Acoustoplasmonic Metasurfaces for Tunable Acoustic Wavefront Shaping with Polarized Light

Abstract

Plasmonic nanoparticles exhibit strong optical scattering and absorption due to enhanced coupling to incident electromagnetic waves, while their efficient photothermal heating enables effective conversion of electromagnetic to mechanical energy. In this work, we put forward a theoretical framework for acoustoplasmonics, where plasmonic nanoparticles control the acoustic wavefront with light. We model the coupled optical, thermoelastic and acoustic mechanisms for gold nanospheres (AuNSs) and nanoellipsoids (AuNEs), and find that each physical mechanism entails a distinct toolbox of parameters, which can be tailored for effective acoustoplasmonic design. Simple analytical studies are performed for AuNSs, both validating numerical models and enabling quasi-analytical wavefront shaping under long laser pulse durations. AuNEs introduce optical anisotropy, and we numerically demonstrate that the polarization-dependent optical absorption in AuNEs can lead to selective photoexcitation and subsequently polarization-tunable acoustic wave generation. Moreover, we investigate the varying acoustoplasmonic frequency regimes, where optical resonance arises due to electromagnetic frequency, while acoustic resonance relates to laser pulse duration. We demonstrate proof-of-concept acoustoplasmonic metasurface designs using these mechanisms for tunable acoustic wavefront shaping in the form of lensing and beam steering. We suggest that future acoustoplasmonic systems, optimized using the physical mechanisms discussed here, will find use in a variety of applications, including miniaturized ultrasonic imaging and high-frequency signal processing.