaces with Angstrom-scale roughness, and further show that completely diffuse reflections occur with surface imperfections on the scale of only a few atomic planes. Our work provides direct experimental insights into the roughness scattering mechanism for phonons that will inform applications such as the coherent manipulation of thermal phonons and passive cooling of electronics.