Jeroen Dekker

Thanks for your swift reply Guilin!

-A while ago I did convergence testing for the PML and Z-boundary height (where the PMLs reside), in both cases their error was a few orders lower than that of the meshing. As a sanity check I increased the number of PML layers from 32 to 56 but this did made no difference.

- I did convergence testing in the range of 10 to 50 meshcells per wavelength with steps of 1 with conformal variant 1 method, for metasurface 1 the refining it nicely converges to the expected value and at 50 meshcells per wavelength I declared that the resonance was fully reproduced. The original work had a resonance with an electric field enhancement of 140 and in lumerical this is 135. For metasurface 2 the resonance very quickly converges to 50 at 30 meshcells per wavelength while in comsol the same metasurface gives an electric field enhancement of 85 .

- Since I do have acces to the original COMSOL files of the simulations I compared the meshing. The meshing seems to be as fine, if not finer, as their simulations.

-I set the x and y symmetry setting on, but made no difference.

- I looked at the points in this post you shared
Only point 8 I can not answer with certainty (since I am not very familiar with COMSOL). But given that the second metasurface COMSOL file is based on the first COMSOL file I would assume they are exactly the same. If the settings were wrong this should most likely not give a reproduction for metasurface 1.  

- There is a small empty region of background material below the gold layer, but testing gave virtually no difference for the resonance peak.

Honestly I am out of ideas for finding the reason for the difference. It would have been easier to accept if I was not able to reproduce metasurface 1 too.