High efficiency reflective binary diffractive optical elements: finite conductivity

It is well known that the efficiency of binary structures can reach > 90% efficiency in TM-polarisation in near Littrow reflection [1]. For infinite conductivity, the optimal size of the depth and width of binary structures for given periods and incidence angles was given in [2]. Detailed investigations for Littrow- and non-Littrow mounts undertaken here, show that the conditions deteriorate for real conductivity: - optimum line depth is no longer quasi-constant for 0.5 < period/wavelength < 1.55, - optimum line width interval for high efficiency >95% is reduced to ½ to 1/3 of that for infinite conductivity, - incidence angle tolerance is the most restricting parameter for non-Littrow high TM-efficiency. The full period/wavelength- and depth/wavelength- dependencies of the efficiency behaviour is given for some gold gratings and the reasons for the deterioration and the effects on the design are discussed. [1] E. G. Loewen, M. Nevière, and D. Maystre (1979), "Efficiency optimization of rectangular groove gratings for use in the visible and IR regions," Appl. Opt. 18, 2262-2266. [2] E. Noponen, J. Turunen (1994), J. Opt. Soc. Am. A 11, 1097-1109.