Multi-band infrared camouflage compatible with radiative cooling and visible colors via a simple multilayer film structure

Abstract

Conventional infrared camouflage materials are typically confined to special spectral ranges. To address challenges posed by the combination of both active and passive detection operating in different bands, a multi-band compatible camouflage material is highly required. Combining rigorous coupled-wave analysis and genetic algorithm, a 9-layer film structure composed of 3 thermally stable materials (SiO2, Ge and TiO2) on a quartz substrate is designed, which achieves multi-band infrared camouflage compatible with radiative cooling and visible colors. Specifically, the structure exhibits low average emissivity of 0.12 and 0.21 in the two atmospheric window bands of 3∼5 µm and 8∼14 µm, while it shows spectral emissivity favorable of radiative cooling (an average of 0.67) in the non-atmospheric window band of 5∼8 µm. The structure also achieves high absorptivity at two widely applied laser wavelengths of 1.06 µm (Nd:YAG lidar) and 10.6 µm (CO2 lidar). In addition, by manipulating the thickness of the top layer, the structure exhibits tunable visible colors covering green, blue and yellow, et al., without much influence on the above selective absorption and emission in the infrared bands. The multi-band selective absorption/emission characteristic is also applicable to large incident/emission angles. The underlying physics are analyzed in detail. The selective emission in the mid-infrared band is mainly caused by the forbidden bands formed by the multilayer film, while the high absorption at both laser wavelengths is attributed to the Fabry-Perot resonance supported in the structure and the intrinsic absorption of the materials. Finally, the overall camouflage performance of the structure is evaluated to make the results more intuitive. The significance of this work lies in the balance between the multi-band compatibility and structural simplicity of camouflage materials.