Polarization Spin Inversion with Nonlinear Plasmon Scattering

We report on circularly polarized Gaussian beam spin angular momenta that can be inverted upon scattering with quadrupole plasmon modes. The conditions for such conversion are met with high-angle collection, dark-field scattering microscopy on spherical plasmonic particles. We further report that silvered nanoporous silica microparticles exhibit a strong nonlinearity in their scattering, specifically a reverse saturated scattering (RSS), when exposed to high laser power densities on the sample of ca. 5 GW/cm². Handedness conversion by these microparticles is only observed at wavelengths tuned to the quadrupole modes. Conversely, the scattering remains linear, and the handedness is unchanged, when the same particles are illuminated with low laser power densities of ca. 10 W/cm². We infer that RSS tuned to the quadrupole modes sufficiently enhances their contribution so that they dominate the high-angle scattering, thereby justifying the light spin inversion. Moreover, the addition of a self-assembled monolayer of ethynylaniline (EA) on the microparticles results in handedness conversion for both low and high incident power, as expected from preferable dipole damping and plasmon mode red shift. This demonstrates that optical nonlinearity in scattering can be exploited for polarization tuning in plasmonic metamaterials.