The boundary-value problem of diffraction from shallow metal gratings with finite conductivity is solved numerically by using an integral equation method. The electromagnetic field in the material is described by means of a series expansion of evanescent waves. As suggested by Hessel and Oliner, grating anomalies can be divided into two types: a Rayleigh type and a resonance type. The Rayleigh-type anomaly characterized by its appearance just at the Rayleigh wavelength λR occurs for both polarizations, but its appearance has some differences, depending on polarization. In the case of E∥ polarization, the grazing mode is suppressed, while in case of H∥ polarization, it is not. The resonance-type anomaly can be observed only for H∥ polarization as a strong peak or a dip at the longer-wavelength side of λR. It can be explained as the result of the resonance excitation of one surface mode which corresponds to a surface plasmon, and its marked features are the extra absorption of light and the relative phase shift of the diffracted field, which are not observed in Rayleigh-type anomalies.
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