Multiple Resonance Metamaterial Emitter for Deception of Infrared Emission with Enhanced Energy Dissipation

Multiresonant Selective Emitter with Enhanced Thermal Management for Infrared Camouflage

Tailoring the optical properties of metamaterials is crucial for improving the performance of infrared (IR) applications. Generally, IR camouflage materials are required to have low IR-emission properties for the detected bands (3-5 and 8-12 μm), in which IR detection is accomplished. However, the heat residue by suppressed thermal radiation degrades the thermal dissipation capacity and thermal stability of IR camouflage materials. Herein, a multilayer metal-dielectric-metal (MDM) selective emitter with high IR-emission performance in the undetected band for thermal management and low IR-emission performance in the detected band for IR camouflage is introduced. Compared to a conventional selective emitter and a low-emission material (Au film), the multiresonance selective emitter exhibited 125 and 2910% increases in heat dissipation within the undetected band, respectively. In addition, the proposed camouflage material exhibited a substantial reduction in emissive energy within the detected bands of 3-5 and 8-12 μm, with reductions of 72 and 83%, respectively, compared to that of a high-emission surface. The effectiveness of our IR camouflage was demonstrated by IR camera measurements. When the surface temperature was 360 K, the radiance temperatures of the multilayer multipeak selective emitter were 314 and 309 K for the 3-5 and 8-12 μm bands, respectively. Thermal management experiments demonstrated the enhanced thermal stability of the multiresonance selective emitter, especially in conditions of low pressure and high heat flux, when compared to that of the low-emissivity film. This work provides a practical strategy to enhance the thermal emission of a selective emitter, expanding its potential beyond IR camouflage to various energy applications.

Self-Cleaning and Spectral Selective Membrane for Sustainable Radiative Cooling

Radiative cooling materials have attracted great attention due to their superiority in energy-free cooling, especially for outdoor applications. However, outdoor cooling performance is threatened by surface pollution. Herein, we demonstrate a ternary compound system, including polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), boron nitride nanosheets (BNNS), and hydrophobic silicon dioxide (SiO2), to synchronously achieve self-cooling and self-cleaning properties through biomimetically building a lotus-like papillomatous structure. The optimized membrane has a high infrared emissivity of 0.93, a sunlight reflectivity of 97.2%, and a water contact angle of 150.5°and not only efficiently cools the object to a suitable temperature but also protects the membrane from polluting and keeps cooling for a long time. The result shows that the membrane can cool a nonfebrile object by 30.5 and 1.7 °C for noon and night, respectively, and the noon and night-time temperature drops are 10.8 and 13.5 °C for the self-heating object, compared to the bare state. Meanwhile, the membrane always keeps self-cleaning if slurry is splashed onto its surface or it is exposed to slurry. Importantly, the integration of superhydrophobic and radiative cooling properties ensures that the membrane has permanent cooling performance by protecting it from being contaminated, which is significant for outdoor applications.

Epsilon-near-zero material-based bi-layer metamaterials for selective mid-infrared radiation

Selective mid-infrared (MIR) radiation is highly desirable in many applications. However, there are still great challenges to simultaneously achieve MIR camouflage and radiative cooling utilizing simple structure. This work theoretically and experimentally proposes a bi-layer metamaterial composed of aluminum doped zinc oxide (AZO) nanoparticles embedded in Al2O3matrix on the aluminum film. The bi-layer metamaterial exhibits high performance in MIR camouflage with radiative cooling, a low emissivity (ε3-5μm= 0.11,ε8-14μm= 0.20) in atmospheric windows and a high emissivity (ε5-8μm= 0.81) in non-atmospheric windows. The interaction of the epsilon-near-zero (ENZ) mode and localized surface plasmon resonance (LSPR) mode is responsible for the perfect emission over the wavelength range of 5-8μm. Additionally, the proposed selective MIR emitter supports large-angle incidence and has great polarization insensitivity. This demonstrates that epsilon-near-zero material-based bi-layer metamaterial is highly promising for the development of selective mid-infrared radiation.

Flexible Metasurface for Microwave-Infrared Compatible Camouflage via Particle Swarm Optimization Algorithm

Metamaterials have the powerful ability to freely control multiband electromagnetic (EM) waves through elaborately designed "artificial atoms" and are hence in the limelight in various fields. Typically, camouflage materials manipulate wave-matter interactions to achieve the desired optical properties, in particular, various techniques are used for multiband camouflage materials in both infrared (IR) and microwave (MW) ranges to overcome the scale difference between these bands. However, in the context of components required for microwave communications, simultaneous control of IR emission and MW transmission is required, which is challenging owing to differences in the wave-matter interactions in these two bands. Herein, the state-of-the-art concept of flexible compatible camouflage metasurface (FCCM) is demonstrated, which can manipulate IR signatures while maintaining MW selective transmission simultaneously. For achieving maximum IR tunability and MW selective transmission, it is performed optimization using the particle swarm optimization (PSO) algorithm. Consequently, the FCCM exhibits compatible camouflage performance with both IR signature reduction and MW selective transmission is demonstrated, with 77.7% IR tunability and 93.8% transmission achieved for a flat FCCM. Furthermore, the FCCM reached the 89.8% IR signature reduction effect even in curved situations.

