Strip gratings at a dielectric interface and applicaton of Babinet's principle.

In this paper, we consider the transmission-line model used to calculate the transmittance of thin metallic strip gratings (at wavelengths longer than the grating period) to resolve a conflict of published expressions for the effect of a thick dielectric substrate on the equivalent circuit capacitance of capacitive gratings. By using rigorous diffraction theory we establish the correct expression and derive a modified form of Babinet’s principle for use with strip gratings on dielectric boundaries. It is found that the equivalent circuit capacitance of a strip grating on the boundary between media of refractive indices n1 and n2 is larger than its free-space value by a factor (n12+n22)/2. The result is applicable in general to the capacitive part of the equivalent circuit of grid reflectors, which are widely used at submillimeter wavelengths. A useful set of rigorously calculated transmission curves for strip gratings is presented, and these are used to establish the range of validity of the transmission-line model.

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