Thank you for the comment. I did not mention that the power monitors are at the periodic boundaries (though if the unit cell is asymmetric, there will be power flow at the boundary). The monitors are acually within my structure at different points. The question is how can I get transmitted power in different parts of a simulation when I use the RCWA, not just reflection and transmission? The field and power monitors seem to not work in RCWA the way they work in FDTD. Sorry if it was not clear!