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.

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.

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.

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.

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

Artificial camouflage surfaces for assimilating with the environment have been utilized for controlling optical properties. Especially, the optical properties of infrared (IR) camouflage materials should be satisfied with two requirements: deception of IR signature in a detected band through reduced emissive energy and dissipation of reduced emissive energy for preventing thermal instability through an undetected band. Most reported articles suggest the reduction of emissive energy in the detected band; however, broadband emission for enough energy dissipation through the undetected band simultaneously is still a challenging issue. Here, we demonstrate the multiresonance emitter for broadband emission with IR camouflage utilizing the electromagnetic properties of dielectric material. We reveal that the interaction between the magnetic resonance and dielectric layer's property in a metal-dielectric-metal structure induces the multiple resonance at the specific band. We present an IR camouflage behavior of multiresonance emitter on a curved surface through the IR camera (8-14 µm). We evaluate the energy dissipation in the undetected band, which is 1613% higher than metal and 26% higher than conventional selective emitters. This study paves the way to develop broadband emitters for radiative cooling and thermophotovoltaic applications.

Metamaterial-Selective Emitter for Maximizing Infrared Camouflage Performance with Energy Dissipation.

Camouflage is a method evading predators in nature by assimilating into the environment. To realize an artificial camouflage surface for displays and sensors, many researchers have introduced several concepts including a metamaterial-selective absorber/emitter (MSAE). When an MSAE is adopted for camouflage at infrared (IR) wave, the energy dissipation of reduced emitting energy, as well as the reduction of emitting energy to deceive the IR signature from the surface, must be considered from the viewpoint of energy balance due to thermal instability. The integrated investigation of radiative heat-transfer characteristics and IR signature control of MSAE remains, however, poorly understood. Here, we investigate MSAE for IR camouflage by considering the energy balance in terms of reduction of emitting energy and dissipation of reduced emitting energy. On the basis of the atmospheric transmittance at an IR band, we designate the detected band as having wavelengths of 3-5 and 8-14 µm and the undetected band as having a wavelength of 5-8 µm. We investigate, via experiments and simulations, the optical characteristics required for IR camouflage and extract the factor that controls the emissive power. Furthermore, we suggest an integrated factor for evaluating the camouflage performance based on the concept of energy balance and propose a design guideline for MSAE with the aim of maximizing the camouflage performance at the IR band. This study will help to expand the range of applications (such as energy harvester and sensors) and others that are based on selective absorption/emission.