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

Abstract

Understanding the properties of two-dimensional materials interfaces with the substrate is necessary for device applications. Surface acoustic wave propagation through the layered material flake on a substrate could provide unique information on the transverse rigidity of the flake-to-substrate interaction. We generate ultrasonic waves by a focused femtosecond laser pulse at the surface of the model system -- fused silica with h-BN flake transferred above. Using an all-optical spatially resolved pump-probe interferometric technique, we measure the spatial dependencies of the surface vertical velocity profiles. Our measurements reveal the appearance of the surface acoustic wave dispersion in the hBN flake region compared to fused silica surface. Multilayer modeling allows us to gain access to longitudinal and shear elastic coupling constants $c^*_{33}$ and $c^*_{44}$ between hexagonal BN and substrate.

Figures (4)

• Figure 1: (a) Reflectivity scan of the studeied sample. (b) hBN contour plot (c) Color map of the scanned simultaneous surface wavefield near the edge of the hBN flake.
• Figure 2: Spatial dependence of the instant wavefield along the white horizontal line ($Y=0$) in Fig. \ref{['ColorMap']}c.
• Figure 3: Measured spatial spectra $\Phi(k,t)$ of the SAW pulses on hBN/SiO$_2$ at $t=T$ (red), $t=2T$ (blue) and $t=3T$ (green). The spectra are shifted vertically for clarity.
• Figure 4: Wave vector dependence of the phase velocity. Green colors - bare SiO$_2$ surface; Blue colors - SiO$_2$/hBN layered structure. Solid lines denote experimental data, dashed lines are theory (constant velocity for SiO$_2$ and result of multilayered modelling for SiO$_2$/hBN). Red dotted curve is a simulation of the SiO$_2$/hBN SAW dispersion within the acoustic mismatch model.