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We derive closed-form expressions for the effective index of subwavelength gratings up to the fourth and the second order for TE and TM polarization, respectively. These expressions are valid for arbitrary grating structures and are a generalization of previous results obtained for lamellar gratings with one groove per period (a structure often called a two-component layered medium). The effective-medium-theory predictions are carefully validated with exact electromagnetic theories for slanted and unslanted sinusoidally modulated volume gratings and for classical mounting. It is shown that, even for large period-to-wavelength ratios near the cutoff value, the form birefringence is accurately predicted at any angle of incidence.
Lalanne et al. (Wed,) studied this question.