Surface acoustic wave enabled all-optical determination of the interlayer elastic constants of a van der Waals interface

Understanding the properties of two-dimensional material interfaces with the substrate is necessary for device applications. Surface acoustic wave propagation through a layered material flake on a substrate could provide unique information on the transverse rigidity of the flake-to-substrate interaction. Ultrasonic waves were generated by a focused femtosecond laser pulse at the surface of a model system—fused silica with hBN flake transferred above. Using an all-optical spatially resolved pump–probe interferometric technique, the spatial dependencies of the surface vertical velocity profiles were measured. The measurements reveal the appearance of surface acoustic wave dispersion in the hBN flake region compared to the fused silica surface. Multilayer modeling allows us to access the longitudinal and shear elastic coupling constants c33* and c44* between hexagonal BN and the substrate.