Impedance mismatch-based enhancement of broadband reflectance of tungsten with bio-inspired multilayers

Official URL: https://dx.doi.org/10.1016/j.jqsrt.2021.107899

Optical coatings composed of thermomechanically superior materials with high reflectance in the broadband spectrum have recently attracted attention. Tungsten (W), among other VIB group materials, suits well for applications that require high thermomechanical stability but suffer from low reflectivity below 1 µm. This paper proposes a bio-inspired multilayer structure that lowers Tungsten's absorption over a broadband spectrum. The proposed design mimics the patterns observed on Morpho butterfly wings and reduces the absorption of W over a broad spectrum. At the initial stage, the impedance mismatch method is implemented to optimize the number and thickness of the homogeneous multilayers over the broadband spectrum. Then, the proposed pattern is incorporated into the homogenous structure. The effect of inhomogeneities on the transverse directions is investigated. The resulting spectral characteristics of both homogeneous and inhomogeneous structures are analyzed by wave impedance analysis. The total optical path was extracted with this analysis. Using this analysis, the mechanisms that trigger sharp reflection dips in homogenous layers are uncovered. S-parameters of the inhomogeneous structures are extracted and evaluated. Our findings indicate that the increased rate of change of the S-parameters phase in inhomogeneous structures is responsible for increased dips over the spectrum.