Design and fabrication of blazed binary diffractive elements with samplingperiods smaller than the structural cutoff

We report here on the theoretical performance of blazed binary diffractive elements composed of pillars carefully arranged on a two-dimensional grid w hose period is smaller than the structural cutoff. These diffractive elemen ts operate under unpolarized light. For a given grating geometry, the struc tural cutoff is a period value above which the grating no longer behaves li ke a homogeneous thin film. Because the grid period is smaller than this va lue, effective-medium theories can be fully exploited for the design, and s traightforward procedures are obtained. The theoretical performance of the blazed binary elements is investigated through electromagnetic theories. It is found that these elements substantially outperform standard blazed eche lette diffractive elements in the resonance domain. The increase in efficie ncy is explained by a decrease of the shadowing effect and by an unexpected sampling effect. The theoretical analysis is confirmed by experimental evi dence obtained for a 3 lambda-period prismlike grating operating at 633 nm and for a 20 degrees-off-axis diffractive lens operating at 860 Mm. (C) 199 9 Optical Society of America [S0740-3232(99)00105-2].