Optical transparent metamaterial with multi-band compatible camouflage based on inverse design

Infrared (IR) thermal camouflage and management are deeply desirable in the field of military and astronomy. While IR compatible with laser camouflage technology is extensively studied to counter modern detection systems, most existing strategies for visible light camouflage focus on color matching, which is not suitable for scenarios requiring transparency. In this work, we propose an optically transparent metamaterial with multi-band compatible camouflage capability based on the inverse design. The metamaterial consists of Ag grating, Si3N4 dielectric spacer layer, Ag reflection layer, and Si3N4 anti-reflective layer. An ideal multi-band compatible spectrum is involved in the inverse design algorithm. Calculated results demonstrate high transmittance (T0.38-0.78µm = 0.70) in the visible region, low reflectance (R1.55µm = 0.01) in laser working wavelength, high reflectance (R3-5µm = 0.86 and R8-14µm = 0.92) in the dual-band atmospheric window, and high emissivity (ɛ5-8µm = 0.61) for the non-atmospheric window. The radiative heat flux in the detected band is 31W/m2 and 201W/m2 respectively. Furthermore, the incident and polarized insensitivity of the proposed metamaterial supports applicability for practical situations. This work, emphasizes an effective strategy for conducting optically transparent design with compatible IR-laser camouflage as well as radiative cooling properties by an automated design approach.

Deep Learning Assisted Optimization of Metasurface for Multi-Band Compatible Infrared Stealth and Radiative Thermal Management

Infrared (IR) stealth plays a vital role in the modern military field. With the continuous development of detection technology, multi-band (such as near-IR laser and middle-IR) compatible IR stealth is required. Combining rigorous coupled wave analysis (RCWA) with Deep Learning (DL), we design a Ge/Ag/Ge multilayer circular-hole metasurface capable of multi-band IR stealth. It achieves low average emissivity of 0.12 and 0.17 in the two atmospheric windows (3~5 μm and 8~14 μm), while it achieves a relatively high average emissivity of 0.61 between the two atmospheric windows (5~8 μm) for the purpose of radiative thermal management. Additionally, the metasurface has a narrow-band high absorptivity of 0.88 at the near-infrared wavelength (1.54 μm) for laser guidance. For the optimized structure, we also analyze the potential physical mechanisms. The structure we optimized is geometrically simple, which may find practical applications aided with advanced nano-fabrication techniques. Also, our work is instructive for the implementation of DL in the design and optimization of multifunctional IR stealth materials.

Multispectral Optical Confusion System: Visible to Infrared Coloration with Fractal Nanostructures

Optical confusion refers to a camouflage technique assimilated with the surroundings through manipulating colors and patterns. With the advances in multispectral imagery detection systems, multispectral camouflage studies on simultaneous deceptions in the visible to infrared ranges remain a key challenge. Thus, creating pixelated patterns is essential for mimicking background signatures by assimilating both colors and patterns. In this study, a multispectral optical confusion system (MOCS) comprising pixelated silicon-based fractal nanostructures (Si-FNSs) is introduced to realize multispectral optical confusion. We analyzed the fractality of the Si-FNSs to understand the relationships between structural characteristics and optical properties with the aggregation phenomenon. The aggregation phenomenon changes the morphological heterogeneity by up to 38.5%, enabling a controllable range of visible reflectivity from 0.01 to 0.12 and infrared emissivity from 0.33 to 0.90. Visible and infrared colors were obtained by controlling the wet-etching time from 10 to 240 min and temperature from 40 to 100 °C. Finally, the MOCS consisting of pixelated Si-FNSs was designed and created by extracting the pattern from the simultaneously captured visible and infrared background images. Using the artificial backgrounds representing these images, we evaluated and compared the multispectral optical confusion performance of the MOCS with conventional camouflage surfaces.

