Enhancing the spectral reflectance of refractory metals by multilayer optical thin-film coatings

Good thermomechanical properties of refractory metals, including tungsten, tantalum, molybdenum, and niobium, make them attractive candidates for operation in extreme environments. In addition, their ability to reflect thermal radiation in the infrared region beyond 1.5 mu m makes them attractive for use as reflective surfaces in extreme environments. These refractory metals, however, have relatively poor reflectivity in the visible and near-infrared spectral regions, making them absorb incident thermal radiation. Average absorption percentages of these metals at the 300-1500 nm spectrum is in the range of 40%-50%. In this paper, we propose and demonstrate that by using periodic thin-film dielectric coatings deposited over refractory metals, the absorption of thermal radiation can be drastically reduced. Various thin-film optical filter designs are investigated to engineer and improve the spectral reflectivity of refractory metals in the visible and near-infrared spectral regions without deteriorating the performance beyond 1.5 mu m. TiO2, Al2O3, and SiO2 are used as materials, which are dielectrics that do not absorb incident radiation in the visible and near-infrared with high melting points and Young's modulus. Our results indicate that a combination of several periodic segments, designed at different wavelengths around which high reflectivity is desired, can be utilized to generate high reflectivity in the broadband spectrum. Bandwidth and magnitude of the reflectivity over the spectrum are highly dependent on materials, number of segments, and number of layers in the segments of the filters. (C) 2018 Optical Society of America