Brillouin Oscillations from Single Au Nanoplate Opto-Acoustic Transducers.

Brillouin oscillations, which are GHz frequency waves that arise from the interaction of light with acoustic waves, are experiencing increasing applications in biology and materials science. They provide information about the speed of sound and refractive index of the material they propagate in, and have recently been used in imaging applications. In the current study, Brillouin oscillations are observed through ultrafast transient reflectivity measurements using chemically synthesized Au nanoplates as opto-acoustic transducers. The Au nanoplates are semitransparent, which allows the Brillouin oscillations to be observed from material on both sides of the plate. The measured frequencies are consistent with the values expected from the speeds of sound in the different materials, however, the attenuation constants are much larger than those reported in previous studies. The increased damping is attributed to diffraction of the acoustic wave as it propagates away from the excitation region. This effect is more significant for experiments with high numerical aperture objectives. These results are important for understanding the relationship between frequency and spatial resolution in Brillouin oscillation microscopy.