Simplified modal method for slanted grating

We report the simplified modal method for the slanted grating based on the accurate dispersion equation. The vividly physical insight is presented to interpret the diffraction process for slanted grating. We also present that the simplified modal method with the two-lowest mode condition is effective for a large slanted angle up to 26°. By examining the eignefunction, the mode index, and the two-lowest mode condition, we provide new evidences to verify the assumption that a slanted grating with subwavelength period can be analyzed as an equivalent rectangular grating using the simplified modal method, which is right and convenient to use for a small slanted angle up to 20°. Numerical simulations of the simplified modal method are coincident with rigorous coupled wave analysis for small slanted angle gratings. Thus the simplified modal method can be used for small slanted angle grating since the equivalence of slanted grating and rectangular grating is verified due to its vivid physical analysis.