Surface-plasmon polariton resonances in triangular-groove metal gratings

Abstract

Electromagnetic resonances of triangular-groove gold gratings illuminated with monochromatic light are studied theoretically. The calculations performed are based on the Green's-function surface-integral equation method with the periodic Green's function. Local-field-enhancement spectra and near-field calculations reveal three types of resonances, namely, geometric resonances determined by the shape of individual grooves, standing-wave surface-plasmon polariton (SPP) resonances due to SPPs reflected by the neighbor grooves, and very sharp resonances (Rayleigh anomalies) at wavelengths near the cutoff wavelength of higher grating-reflection orders, which can be tuned simply by changing the angle of incident light. These resonances are also found to be observable in the reflection spectra, whose minima correspond to peaks in the enhancement spectra. Typical enhancements of the electric field magnitude inside the grooves are larger than 20, reaching in some cases the level of $\ensuremath{\sim}35$. In the case of Rayleigh anomalies, the total reflection can be almost completely suppressed. The resonances can be realized in the wavelength range from visible to infrared by varying the groove height, angle, and periodicity, a feature that makes this configuration promising for a wide range of practical applications, for example, within surface-enhanced spectroscopies.