Thermal radiation from high-temperature application conditions of infrared windows can cause serious interference with infrared imaging. To suppress the thermal radiation from the window, the modulation effects of the optical properties and curvature characteristics of the two surfaces on the thermal radiation distribution from the window are investigated, respectively. The film–substrate–film radiation system is modeled. And the backward emissivity in the different cases of surface optical properties is investigated by the application of zinc sulfide (ZnS) windows and Y2O3 thin films as an example. It is shown that the backward emissivity is suppressed when the outer surface reflectivity is lower and the inner surface reflectivity is higher. A new definition of spectral directional locational emissivity is proposed to characterize the nonplate window emissivity. A model of thermal radiation propagation from the spherical substrate is developed. The emissivity distribution of the window with different curvature characteristics is investigated. It is shown that the curvature radius of the two surfaces can be matched to change the spatial distribution of the emissivity to reduce the effect of thermal radiation on the imaging of the backward optical system.

We investigate niobium-silicon mixed films prepared by plasma-assisted reactive magnetron sputtering. Multilayer films composed of niobium-silicon oxide with various Nb fractions were fabricated. The Nb fractions were calculated using the Brüggemann model and measured by x-ray photoelectron spectroscopy. The morphology of the samples shows that the stress in mixed monolayers depends on the Nb fraction and annealing temperature. A high-reflectivity (HR) multilayer film was fabricated from two mixed-oxide materials with Nb fractions of 20% and 95%, which are optimal for stress self-compensation and a maximum difference in refractive index to facilitate film design. The residual stress of this mixed-oxide multilayer HR film is completely self-compensated through annealing. Although annealing this film redshifts the transmission spectra and increases the surface roughness of the film, the results of cavity ring-down tests indicate that the reflectance remains high in the wavelength band of interest (1064 nm) and spatially uniform over the substrate surface. This detailed characterization of these films shows that the mixed-oxide multilayer HR mirror with self-compensated stress would be appropriate in numerous applications.

Multilayer structures with a thin chromium (Cr) layer embedded in the high reflective silver layer and dielectric layers, such as SiO2 and Ta2O5, are proposed. Reflectivity can be easily manipulated by adjusting the thickness of the Cr and dielectric layers. To demonstrate the potential for enhancing the flatness of reflection spectra, multilayers with average reflectivity value of 45% and 32% in the range of 450 to 900 nm are constructed, respectively. The reflectivity value exhibits a change of less than ±2.5%, and the transmittance is nearly negligible for both multilayer architectures. They can be fabricated using the physical vapor deposition technique.

Radiative cooling is a passive cooling strategy that can radiate heat to outer space through the 8 to 13 μm waveband (atmospheric window) and is now widely used for buildings, wearable fabric, solar cells, and electronic devices. Daytime radiative cooling requires both high reflection in the solar spectrum and high absorption/emission in the 8 to 13 μm range. Previous multilayered structures were optimized by changing the thickness ratio of the layers, but the optical properties of multilayer thin films, such as absorptivity, transmissivity, and reflectivity, are determined by complex factors. In this work, several initial multilayer structures were selected and then the thickness of each layer was globally optimized; the theoretically smallest thickness with the best absorption performance was achieved in the 8 to 13 μm range, which significantly improved the cooling performance and reduced costs. We developed an optimized SiO2-Ta2O5 alternating multilayer photonic radiative cooling thin film and fabricated it using ion source assisted electron beam evaporation with an average emissivity of 0.876 within the 8 to 13 μm range and an average reflectivity of 0.963 in the 0.3 to 2.5 μm waveband; it achieved an average temperature reduction of 20.1°C lower than the uncoated substrate and 3.2°C lower than the ambient temperature under direct sunlight with an average solar power of 859.3 W/m2.

Mirror stability is crucial for their applications, especially in the far ultraviolet (FUV) where Al mirrors protected with fluoride are the preferred choice. This study evaluates the effect of storage conditions on the performance of Al/LiF/MgF2 mirrors and compares them with previous investigations of Al/LiF/eMgF2 mirrors. This study focuses on FUV reflectivity, structural characteristics, and the long-term stability of mirrors stored in various environments. Coatings were deposited on the super-polished silicon wafers using thermal evaporation. Three sets of mirrors were stored in the environment with 20% relative humidity (RH), 40% RH, and 80% RH, respectively. Additionally, three other sets were stored in vacuum, nitrogen (N2), and oxygen (O2), respectively, in sealed bags kept in a 40% RH environment. Mirrors stored in a 20% RH environment demonstrated the best performance, exhibiting the smallest decline in reflectivity and a relatively stable surface morphology. Conversely, mirrors stored in 80% RH experienced a significant reduction in reflectivity, accompanied by the most pronounced deterioration in surface morphology. It was concluded that the chemical changes and high roughness of the films contributed to the decrease in the reflectivity.