Tunable mid-wavelength infrared (MWIR) polarizer by ORMOCHALC composite with improved thermomechanical stability

Mid-wavelength infrared (MWIR, λ = 3 – 5 μm) materials are of great importance due to their applications in optical sensors and devices for military, industry, and non-invasive medical diagnostics. Specifically, MWIR polarimetry are used in biometric recognition and camouflaged detection. Recently, sulfur based organically modified chalcogenides (ORMOCHALC) polymers have been utilized to fabricate MWIR polarizers with competitive extinction coefficient to commercial polarizers, which are mostly made of expensive, brittle, and heavy inorganic materials. On the other hand, to adjust or reinforce the refractive index of the polymer, the chalcogenide content (i.e., sulfur, selenium) needs to be increased, which may result in adverse effects on the thermomechanical characteristics, including Young's modulus and lower glass transition temperature. This reduced thermomechanical stability compromises the structural integrity of ORMOCHALC-based optical devices. In this study, an ORMOCHALC polymer, poly(S–r–DIB), was reinforced with the zinc sulfide (ZnS) nanoparticles to simultaneously improve the refractive index and the thermomechanical properties. The addition of 20 wt% ZnS nanoparticles improves the pure polymer's glass transition temperature (Tg) from 9.6 °C to 31.4 °C, and the refractive index by Δn= 6.58 %. Then, subwavelength wire-grid polarizers were fabricated based on the pure ORMOCHALC first by a simple thermal imprinting method followed by metal deposition. The resulted MWIR polarizer showed a high extinction coefficient which is comparable with commercial polarizers. However, the structural integrity of the polarizers was compromised due to lower glass transition temperature. Finally, finite element analysis was conducted on the possible wire-grid polarizer fabrication utilizing optically and mechanically reinforced composites, as demonstrated in this study. If fabricated, these nanoparticles reinforced polarizers will possess superior structural integrity compared to ORMOCHALC polarizers. Moreover, these polarizers show a spectral selectivity as the transmission curve's resonance wavelength depends on the composite's refractive index, which is tunable by controlling the nanoparticles content. The resonance peak redshifted from 3.12 μm to 3.3 μm for the pure polymer to 20% ZnS reinforced ORMOCHALC polarizer. The extinction coefficient of the polarizer also improved in multiple wavelength bands in the MWIR. These polarizers with superior extinction coefficient, spectral selectivity, and improved thermomechanical stability demonstrate a broader prospect in MWIR optics.