Resonant Effects in Subwavelength Diffraction Gratings with Varying Period in the Case of Oblique Incidence

Optical properties of a resonant diffraction grating with a period varying in the periodicity direction are studied at oblique incidence of light. Using rigorous numerical simulations based on the Fourier modal method, it is shown that in the case of relatively compact varying-period gratings, the period change rate must be taken into account, and the local periodic approximation commonly used for the description of such structures becomes inapplicable. Coupled-mode equations with varying parameters are obtained for the case of oblique incidence and solved analytically in terms of the complementary error function. The predictions of the developed coupled-mode theory appear to be in good agreement with the rigorous numerical results.