This problem is quite challenge, since your layer thickness is less than 1/10000 wavelength. The mesher might have a limit of max/min element size ratio and won't mesh it correctly if its too thin. Even though it meshed successfully, to get a accurate answer will take you a while. I assume your problem is an open problem, then the PML or ABC will have trouble for this kind of issue as well. Special techniques has to be developed to address this. Google some papers for graphine and FSS (frequency select surface) for some modeling hints.