Hello everyone, We are trying to fabricate a copper layer that is 41% reflective to a beam of 1521nm wavelength. The formula I have used to calculate the thickness is: R = 1 - e-αt Where R: desired reflectance of mirror t: thickness of mirror that would produce said reflectance α: material- and wavelength-specific absorption coefficient. (for copper at 1521nm: α = 8.4327e+5 cm-1, from http://refractiveindex.info/?shelf=main&book=Cu&page=Rakic) For these values, I get t=6.25nm. When they fabricated it at the lab, there was almost no reflection at all. Is the formula wrong? I based it on the transmission of a mirror being approximately T=e-αt and R being 1-T for a mirror. Is there a siginificant amount of energy absorbed by the copper itself? Your help is highly appreciated.