Acoustoplasmonic Metasurfaces Based on Polymer-Grafted Nanoparticles

Nanocomposites assembled from polymer-grafted plasmonic nanoparticles (PGNs) can combine strong light-matter interactions with soft-matter functionalities and a high degree of translational symmetry. This work explored the potential of gold nanoparticles (16 nm diameter) grafted with polystyrene chains (degree of polymerization, N ≈ 63) as building blocks for acoustoplasmonic metasurfaces. We have decorated inorganic surfaces─crystalline silicon and SiO₂ glass─with PGN monolayers and explored their surface acoustic waves with micro-Brillouin Light Scattering (μ-BLS) at various photon energies. Aided by finite-element-method calculations of acoustic phonons, plasmons and optomechanics, we show that PGNs sustain coupled sphere modes with rattling, torsional, and quadrupolar features. The coupled sphere modes exhibit plasmon-enhanced BLS and form a wide acoustic band gap below the line of sound. In the long wavelength limit, the coupling to the substrate leads to the emergence of shear-horizontal and Sezawa waves, whose dispersion relationships yield the local scale elasticity of ultrathin PGN monolayers.