Influence of light incidence angle and polarization on the refractive index sensitivity of tightly-packed ellipsoidal gold nanoparticle layers

The effect of excitation angle and polarization on the bulk refractive index sensitivity (RIS) of hexagonally-arranged gold nanoparticles is studied through numerical simulations and experiments. Analytical approximations (based on Rayleigh-Gans theory) and boundary element method (BEM) simulations proved that the RIS of single ellipsoidal particles depends intrinsically on the illumination angle. Compared to normal incidence with linear polarization, the RIS increases by 5–10 % at 70° incidence angle, depending on the particle geometry. Finite element method (FEM) simulations explored near-field coupling in tightly-packed particle assemblies and the effect of a substrate beneath the nanoparticles. The simulations showed that coupling between the particles significantly improves the sensitivity to the incidence angle, resulting in a 17–25 % increase in RIS at 70° illumination compared to normal incidence. Experiments performed on three realized nanoparticle arrangements with varying particle density were carried out in a transmission-based setup with linear, circular polarization, and unpolarized light. Results show significantly increased RIS in the 60°<θ<75° incidence angle range, namely between a 12–35 % improvement compared to normal incidence, depending on the particle density. These results underline the importance of optimizing the excitation parameters to maximize the sensitivity of nanoplasmonic sensors for localized surface plasmon resonance (LSPR) applications.