Design and fabrication of an encapsulated broadband infrared wire-grid polarizer for an infrared imaging system

Wire-grid polarizers (WGPs) operating in the long-wave infrared (LWIR) bands are widely used in infrared polarimetric imaging systems. The increasing complexity of modern imaging environments presents additional challenges for these polarizers. This study introduces a design method and fabrication process for an encapsulated WGP. We investigate the fabrication process of an Au/Si bilayer wire grid including holographic interference lithography, reactive etching, and the deposition of zinc sulfide (ZnS) film onto the grating structure. This film not only protects the underlying metal wire-grid structures but also enhances the transmittance as anti-reflective coating (ARC). Experimental results indicate that the average transmittance of transverse magnetic (TTM) polarized light exceeds 70% at 7–12.1 µm, with the TTM averaging over 80% between 7.6 µm and 10.4 µm. This design is particularly effective in suppressing the resonance absorption peaks that typically occur for the silicon element around 9 µm. The average extinction ratio (ER) exceeds 30 dB across the LWIR, reaching up to 35 dB within the range of 7–10.6 µm. The encapsulated polarizers are desired to improve the durability of the wire grid by sealing the metal wire-grid structures while increasing both TTM and ER.