Coherent Acoustic Phonon Dynamics and Coupling in Metal-van der Waals Heterostructure Nanocavities

The development of acoustic nanocavities with resonant frequencies in the gigahertz to terahertz range has enabled advancements in quantum information processing, acoustic sensing, and advanced optoacoustic devices. Here, we demonstrate the generation and strong coupling of coherent acoustic phonons within metal-van der Waals (vdWs) heterostructure nanocavities, constructed from semiconductor MoS₂ and insulating h-BN thin films, integrated with chemically synthesized Au nanosheets. Both heterostructures exhibit extended coherent phonon spectra, as observed through ultrafast femtosecond pump-probe spectroscopy. The inhomogeneous broadening features of these spectra are accurately reproduced using finite element method simulations and continuum mechanics calculations. A detailed analysis of the phonon coupling mechanism using a spring model reveals distinct coupling strengths of 78 and 55 GHz for the MoS₂/Au and h-BN/Au nanocavities, respectively. Notably, the presence of a thin polymer (PVP) spacer layer at the metal-vdWs interface significantly influences the interfacial coupling strength and phonon lifetime. These findings provide insights into phonon coupling optimization in metal-vdWs nanocavities, contributing to the design of high-performance phononic devices.