Abstract
We report a spectroscopic Mueller matrix experimental study of a plasmonic photonic crystal consisting of gold hemispheroidal particles (lateral radius 54 nm, height 25 nm) arranged on a square lattice (lattice constant 210 nm) and supported by a glass substrate. Strong polarization coupling is observed for ultraviolet wavelengths and around the surface plasmon resonance for which the off-block-diagonal Mueller matrix elements show pronounced anisotropies. Due to the Rayleigh anomalies, the block-diagonal Mueller matrix elements produce a direct image of the Brillouin Zone (BZ) boundaries of the lattice and resonances are observed at the M-point in the first and at the X-point in the second BZ. These elements show also the dispersion of the localized surface plasmon resonance.
Highlights
Ellipsometric characterization of plasmonic nanoparticles on surfaces is an interesting problem dating back to the original works of Drude [1]
Note that the m21 element is nominally identical to m12, but we have on purpose chosen a different colorbar and rescaled m21 below 2.5 eV in order to better visualize the features weaker than the strong localized surface plasmon resonance (LSPR)
It is noted that the elements m34 and m43 are essentially equal in magnitude but have opposite signs, so that the Mueller matrix has the expected symmetry
Summary
Ellipsometric characterization of plasmonic nanoparticles on surfaces is an interesting problem dating back to the original works of Drude [1]. Within the limitations of nanofabrication, the macroscopic effective properties of metamaterials and even arrays of plasmonic particles, share many features with photonic crystals [2], and it is necessarily the combined response that will be observed by spectroscopy across a larger spectral range. For highly ordered arrays of plasmonic/metallic particles, the lattice plays a major role, and a number of reports show experimentally and numerically that the localized surface plasmon resonance (LSPR) is strongly perturbed by the Rayleigh/Wood modes that occurs in photonic crystals [6, 13], i.e. for lattice constants with nominally weak dipole-dipole interactions [14]. The plasmon modes occurring as a result of the lattice for larger lattice constants, can be expected to both convey polarization coupling and resonances not accounted for in the Bedeaux-Vlieger model, and reflect the two-dimensional photonic crystal character of the surface. Through the use of Focused Ion Beam milling (FIB) of a thin Au film, the ITO layer can be omitted, which allows to reveal the complete picture, since Rayleigh modes can freely propagate in the uv-range in fused-SiO2
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