Optical frequency comb generated from nonlinear ultrasound.
(A) Optical frequency comb generated from nonlinear ultrasound propagating in bulk water (theory). In this particular example, the spacing between the comb peaks equals 1 MHz, which is set by the frequency of the incident sound. Note an enhancement (red solid curve) in the peak intensity due to plasmon nanoparticles (NPs) acting as optical antennas with respect to bulk water without nanoparticles (green dashed curve). (B) Fourier transform of the spectrum in (A). Adapted with permission from . Copyright 2016, Optical Society of America.