How do I simulate a rectangular amplitude grating using the RCWA DLL. I used srg_trapezoid_RCWA.dll for the simulation. I set Index Grate to 1, Index Env to 0.7-7.0i to simulate transmissive and opaque slits, and both substrate and outside refractive index are set to 1. However, I found that the diffraction efficiency calculated by it is quite different from that calculated by using scalar theory in the paraxial region. Also, when I change Index Env (e.g., 0.7-20i) and Depth, the diffraction efficiency changes more significantly. How can I simulate a rectangular amplitude grating that is consistent with scalar theory?