Multiple surface-plasmon-polariton waves guided by a chiral sculptured thin film grown on a metallic grating

Theoretical and experimental studies were undertaken to investigate the grating-coupled excitation of multiple surface-plasmon-polariton (SPP) waves guided by the periodically corrugated interface of a metal and a chiral sculptured thin film (STF). The rigorous coupled-wave approach was adapted to calculate the absorptance spectrum of a structure comprising a chiral STF atop a rectangular metallic grating when that structure is illuminated by a linearly polarized plane wave whose wavevector lies wholly in the corrugation plane. The incidence direction could be either normal or oblique with respect to the thickness direction of the structure. High-absorptance bands for both s- and p-polarized incident plane waves were found to be correlated with the SPP wavenumbers calculated from the solution of the underlying canonical boundary-value problem, indicating that multiple distinct SPP waves can be excited at a specific frequency. The resistive-heating thermal evaporation technique was used to deposit chiral STFs of zinc selenide on gold gratings made by electron-beam lithography, and transmittance and reflectance spectra of the fabricated structures were measured in a variable-angle spectroscopic system in order to qualitatively validate the theoretical understanding. Several high-absorptance bands were found to be almost unaffected by the number of periods of the chiral STF. The existence of these bands indicated that as many as three distinct SPP waves were excited at a specific frequency in several spectral regimes by s- and/or p-polarized incident light.