Parallel Wood’s Anomalies of Optical Gratings

Abstract

Wood’s anomalies of gratings consisting of imperfect conductors are discussed in the framework of the Rayleigh–Fano theory. Approximate expressions for the intensity of diffracted waves have been obtained around singular points. The parallel anomaly can exist at the same location as the perpendicular one, though the former is less distinct than the latter and dark bands can be expected as well as bright bands.Wood’s anomalies have been measured on transmission gratings consisting of low-refractive-index materials. The experimental results have agreed well with the predictions derived from the Rayleigh–Fano theory. For instance, the existence of the parallel anomaly for gratings with shallow and narrow grooves has been confirmed, and the anomalies which seem to be closely related to the occurrence of dark bands have been found.