I am trying to simulate the reflectance/absorptance of a metallic grating. The system consists of rectangular Au grating, with a thin polymer layer on top. Above the polymer is air. I have attached the mph file.
The problem is that when I change the thickness (height) of the air layer, the reflectance vs incident angle graph changes, The first absorptance peak (smallest incident angle) stays the same, but the (smaller) peaks at larger incident angle change with the air layer thickness. When I kept the air layer thickness the same and change the thickness of the Au bulk, the results are identical even for absorptance peaks at larger incident angles. This seems strange as I expect changing the thickness of the semi-infinite layers to have no impact of the reflectance.
Are there some problems in my simulations, or am I missing some important physics (i.e. the reflectance does change with air thickness)?
thanks