Porous Nanostructured Composite Film for Visible-to-Infrared Camouflage with Thermal Management

Progressive advancement in modern detection technologies entails multispectral compatible camouflage. Previously, infrared camouflage materials, such as photonic crystals and metamaterials, have been developed, but improved multispectral compatibility, easy fabrication, and cost-effectiveness remain a challenge. Here, we report a nanostructured composite film based on oxalate-rich porous alumina (OPA) for visible-to-infrared compatible camouflage and simultaneous thermal management. The nanostructured composite film consists of a visible-transparent OPA layer, a composite layer of OPA/metal oxides, and an aluminum substrate. Each functional layer exhibits desirable reflection/emission properties for infrared and visible camouflage. Infrared camouflage is realized by the high reflection (low emission) of the metal substrate in both infrared-detected bands (3-5 and 8-14 μm). Meanwhile, radiative cooling arising from the intrinsic absorption of oxalate in the undetected band (5-8 μm) enhances surface heat dissipation. In addition, background-matching colors can be tuned by the metal oxides in the composite layer for visible camouflage, such as green for forest and brown for desert. This work provides a facile strategy to modulate multispectral absorption/emission properties with much flexibility and thus has great potential for energy conversion and stealth applications.

Flexible Thermocamouflage Materials in Supersonic Flowfields with Selective Energy Dissipation

Camouflage refers to a creature's behavior to protect itself from predators by assimilating its signature with the environment. In particular, thermal camouflage materials in the infrared (IR) wave are attracting interest for energy, military, and space applications. To date, several types of camouflage materials such as photonic crystals and metal-dielectric-metal structures have been developed. However, flexible camouflage materials still face challenging issues because of the material's brittleness and anomalous dispersion. Herein, we propose flexible thermocamouflage materials (FTCM) for IR camouflage on an arbitrary surface without mechanical failure. Without using a polymer as a dielectric layer, we realized FTCM by changing the unit cell structure discretely. By imaging methods, we verified their flexibility, machinability, and IR camouflage performance. We also measured and calculated the spectral emissivity of FTCM; they showed electromagnetic behavior similar to a conventional emitter. We quantified the IR camouflage performance of FTCM that the emissivity in the undetected band (5-8 μm) is 0.27 and the emissivity values in detected bands are 0.12 (3-5 μm) and 0.16 (8-14 μm) in the detected bands, respectively. Finally, we confirmed the IR camouflage performance on an arbitrary surface in a supersonic flowfield. FTCM are expected to help to improve our basic understanding of metamaterials and find widespread application as IR camouflage materials.

Multifunctional Membrane for Thermal Management Applications

Space cooling and heating consume a large proportion of global energy, so passive thermal management materials (i.e., without energy input), especially dual-mode materials including cooling and heating bifunctions, are becoming more and more attractive in many areas. Herein, a function-switchable Janus membrane between cooling and heating consisting of a multilayer structure of polyvinylidene fluoride nanofiber/zinc oxide nanosheet/carbon nanotube/Ag nanowire/polydimethylsiloxane was fabricated for comprehensive thermal management applications. In the cooling mode, the high thermal radiation emissivity (89.2%) and sunlight reflectivity (90.6%) of the Janus membrane resulted in huge temperature drops of 8.2-12.6, 9.0-14.0, and 10.9 °C for a substrate, a closed space, and a semiclosed space, respectively. When switching to the heating mode, temperature rises of 3.8-4.6, 4.0-4.8, and 12.5 °C for the substrate, closed space, and semiclosed space, respectively, were achieved owing to the high thermal radiation reflectivity (89.5%) and sunlight absorptivity (74.1%) of the membrane. Besides, the Janus membrane has outstanding comprehensive properties of the membrane, including infrared camouflaging/disguising, electromagnetic shielding (53.1 dB), solvent tolerance, waterproof properties, and high flexibility, which endow the membrane with promising application prospects.

Enhanced Broadband Plasmonic Absorbers with Tunable Light Management on Flexible Tapered Metasurface

The growing attention in solar energy has motivated the development of highly efficient solar absorbers, and a metasurface absorber with broadband optical absorption is one of the main research interests. In this study, we developed an efficient metasurface absorber on a flexible film with a simple fabrication process. It consists of a polyimide nanocone substrate coated with gold and tungsten layers, exhibiting over 96% optical absorption in the visible range and a tunable absorption performance in the long wave range. From the analysis of experiment and simulation, the enhanced optical absorption is attributed to the synergistic effects of localized nanoparticle plasmon resonance and cavity plasmon resonance, and tunable light management comes from the strong infrared reflection of a gold layer and intrinsic absorption of variable tungsten layers. Meanwhile, the polarization-independent and omnidirectional optical absorption properties are demonstrated in the fabricated absorbers. Furthermore, this absorber shows the robustness against bending, maintaining the stable and excellent absorption performance after hundreds of bending tests. Our work offers a low-cost and straightforward tactic to design and fabricate flexible solar absorbers, and this metasurface absorber is a promising candidate for many exciting applications, such as emissivity control and flexible energy-related devices.