In a recent paper [J. Opt. Soc. Am. A 16, 1108 (1999)] Logofǎtu et al. demonstrated by experimental and numerical evidence that the 0th-order cross-polarization (s to p and p to s) reflection coefficients of isotropic, symmetrical, surface-relief gratings in conical mount are identical. Here an analytical proof is given to show that the observed identity is merely a manifestation of the electromagnetic reciprocity theorem for the 0th-order diffraction of symmetrical gratings. The above result is further generalized to bianisotropic gratings, to the 0th-order cross-polarization transmission coefficients, and to the -order reflection and transmission coefficients when the wave vector of the incident plane wave and the negative of the wave vector of the reflected order are symmetrical with respect to the plane perpendicular to the grating grooves.
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