Speaker
Description
We demonstrate that spatial dispersion plays an important role in optical properties of layers of plasmonic (interacting) nanoparticles. It results particularly in the dependence of the effective optical properties of such structures on the angle of incidence of the exciting light as well as on the orientation of the lattice in relation to the plane of incidence even though formally such lattices are isotropic. Both effects cannot be described by any standard dielectric tensor and demand the inclusion of spatial dispersion.
Illumination of a square lattice of circular symmetrical particles along any of four planes of symmetry (rows or diagonals) produces no cross-polarization with an effective birefringence at inclined incidence due to the elliptical shape of the Fresnel zones. Such a birefringence is fixed by the plane of incidence, so effective optical constants along two orthogonal axes replace each other at a rotation of the lattice by 90°. At other non-symmetrical orientations cross-polarization arises well seen in the non-diagonal elements of measured Müller matrix. Such a behavior is shown in Figure 1 with color coded values of the m₁₃ element of the Müller matrix for a lattice of disc-like particles with a diameter of 260 nm and a period of 350 nm versus the orientation angle of the lattice in relation to the plane of incidence and the wavelength. Spectra of ellipsometric angles measured along rows (0°) and along columns (rotated by 90°) also shown in the Figure coincide completely and demonstrate total coincidence of the resonance positions with the non-zero values of cross-polarization. Only the gap mode, which field is almost totally concentrated between particles and their image in the substrate and no crosstalk of particles exists is not exhibited in the m₁₃ map. It is an additional argument supporting the creation of effective birefringence and spatial dispersion as a collective effect of interacting particles.