Infrared (IR) adaptation phenomena are ubiquitous in nature and biological systems. Taking inspiration from natural creatures, researchers have devoted extensive efforts for developing advanced IR adaptive materials and exploring their applications in areas of smart camouflage, thermal energy management, biomedical science, and many other IR-related technological fields. Herein, an up-to-date review is provided on the recent advancements of bioinspired IR adaptive materials and their promising applications. First an overview of IR adaptation in nature and advanced artificial IR technologies is presented. Recent endeavors are then introduced toward developing bioinspired adaptive materials for IR camouflage and IR radiative cooling. According to the Stefan-Boltzmann law, IR camouflage can be realized by either emissivity engineering or thermal cloaks. IR radiative cooling can maximize the thermal radiation of an object through an IR atmospheric transparency window, and thus holds great potential for use in energy-efficient green buildings and smart personal thermal management systems. Recent advances in bioinspired adaptive materials for emerging near-IR (NIR) applications are also discussed, including NIR-triggered biological technologies, NIR light-fueled soft robotics, and NIR light-driven supramolecular nanosystems. This review concludes with a perspective on the challenges and opportunities for the future development of bioinspired IR adaptive materials.

Beyond the Visible: Bioinspired Infrared Adaptive Materials.

Interminable surveillance and reconnaissance through various sophisticated multispectral detectors present threats to military equipment and manpower. However, a combination of detectors operating in different wavelength bands (from hundreds of nanometers to centimeters) and based on different principles raises challenges to the conventional single-band camouflage devices. In this paper, multispectral camouflage is demonstrated for the visible, mid-infrared (MIR, 3–5 and 8–14 μm), lasers (1.55 and 10.6 μm) and microwave (8–12 GHz) bands with simultaneous efficient radiative cooling in the non-atmospheric window (5–8 μm). The device for multispectral camouflage consists of a ZnS/Ge multilayer for wavelength selective emission and a Cu-ITO-Cu metasurface for microwave absorption. In comparison with conventional broadband low emittance material (Cr), the IR camouflage performance of this device manifests 8.4/5.9 °C reduction of inner/surface temperature, and 53.4/13.0% IR signal decrease in mid/long wavelength IR bands, at 2500 W ∙ m−2 input power density. Furthermore, we reveal that the natural convection in the atmosphere can be enhanced by radiation in the non-atmospheric window, which increases the total cooling power from 136 W ∙ m−2 to 252 W ∙ m−2 at 150 °C surface temperature. This work may introduce the opportunities for multispectral manipulation, infrared signal processing, thermal management, and energy-efficient applications. Manipulating electromagnetic waves to camouflage objects is an important tool. Here, the authors present a camouflage that covers a wide range of frequencies based on multilayer and metasurface technologies.

/pdf/multispectral-camouflage-for-infrared-visible-lasers-and-5fkpm79n2a.pdf

Multispectral camouflage for infrared, visible, lasers and microwave with radiative cooling.

The intent of this paper is to provide an overview of basic concepts, types, techniques, and experimental studies of the current state-of-the-art Frequency Selective Surfaces (FSSs). FSS is a periodic surface with identical two-dimensional arrays of elements arranged on a dielectric substrate. An incoming plane wave will either be transmitted (passband) or reflected back (stopband), completely or partially, depending on the nature of array element. This occurs when the frequency of electromagnetic (EM) wave matches with the resonant frequency of the FSS elements. Therefore, an FSS is capable of passing or blocking the EM waves of certain range of frequencies in the free space; consequently, identified as spatial filters. Nowadays, FSSs have been studied comprehensively and huge growth is perceived in the field of its designing and implementation for different practical applications at frequency ranges of microwave to optical. In this review article, we illustrate the recent researches on different categories of FSSs based on structure design, array element used, and applications. We also focus on theoretical breakthroughs with fabrication techniques, experimental verifications of design examples as well as prospects and challenges, especially in the microwave regime. We emphasize their significant performance parameters, particularly focusing on how advancement in this field could facilitate innovation in advanced electromagnetics.

Frequency Selective Surfaces: A Review

High-temperature infrared (IR) camouflage is crucial to the effective concealment of high-temperature objects but remains a challenging issue, as the thermal radiation of an object is proportional to the fourth power of temperature (T4). Here, we experimentally demonstrate high-temperature IR camouflage with efficient thermal management. By combining a silica aerogel for thermal insulation and a Ge/ZnS multilayer wavelength-selective emitter for simultaneous radiative cooling (high emittance in the 5–8 μm non-atmospheric window) and IR camouflage (low emittance in the 8–14 μm atmospheric window), the surface temperature of an object is reduced from 873 to 410 K. The IR camouflage is demonstrated by indoor/outdoor (with/without earthshine) radiation temperatures of 310/248 K for an object at 873/623 K and a 78% reduction in with-earthshine lock-on range. This scheme may introduce opportunities for high-temperature thermal management and infrared signal processing. The infrared signal from high-temperature structures such as jet engines and ship funnels could be significantly concealed by a composite material that reduces the radiation temperature of internally hot objects. Such “infrared camouflaging” is especially significant for military and civil applications, to reduce the threat from heat-seeking missiles and evade thermal imaging systems. Researchers in China, led by Qiang Li at Zhejiang University, built their layered thermal camouflage from a silica aerogel and a germanium/zinc sulfide blend. The highly porous aerogel forms effective insulation. The germanium/zinc sulfide layers achieve radiative cooling by emitting energy at wavelengths outside the atmosphere’s window. In tests on objects with an internal temperature of 600 °C, the camouflage material reduced the surface temperature and the radiation temperature to 137 °C and 37 °C, respectively.

/pdf/high-temperature-infrared-camouflage-with-efficient-thermal-37i3te1qaa.pdf

High-temperature infrared camouflage with efficient thermal management.

Broadband multispectral compatible absorbers for radar, infrared and visible stealth application

Flexible and Transparent Microwave–Infrared Bistealth Structure

In this communication, we propose a method to achieve high-efficiency transmission and wideband scattering reduction at two distinctive frequency bands by combining frequency-selective surface (FSS) and electromagnetic bandgap (EBG) surface. Such FSS-EBG surface is composed of two-layered metallic patterns. The bottom FSS layer works as a filter for allowing in-band signal to pass and reflecting out-of-band signal, while the top EBG layer adopts 0 and $\pi $ reflection phase cells with a chessboard distribution to reduce the backward scattering wave. As a proof-of-concept demonstration, the planar and cylindrical surfaces are, respectively, designed to verify such two functionalities. Our FSS-EBG surface can make the antenna signal transmit at S-band with small insertion loss and simultaneously provide wideband low-scattering property at X–Ku band, which is suitable to be used as the stealth antenna radome. In addition, the possibility of integrating polarization conversion function into the passing band is numerically demonstrated.

Combining FSS and EBG Surfaces for High-Efficiency Transmission and Low-Scattering Properties

https://onlinelibrary.wiley.com/doi/pdf/10.1002/admt.201970044

Metamaterials: Flexible and Transparent Microwave–Infrared Bistealth Structure (Adv. Mater. Technol. 8/2019)

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Flexible and Transparent Microwave–Infrared Bistealth Structure

Combining FSS and EBG Surfaces for High-Efficiency Transmission and Low-Scattering Properties

Metamaterials: Flexible and Transparent Microwave–Infrared Bistealth Structure (Adv. Mater. Technol. 8/2019)