Optical response of gold hemispheroidal lattices on transparent substrates

Highlights: • Mueller Matrix Ellipsometry reveal new features of arrays of Au nanoparticles. • Complete description of Rayleigh lines and approach to solve for lattice constants. • Show dispersion of polarization conversion around the LSPR and around Rayleigh lines. • Modelled isotropic response using fast Bedeaux Vlieger formalism. • Modelled optical response using COMSOL, and discussed computation time. - Abstract: Square arrays of gold (Au) hemispheroids deposited on a UV-transparent glass substrate reveal a rich optical response when investigated by spectroscopic Mueller Matrix Ellipsometry. Two samples were studied; the first consisted of hemispheroids of parallel radius of 58 nm and height 30 nm with lattice constant 210 nm; the corresponding parameters for the second sample were 38 nm, 20 nm and 125 nm, respectively. By a full azimuthal rotation of the samples, we observe all the Rayleigh anomalies corresponding to grazing diffracted waves, with strong resonances for co-polarization scattered light near the high symmetry points and cross-polarization scattered light around the Localized Surface Plasmon Resonance. Polarization-conversion becomes particularly important at grazing incidence, and the cross-polarization follows the Rayleigh lines. The optical response (neglecting polarization conversion) is modelled in the quasi-static approximation using the so-called Bedeaux-Vlieger formalism, and the Finite Element Method using COMSOL. The direct inversion of the effective (substrate dependent) dielectric function is discussed.