Flat surface-plasmon-polariton bands and resonant optical absorption on short-pitch metal gratings

The dispersion curves of surface plasmon polaritons (SPP’s) on short-pitch metal gratings consisting of an array of narrow Gaussian grooves are calculated. Very flat SPP bands (large band gaps) are observed in the frequency region where the electromagnetic wavelength is much larger than the grating pitch and grating depth. These flat SPP bands originate from the coupled SPP modes localized in the narrow grating grooves. The interaction between SPP modes and free radiation broadens the SPP modes inside the light line and also leads to strong anticrossings wherever the flat SPP bands cross the light line. Excitation of the SPP modes on these zero-order gratings by incident electromagnetic radiation may result in strong resonant absorption. The line shape of the resonant absorption is explained in terms of the radiative broadening and thermal broadening of the SPP modes.