  Phononic engineering at GHz frequencies form the foundation of microwave
acoustic filters, high-speed acousto-optic modulators, and quantum transducers.
THz phononic engineering could lead to acoustic filters and modulators at
higher bandwidth and speed, as well as quantum circuits operating at higher
temperatures. It can also enable new ways to manipulate and control thermal
transport, as THz acoustic phonons are the main heat carriers in nonmetallic
solids. Despite its potential, methods for engineering THz phonons have been
little explored due to the challenges of achieving the required material
control at sub-nanometer precision and efficient phonon coupling at THz
frequencies. Here, we demonstrate efficient generation, detection, and
manipulation of THz phonons through precise integration of atomically thin
layers in van der Waals heterostructures. We employ few-layer graphene as an
ultrabroadband transducer, converting fs near-infrared pulses to broadband
acoustic phonon pulses with spectral content up to 3 THz. A single layer of
WSe$_2$ is used as a sensor, where high-fidelity readout is enabled by the
exciton-phonon coupling and strong light-matter interactions. By combining
these capabilities in a single van der Waals heterostructure and detecting
responses to incident mechanical waves, we performed THz phononic spectroscopy,
similar to conventional optical spectroscopy which detects responses to
incident electromagnetic waves. We demonstrate high-Q THz phononic cavities
using hBN stacks. We further show that a single layer of WSe$_2$ embedded in
hBN can efficiently block the transmission of THz phonons. By comparing our
measurements to a nanomechanical model, we obtain the important force constants
at the heterointerfaces. Our results could enable THz phononic metamaterials
based on van der Waals heterostructures, as well as novel routes for thermal
engineering.
