We demonstrate a controllable electromagnetic wave reflector/absorber for different polarizations with metamaterial involving electromagnetic resonant structures coupled with diodes. Through biasing at different voltages to turn ON and OFF the diodes, we are able to switch the structure between nearly total reflection and total absorption of a particularly polarized incident wave. By arranging orthogonally orientated resonant cells, the metamaterial can react to different polarized waves by selectively biasing the corresponding diodes. Both numerical simulations and microwave measurements have verified the performance.

Switchable metamaterial reflector/absorber for different polarized electromagnetic waves

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.

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.

Hollow CuS microflowers anchored porous carbon composites as lightweight and broadband microwave absorber with flame-retardant and thermal stealth functions

Flexible and Transparent Microwave–Infrared Bistealth Structure

In this communication, a broadband and tunable radar absorber is proposed by integrating graphene capacitor with resistive frequency-selective surfaces (RFSS). The microwave reflectivity can be dynamically controlled by changing the effective sheet resistance of graphene through an electrostatic field bias. The simulated results indicate that this structure can tune its reflectivity over a wide frequency band ranging from 2.9 to 15.9 GHz under the normal incidence, when the graphene resistance is varied from 120 to $700~\Omega $ /sq. The tuning range of the average reflectivity is from −4.6 to −15.3 dB. The physical mechanism of this tunable absorber is discussed by studying the electric field distribution and power loss density. We fabricated this absorber and measured its tunable reflectivity by applying different biasing voltages. The experimental results are in a good agreement with those of the simulation results. In addition, this absorber is also verified to have a good angular stability and polarization insensitivity, which will have broad prospects in the application of stealth technology.

Broadband and Tunable Radar Absorber Based on Graphene Capacitor Integrated With Resistive Frequency-Selective Surface

Multifunctional and Tunable Radar Absorber Based on Graphene‐Integrated Active Metasurface

Graphene‐Integrated Reconfigurable Metasurface for Independent Manipulation of Reflection Magnitude and Phase

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

Jinqiang Peng

Papers

Flexible and Transparent Microwave–Infrared Bistealth Structure

Broadband and Tunable Radar Absorber Based on Graphene Capacitor Integrated With Resistive Frequency-Selective Surface

Multifunctional and Tunable Radar Absorber Based on Graphene‐Integrated Active Metasurface

Graphene‐Integrated Reconfigurable Metasurface for Independent Manipulation of Reflection Magnitude and Phase

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