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.
The optical excitation and propagation of converging surface acoustic waves on silicon with orientations (001) and (111) have been experimentally studied. An axicon-assisted formation of an annular irradiated region on the sample surface served as a source for converging surface waves. Surface wave patterns at different times were recorded using a Sagnac interferometer with spatial resolution. A study of the field distribution at the focus showed that, in spite of elastic anisotropy, which generally leads to aberrations, the acoustic energy can be concentrated into a spot with dimensions close to the diffraction limit. An asymmetric excitation distribution makes it possible to control the structure of the converged wave field at the focus, providing an effective tool for all-optical diagnostics of the local crystal structure as well as electronic properties of quantum objects embedded in the solid-state matrix.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.