I know this thread is already some months old, but it was the only one that I found here concerning with the Q-factor calculation of L3 photonic crystal cavities in membrane waveguides.
I am a student and new to FDTD simulations in general and to the extraction of Information about the Q-Factor of those cavities with lumerical, so apologies if any of my questions are answered elsewhere or are kind of obivous.
What I am trying to simulate is this structure and material system published by Nakamura at al, and to reproduce and confirm their simulations: DOI:10.1364/OE.24.009541. I will attach my attempt on modeling this scenario in the end of my post.
The radius of their holes is r = 108 nm, with a period length of r = 420 nm in a slab waveguide with thickness of t = 220 nm and a refractive index of n = 3.46. All these parameters should provide operation of this cavity in the 1.55 µm telecommunication band.
They start from an unaltered L3 cavity and observe the Q-Factor of the (ground) mode in respect to shifting specific holes around the cavity, including the well known increase in Q-Factor by shifting the two inner cavity holes.
Now to my problem. They mention a Q-Factor of 5300 already for the unaltered design (whereas I just got values of something of around 2700) and if I shift the two holes by their proposed amount of 0.2*a I get suddenly negative Q-Factors (in this case -139505), which makes obviously no sense.
As I said I have just started to use lumerical and what I have chosen here for the settings is mainly based on the cavity tutorial for the H1 cavity that is shown on your website, with the only difference, that I am using just one source, right in the center of the cavity and Qanalysis in the center of the cavity, too. The mesh sizes are dx = 0.1a, dy = 0.1asqrt(3)/2 and dz = 0.1t. Maybe I have missed something important?
Nakamura_silicon_L3.fsp (3.7 MB)