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Hello everybody.

It would be great if anybody could give me an input. I simulate the light reflected from a copper grating. I use a 2D model in which the system consists copper substrate with one squared protrusion (size 500nm and height 150nm) and an air column above it. Floquet periodicity is applied at the boundaries and light is emitted by a periodic port at the top of the air column which is supposed to be 4 microns high. Here comes my problem. The actual air column height is a mistery for me because the spectra I obtain for total reflectance and different diffraction orders actually vary with it - namely distance between the port source and the metal surface.

I had the same issue when i was studying the reflectivity at 0 degrees angle of incidence because I was not taking into account the diffraction orders After considering them, the air height did not play a role anymore and total reflectance (and order of diffractions) plots vs wavelength would perfeclty coincide for varying air height (no difference at all). However, now I changed the incident angle to 5 deg and the problem comes back. for different air column height I have different results. Anyone knows why this happens? how to get around it? The mph file already processed can be found at the following link https://drive.google.com/file/d/1RgbtBpKSCw0MSvDZA26lbihTqacrPnBr/view?usp=sharing

Many thanks, Marco