Propagation of Laser-Induced Hypersound Waves in Polycrystalline Diamond with Submicron Crystallites

We investigate the propagation of laser-induced hypersound waves in polycrystalline diamond films by analyzing the spatial changes in the phase and amplitude of the probing laser beam reflected from the surface. We show that the velocity of bulk (longitudinal) acoustic waves is 14300 m/s in films with an average crystallite size of ≈ 0.2 μm and monotonically increases to 17200 m/s with increase in the crystallite size up to ≈ 2 μm. For surface waves, anomalous dispersion is observed, leading to a change in the phase velocity from ≈ 4800 to ≈ 6600 m/s, which is explained by the effective localization of the surface wave field in the diamond films. The values obtained can be used for the design of diamond films and membranes for applications in diamond photonics and acousto-electronics.