Subwavelength transmission grating retarders for use at 10.6 μm

Designs are given for gallium-arsenide subwavelength grating retarders operating at 10.6 μm. A design procedure is detailed that takes into account the reflections at all surfaces and that uses numerical optimization to improve the transmittance of the retarders to nearly 100%. It is shown that the homogeneous uniaxial layer model for subwavelength gratings can be used to provide starting points for the Nelder-Mead simplex optimization, obviating the need for stochastic optimization techniques such as simulated annealing. An analysis of the designs with respect to wavelength, angle of incidence, and fabrication tolerances indicates that such grating retarders will perform favorably compared with commercial alternatives.