I am simulating a photonic crystal nanobeam and determining the Q factor via the built in Q analysis group. I have noticed when I run my simulation in 2.5varFDTD the resonant wavelength found by the analysis group is always ~40-50nm larger than that found when ran in the 3D FDTD solver. I have also noticed that in my simulation for determining transmission (using a mode source at the input of the nanobeam as opposed to a dipole at the antinode) the resonant frequency wavelength according to the Q analysis group is ~5-10 nm larger than in my Q determination set up.
I am curious what the cause and significance is of this discrepency, and if it is to be expected. I would think the reso wl should be independent of the source, but I am not seeing this.
Also, given that this discrepancy is to be expected, what is the best way to confirm I am extracting the same resonant mode between simulations? Would the mode profile found via the field and profile monitor be the most useful?
Thank you in advance,