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3D Hypersound Microscopy of Van der Waals Heterostructures

We employ here a picosecond ultrasonic technique to study Van der Waals heterostructures. Temporal variation of the reflection coefficient of the Al film that covers Van der Waals hBN/WSe$_2$/hBN heterostructures on a sapphire substrate after the femtosecond laser pulse excitation is carefully measured using an interferometric technique with spatial resolution. The laser pulse generates a broadband sound wave packet in aluminum film propagating perpendicular to the plane direction and partially reflecting from the heterostructural interfaces. The demonstrated technique has enough sensitivity to resolve a WSe$_2$ monolayer embedded in hBN. We apply a multilayered model of the optical and acoustical response that allows to evaluate the mechanical parameters, in particular, rigidity of interfaces, inaccessible from the other measurements. Mapping of the Fourier spectra of the response clearly visualizes different composition regions and can therefore serve as an acoustic tomography tool. Our findings demonstrate almost zero acoustic phonon dissipation below 150 GHz at the interfaces and in the layers that makes Van der Waals heterostructures perspective for nano-acoustical applications.