Taylor Robertson
Ansys Employee
Hello Lizzie,nI think that it is more accurate to simulate the gratings with FDTD. The central assumption for varFDTD is that there will not be light scattered into or out of the device plane. This is not the case with subwavelength gratings. The most accurate way of doing this would be similar to this example https://support.lumerical.com/hc/en-us/articles/360042304394-Bragg-grating-initial-design-with-FDTD.nYour mode analysis is likely pretty accurate, but you are simply taking the average effective index. This will be close to the composite effective index, but the simple average will not tell the entire story. Large variations from this would suggest to me that the varFDTD simulation is not set-up correctly. I would suggest using FDTD or EME to more accurately model this structure.nBest,n