Effective medium theory is useful for designing optical elements with a form-birefringent subwavelength structure. We describe a way to determine the effective refractive indices and the directions of their principal axes for two-dimensional (2-D) subwavelength gratings in the normal incidence case. The effective indices and the directions are calculated from finite coupled-coefficient equations that are used in 2-D rigorous coupled-wave analysis. We use the effective medium theory to calculate transmittance of several kinds of 2-D periodic subwavelength gratings. These results are compared with results of rigorous grating analysis to confirm the predictions of the effective medium theory for 2-D subwavelength gratings.
© 1998 Optical Society of America
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