Engineered near- and far-field optical response of dielectric nanostructures using focused cylindrical vector beams

Abstract

Near- and far-field optical properties of silicon nanostructures under linear polarization (Gaussian beam) and azimuthally or radially focused cylindrical vector beams are investigated by finite-difference time-domain method (FDTD) in Meep open-source software. A python toolkit allowing FDTD simulations in Meep for using those excitation sources is provided. In addition to the preferential excitation of specific electric or magnetic resonance modes as a function of the excitation beam polarization, it is shown in the case of spheroids that shape anisotropy affects the resonance wavelength and the dipole orientation of the magnetic or electric dipole mode. Depending on the spheroid symmetry axis with respect to the electric field orientation, the electric dipole resonance can be split into two peaks, giving quasi-unidirectional scattering, separated by an anapole mode. The optical properties in both far-field (scattering pattern) and near-field (electric and magnetic field hot spots) can be tuned by changing the excitation polarization at a fixed wavelength and selecting properly the spheroid shape and dimensions. These numerical simulations are extended to top-down fabrication-friendly nanostructures such as nanocylinders with circular or elliptic sections.