The increasing sophistication of multispectral surveillance and reconnaissance technologies poses a significant threat to military assets and personnel. Existing camouflage technologies typically focus on a single spectral band (visible, infrared, or microwave), rendering them inadequate against multi-band detection systems. This paper addresses this challenge by developing a multispectral camouflage device capable of concealing objects from detection across a wide range of electromagnetic wavelengths. The device must meet several demanding criteria: low emissivity in the MIR atmospheric windows (3–5 µm and 8–14 µm) to evade thermal imaging and heat-seeking missiles; characteristic reflection in the visible range (380–780 nm) for background matching; high absorbance in the microwave radar band (8–12 GHz) to reduce the radar cross-section (RCS); and high absorbance at specific laser wavelengths (1.55 µm and 10.6 µm). Furthermore, efficient radiative cooling in the non-atmospheric window (5–8 µm) is crucial to mitigate heat build-up from absorbed energy, which is exacerbated by the need for low emissivity in the MIR atmospheric windows. Previous work has addressed aspects of multispectral camouflage, but a material satisfying all these criteria simultaneously has been lacking. This study aims to fill this gap by designing and fabricating a device that incorporates wavelength-selective emission for the visible and infrared regions, along with a microwave-absorbing metasurface. The effectiveness of the device will be evaluated through a comprehensive analysis of its thermal and electromagnetic properties.

The authors extensively review existing camouflage technologies, highlighting the limitations of single-band approaches in the face of advanced multispectral detection systems. They discuss various methods for manipulating electromagnetic waves across the visible, infrared, and microwave spectrums, noting the challenges of achieving compatibility across such a broad range. The literature review emphasizes the importance of radiative cooling to mitigate heat buildup from absorbed energy, particularly in military applications where high-temperature objects need camouflage. The use of wavelength-selective emitters and metamaterials for radiative cooling and camouflage in individual spectral bands is also extensively discussed, setting the stage for the novel approach presented in this study, which aims to achieve multispectral camouflage with radiative cooling in a single device.

The researchers designed a multispectral camouflage device consisting of two key components: a Ge/ZnS multilayer wavelength-selective emitter (SE) and a Cu-ITO-Cu microwave absorptive metasurface. The Ge/ZnS multilayer acts as a one-dimensional photonic crystal, creating forbidden bands in the mid-wave and long-wave infrared atmospheric windows, thus minimizing emissivity in these ranges. Simultaneously, it allows high emissivity in the non-atmospheric window (5–8 µm) to promote radiative cooling. The visible spectrum reflectance is controlled by the thickness of the top ZnS layer, enabling background color matching. Laser absorption at 1.55 µm and 10.6 µm is achieved by manipulating the thicknesses of specific Ge/ZnS layers. The Cu-ITO-Cu metasurface, fabricated using common printed circuit board (PCB) techniques, effectively absorbs microwave radiation in the X-band (8–12 GHz). The thicknesses of the Ge/ZnS layers were optimized using a genetic algorithm to maximize performance across all spectral bands. The fabrication process involved E-beam evaporation for the multilayer film deposition. The optical characterization involved using a spectrophotometer for visible/NIR, an FTIR microscope with spectrometer for MIR, and a vector network analyzer for microwave reflectance measurements. The thermal characterization involved using MWIR and LWIR cameras to measure radiation temperature and thermocouples/thermal resistors to measure absolute temperature under both normal pressure and vacuum conditions. The heat transfer simulations were performed using COMSOL Multiphysics, along with simulations of the microwave metasurface.

The fabricated device demonstrated effective multispectral camouflage across the visible, MIR, laser, and microwave bands. The Ge/ZnS multilayer SE achieved low emittance (0.11/0.12) in the MWIR/LWIR bands and high emittance (0.61) in the non-atmospheric window, allowing for simultaneous IR camouflage and radiative cooling. The visible reflectance could be tuned to match different background colors by adjusting the thickness of the top ZnS layer. The Cu-ITO-Cu metasurface achieved high absorbance (>0.9) in the X-band. Compared to a conventional broadband low-emittance Cr film, the SE resulted in a significant reduction in both surface and internal temperature (8.4/5.9 °C at 2500 W m⁻² input power density). In IR camouflage experiments, the SE exhibited 53.4% and 13% reductions in IR signal intensity in the MWIR and LWIR bands, respectively. The study also demonstrated that radiative cooling in the non-atmospheric window enhances natural convection, leading to increased cooling power. This enhanced cooling power is particularly relevant in applications where forced convection is limited. The combination of the Ge/ZnS multilayer and the Cu-ITO-Cu metasurface achieved an absorbance higher than 85% in the microwave range, even after combining them.

The results demonstrate the successful realization of a multispectral camouflage device that effectively conceals objects from detection across a wide range of electromagnetic wavelengths, while also actively mitigating heat buildup through radiative cooling. The key to this success lies in the careful design and optimization of the Ge/ZnS multilayer structure, which simultaneously enables low emissivity in the IR atmospheric windows and high emissivity in the non-atmospheric window for efficient cooling. The use of a separate microwave-absorbing metasurface further enhances the device’s multispectral capabilities. The finding that radiative cooling in the non-atmospheric window enhances natural convection provides valuable insights into thermal management strategies for camouflage and other applications. The ease of fabrication using established techniques such as E-beam evaporation and PCB processing suggests significant potential for large-scale applications.

This research presents a significant advance in multispectral camouflage technology. The developed device successfully integrates IR camouflage, laser camouflage, microwave camouflage, and radiative cooling into a single structure, addressing the limitations of existing single-band approaches. The findings on the enhancement of natural convection by radiative cooling offer new avenues for thermal management. Future research could focus on further optimizing the device’s performance, exploring different materials and structural designs, and investigating the device's effectiveness under diverse environmental conditions. The scalability of fabrication techniques suggests potential applications in various fields beyond military camouflage, including thermal management and energy-efficient technologies.

While the device demonstrated significant improvements in multispectral camouflage and radiative cooling, certain limitations exist. The experiments were conducted under controlled laboratory conditions; further testing in realistic field environments would be beneficial. The optimization of the device’s performance across all spectral bands was achieved via genetic algorithm, which can be computationally intensive. Future studies could explore more efficient optimization methods. The current design might not be optimal for all possible background scenarios; additional work could focus on adaptability and tunability to different environmental conditions and background colors.