The
mechanical properties of the layered crystals in the few layer
limit are largely unexplored. We employ a picosecond ultrasonic technique
to access the corresponding mechanical parameters. Temporal variation
of the reflection coefficient of the Al film that covers hBN/WSe2/hBN (where hBN is hexagonal boron nitride) 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
propagating perpendicularly to the Al plane and partially reflecting
from the heterostructural interfaces. The demonstrated technique allows
one to resolve a WSe2 monolayer embedded in
hBN. We apply a multilayered model of the optoacoustical response
to evaluate the mechanical parameters, in particular, the rigidity
of the interfaces. Mapping of the Fourier spectra of the response
visualizes different composition regions and may serve as an acoustic
tomography tool. Almost zero phonon dissipation below 150 GHz demonstrates
the van der Waals heterostructures’ potential for nanoacoustical
applications.
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