Design and fabrication of sub-wavelength metal wire-grid and its application to experimental study of polarimetric imaging

Metal wire-grid whose period is far less than the illuminating wavelength exhibits strong polarization selectivity within a large field of view over a wide wave length range. Thus it could be used as the polarization-controller in the polarimetric imaging system. For infrared polarimetric imaging system, an aluminum wire-grid on the CaF substrate with period of 200 nm and depth of 100 nm was designed based on effective medium theory. The calculation results demonstrate that the designed wire-grid has the extinction ratio larger than 35 dB within a field of view ±20° over both middle wave infrared (MWIR) (3-5 μm) and long wave infrared (LWIR) (8-12 μm). The aluminum wire-grid was fabricated by electron beam lithography, reactive ion etching and plasma stripping. A clear-cut polarimetric image was obtained when wire-grid was placed in front of a MWIR thermal infrared imager.