Spatial signature of the photoelastic effect in the acoustic-plasmonic coupling revealed by space responsivity induced by polarized optical excitation.

Acoustic-plasmonic coupling in metallic nanoparticles can significantly alter their optical absorption and scattering characteristics. However, almost all previous investigations on acoustic-plasmonic coupling so far have been focused on the spectral response of particles in a vacuum. In this report, a spatial photon scattering mode taking count in the acoustic-plasmonic coupling of individual gold nanoparticle (GN) on a silicon substrate under ultrasonic influence was presented. The acoustic-plasmonic is visualized with parametric images with spatial scattering patterns of the particle under the excitation of polarized light along the Poincare's equatorial trajectory. The ultrasonic sources can be sensitively extracted from the parametric sin images, providing clear evidence of the extremely weak influence of ultrasound wave directivity on the spatial characteristics of the scattering of the particle through acoustic-plasmonic coupling. Experiment and simulation results reveal that, in general, the coupling is the strongest, when the maximum electric field (plasmon vibration mode) aligns with the ultrasonic propagation direction. This study provides a new angle to observe and deepen the understanding of the acoustic-plasmonic effect of nanoparticles, in addition to the conventional manner of investigation on their scattering spectra. It emphasizes the possibility of determining the spatial distribution of nanoparticles via photon state scattering when they are in a weakly oscillating environment, providing valuable guidance for future potential applications exploiting the acoustic-plasmonic effect of nanostructures.