Do you mean this graph:
left or right? might be the right graph?
As long as you can get the dispersion relation and the Rabi splitting is in the dispersion relation, I see no reason you cannot get it, as long as you have correct settings for the simulation.
I guess the change of the depth leads to the shift of the Rabi splitting frequency. In the caption of the right figure it says "b Simulated 2D contour plot of the reflectance for various NG depths" it means you need to simulate the reflectance by sweeping the depth and then image it. Please note accurate reflection can only be obtained with plane wave, at specific K value (so it is angled incidence, you will need to use Bloch BC in the plane of angle) with long simulation to let the signal decay. It will not be like the band structure simulation.
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