A systematic procedure using the effective index method and impedance matching has recently been developed Appl. Opt. 26, 3123 ( 1987)] for the design of antireflection high-spatial-frequency rectangular-groove gratings on lossy materials including high conductivity metals. The design procedure in turn can be used as a starting point to design antireflection metallic gratings with lower spatial frequencies using rigorous coupled-wave analysis. These lower spatial-frequency gratings have the advantage of being easier to fabricate. In the present work, a particular antireflection gold grating design (having a period of 1.0 μm, a filling factor of 50%, and a groove depth of 147.5 nm for use at a freespace wavelength of 500 nm, normal incidence, and polarization parallel to the grooves) was fabricated and its diffraction characteristics experimentally measured. The grating indeed showed very nearly zero specular reflection in the blue region of the spectrum. Unlike previously reported antireflection anomalies, the effect is broadband occurring over a broad range of wavelengths and angles of incidence, and for both orthogonal polarizations. This work clearly shows that the systematic design of zero specular reflection grating surfaces is possible.
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