Metasurfaces with Enhanced Optomechanical Coupling through the Colocalization of Plasmonic and Acoustic Fields.

Plasmons can enhance inelastic light scattering by terahertz molecular vibrations, enabling single-molecule detectors, molecular optomechanics in nanocavities, and vibrational strong coupling phenomena. Yet plasmon-enhanced optomechanical coupling of propagating gigahertz (GHz) acoustic waves/phonons remains elusive, despite its significance for signal-processing and sensing applications. In this work, we present an acoustoplasmonic metasurface, i.e., a structure that combines plasmonic resonances and acoustic stopbands, offering enhanced optomechanical coupling through colocalization of plasmons and GHz acoustic phonons. The metasurface consists of a 30 nm layer of Au, deposited on a thermal oxide-on-silicon wafer and patterned with a square lattice of holes. Using momentum-resolved Brillouin light scattering at two light wavelengths, on- and off-resonance with plasmons, and finite-element-method optomechanical calculations, we show that the nanoholes induce colocalization of plasmonic hot spots and GHz acoustic modes. This colocalization leads to plasmonic enhancement of the so-called moving interface effect, in which mechanical motions of optical interfaces modulate the spectrum of scattered light. Our work can be useful for developing plasmon-enhanced optomechanical sensors and modulators operating at GHz frequencies.