We use metal nanostructures (nanoplasmonics) excited with dual frequency lasers to generate and detect high-frequency (>10 GHz) sound wave resonances in water. The difference frequency between the two lasers causes beating in the intensity, which results in a drop in the transmission through the nanostructure when an acoustic resonance is excited. By observing the resonance frequency shifts with changing nanostructure size, the transition from slow to fast sound in water is inferred, which has been measured by inelastic scattering methods in the past. The observed behavior shows remarkable similarities to finite element simulations using a simple Debye model for sound velocity (without fitting parameters).
Keywords: Nanoplasmonics; acoustic resonances; dynamics of water; extremely high frequency waves; sound velocity.