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 has significantly improved 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 made of expensive, fragile, brittle, and heavy inorganic materials. ORMOCHALC materials provided superior MWIR transmission compared to other carbon-based polymers as the IR vibrational absorption from the major component (i.e., S – S bonding) is negligible. While the refractive index of these polymer increases as the chalcogenide (i.e., S) content increases, it results in reduced Young’s modulus and lower glass transition temperature, thus compromising the structural stability and integrity of ORMOCHALC optical devices.In this study, an ORMOCHALC polymer, poly(S-r-DIB) was reinforced with the zinc sulfide (ZnS, diameter ~7.7 nm) nanoparticles, to tune the refractive index, and to improve the thermomechanical properties. Then, subwavelength wire-grid polarizers were fabricated based on the optically and mechanically reinforced composites. The addition of nanoparticles significantly improved the thermomechanical properties, as the glass transition temperature (Tg) improved from 9.6 oC of the pure polymer to 31.4 oC for 20 wt% reinforced ZnS ORMOCHALC. The TGA results also showed that material degradation occurs at a higher temperature for ZnS reinforced ORMOCHALC. The complex refractive indices of composites characterized by the Infrared Variable Angle Spectroscopic Ellipsometer (IR-VASE) also show a significant increment in refractive index­ of Δn = 6.58 % by adding 20 wt% of ZnS to the polymer. The MWIR polarizers were fabricated by the thermal imprint with a grating mold on a thin film of ZNS-ORMOCHALC composite on top of a DSP (double side polished) silicon wafer, and subsequent Au deposition. The MWIR polarizers were characterized by Fourier-transform infrared (FTIR) spectroscopy measurement. The measured extinction ratio (the ratio of the transmission, TTM/TTE) was found to be competitive with commercial polarizers. Moreover, the polarizers show a spectral selectivity due to the refractive index tuning of the ORMOCHALC composite. The resonance of the transmitted-reflected curve was tuned by 5.6%, as the resonant wavelength moves from 3.12 µm to 3.3 µm for the pure polymer to 20% ZnS reinforced ORMOCHALC composite. These polarizers with superior extinction coefficient, spectral selectivity, and improved thermomechanical stability demonstrate a border application in MWIR optics.

Highly Efficient Mid-Wavelength Infrared (MWIR) Polarizer by Ormochalc Composite With Improved Thermomechanical Stability and Spectral Selectivity

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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