All-Optical Generation and Detection of Coherent Acoustic Vibrations in Single Gallium Phosphide Nanoantennas Probed Near the Anapole Excitation

TL;DR

This study demonstrates the generation and detection of coherent acoustic vibrations in gallium phosphide nanoantennas, revealing enhanced optomechanical interactions near the anapole state and potential for photonic-phononic signal processing.

Contribution

It introduces the first exploration of optomechanical resonances in dielectric nanoantennas, highlighting their superior modulation compared to plasmonic counterparts and their ability to couple mechanical waves over distances.

Findings

Maximum detection efficiency near the anapole state.

Dielectric nanoantennas show up to 5 times larger modulation amplitude than plasmonic ones.

Successful launching and coupling of acoustic waves in the substrate.

Abstract

Nanostructured high-index dielectrics have shown great promise as low-loss photonic platforms for wavefront control and enhancing optical nonlinearities. However, their potential as optomechanical resonators has remained unexplored. In this work, we investigate the generation and detection of coherent acoustic phonons in individual crystalline gallium phosphide nanodisks on silica in a pump-probe configuration. By pumping the dielectric above its bandgap energy and probing over its transparent region, we observe the radial breathing mode of the disk with an oscillation frequency around 10 GHz. We analyze the performance of nanoantennas of various sizes in the 300-600 nm diameter range at fixed 125 nm height and find that the detection efficiency is maximum near the fundamental anapole state, in agreement with numerical simulations. By comparing with reference gold nanodisk and nanorod plasmonic resonators, we find that the dielectric nanoantennas display a modulation amplitude up to ~5 times larger. We further demonstrate the launching of acoustic waves through the underlaying substrate and the mechanical coupling between two nanostructures placed 3 {\mu}m apart, laying the basis for photonic-phononic signal processing using dielectric nanoantennas.