A novel finite-element method for calculating the illumination-dependence of
absorption in three-dimensional nanostructures is presented based on the RF
module of the COMSOL software package. This method is capable of numerically
determining the optical response and near-field distribution of sub-wavelength
periodic structures as a function of illumination orientations specified by
polar angle, fi, and azimuthal angle, gamma. The method was applied to
determine the illumination-angle-dependent absorptance in cavity-based
superconducting-nanowire single-photon detector (SNSPD) designs.
Niobium-nitride stripes based on dimensions of conventional SNSPDs and
integrated with ~ quarter-wavelength hydrogensilsesquioxane-filled nano-optical
cavities and covered by a thin gold film acting as a reflector were illuminated
from below by p-polarized light in this study. The numerical results were
compared to results from complementary transfer-matrix-method calculations on
composite layers made of analogous film-stacks. This comparison helped to
uncover the optical phenomena contributing to the appearance of extrema in the
optical response. This paper presents an approach to optimizing the absorptance
of different sensing and detecting devices via simultaneous numerical
optimization of the polar and azimuthal illumination angles.Comment: 15 pages, 4 figure