Amrita Pati
Ansys Employee

Hi Gukhyun,

I apologize for late reply. 

Q1. I ran some quick tests, and my comparison looks like this:

 I agree that there must be much better agreement between the FDTD and RCWA results. I am running some more tests and I will get back to you soon. In the meantime, we have a similar: Photonic Crystal Slab (RCWA)  you may have a look for more information on the general setup.

Q2. We have been hearing similar concerns about the index result. I was in touch with the R&D team, I was told that while looking at the index result, we should be checking it in the transverse plane. For example, if the propagation direction is y, we should be looking at the index in the x-z plane for different layers along the y-axis by changing the following fields (change x-parameter to Plot x-axis change z-parameter to Plot y-axis and change y-parameter Slice):

But since this is a 2D RCWA simulation, doing that should just give us a line. I am still in talks with the Development Team. And I will get back to you on this as well.