THz phonon spectroscopy with van der Waals heterostructures

Acoustic phonons at THz frequencies are one of the main heat carriers in nonmetallic solids, but the coherent generation and detection of THz acoustic phonons have been limited by the transducer thickness and response time. We used the atomically thin nature of van der Waals materials and femtosecond laser pulses in the near IR frequencies to generate broadband THz acoustic phonons and studied their properties. A monolayer WSe₂ was used as a sensing layer, where the WSe₂ exciton energy shift is monitored over time using two-color pump-probe spectroscopy. Exciton-phonon coupling enables a high-fidelity readout of the phonon propagation even with a monolayer, and it provides a way to acoustically control excitons. We estimated phononic properties such as phonon dissipation rates, reflection coefficients, and interlayer coupling constants between different van der Waals materials. Our work opens the possibility to study thermal transport at THz frequencies, and the measured parameters will be useful for designing phononic THz metamaterials.