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Broadband and Ultrathin Infrared Stealth Sheets
Abstract
To effectively hide objects and render them invisible to thermographic detectors, their thermal signatures in the infrared (IR) region of the spectrum may be concealed. However, to conceal broadband spontaneous thermal emission of objects, covering the mid‐ and long‐IR is a major obstacle. Here, metallic‐dielectric nanostructures and microscale IR emitters are integrated and transferred onto thin flexible substrates to realize IR stealth sheets. The nanostructures absorb and scatter a broad band of IR wavelengths to reduce both reflection and transmission to below 5% across a wide range from 2.5 to 15.5 μm, and thus significantly attenuate the amount of IR signals propagating toward the detectors. Results show that the nanostructures with their unique properties can almost completely conceal the thermal emission from objects and blend them into their surroundings. In addition, micro‐emitters thermally isolated from the broadband absorbers can present false thermography to deceive IR detectors and heat‐sensing cameras. Thermal signatures of objects in the infrared (IR) spectrum may be concealed to defeat thermal cameras. In this study, silver nanoparticles with a high absorption coefficient are embedded between silicon nanowires with a strong antireflection ability to achieve a highly absorptive nanostructure, which is then transferred onto a plastic substrate to realize thin, lightweight, and portable IR stealth sheets.
... To control the surface emittance, nanostructure-based surfaces (e.g., metasurfaces 3,8 and metallic-dielectric nanowires 9 ) or films (metal 10 , semiconductor 11,12 , and multilayer films [13][14][15][16][17] ) are demonstrated with low-surface emittance over the whole IR range, and yet the radiative heat transfer is blocked, causing severe heat instability 18 . Wavelength-selective emitters [19][20][21][22][23][24][25] with radiative cooling [26][27][28][29][30][31] in the non-atmospheric window (5-8 μm) 18,20,32 are adopted to mitigate the heat instability without influencing the IR camouflage. ...
... The IR images of samples I and II (Fig. 3f) also show a radiation intensity reduction of sample I by approximately 15% (0.72 dB) compared with sample II in the atmospheric window (200 W/m 2 and 236 W/m 2 for sample I and sample II, respectively). The low-surface temperature and low band emittance (ε [8][9][10][11][12][13][14] contribute to the lower radiation temperature of sample I. The maximum heating temperature for samples IV and V is 573 K, beyond which the samples are oxidized (Fig. S7 in Supporting information). ...
... c Contribution to the IR signal intensity reduction of thermal insulation (orange area), low emittance (ε = 0.05) in 8-14 μm (red area) and high emittance in 5-8 μm (blue area) for Case II in (a), at different object temperatures T o with that of polished stainless steel, which shows lowsurface emittance and is extensively applied in military objectives. For the atmospheric window (8-14 μm), the band emittance (ε[8][9][10][11][12][13][14] , see band emittance in Section S3 in the Supporting information) at 573 K and the absorptance are 0.078 and 0.022, respectively, similar to the simulated absorptance of 0.094. For the non-atmospheric window (5-8 μm), the measured band emittance (ε 5-8 ) at 573 K, the measured absorptance and the simulated absorptance are 0.580, 0.545, and 0.575, respectively. ...
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.
... [25][26][27][28][29][30][31][32] The latter has the advantage of being passive and that does not require an additional energy source. Currently, infrared stealth has been mainly realized by controlling the surface emissivity using metamaterials or metasurfaces [33][34][35][36][37][38][39] , phase-changing materials [40][41][42] and stimuli-responsive structures [43][44][45][46] , resulting in adaptive thermal camouflage [47][48][49][50][51][52] and multispectral camouflage [53][54][55][56] . The thermal metamaterial approach is the main paradigm for infrared camouflage. ...
... 25,35,51,56 However, the reported works have focused on camouflaging a given object with uniform temperature. [33][34][35]37,39,57 To perfectly camouflage a continuously changing thermal field, which widely exists in practical applications, the required emissivity profile is position-dependent and varies continuously. This problem was tackled by employing a form of discretization, in which a step-wise approximation of ideal emissivity parameters was made at the sacrifice of camouflage performance. ...
Infrared camouflage based on artificial thermal metasurfaces has recently attracted significant attention. By eliminating thermal radiation differences between the object and the background, it is possible to hide a given object from infrared detection. Infrared camouflage is an important element that increases the survivability of aircraft and missiles, by reducing target susceptibility to infrared guided threats. Herein, a simple and practicable design is theoretically presented based on a multilayer film for infrared stealth, with distinctive advantages of scalability, flexible fabrication, and structural simplicity. The multilayer medium consists of silicon substrate, carbon layer and zinc sulfide film, the optical properties of which are determined by transfer matrix method. By locally changing the thickness of the coating film, the spatial tunability and continuity in thermal emission are demonstrated. A continuous change of emissive power is further obtained and consequently implemented to achieve thermal camouflage functionality. In addition, other functionalities, like thermal illusion and thermal coding, are demonstrated by thickness-engineered multilayer films.
... Besides conduction and convection, mid-infrared (IR) radiation plays a dominating role in heat loss (e.g., it occupies >50% of the heat loss of the face) (Peng and Cui, 2020;Moghimi et al., 2018). Of note, the emerging super-hygroscopic gels are propelling a series of pioneering works, e.g., digesting moisture as a new hydrogen source for clean energy generation (Zhang et al., 2020a), harvesting moisture-induced electricity energy for self-powered microrobots , collecting moisture for largescale urban rooftop farming . ...
... After samples had been exposed to 90% RH for 1 h, TGA tests were conducted using a thermal analyzer (Perkin-Elmer) with a ramping rate (2 • C min −1 ) under N 2 flow. As the maximum mid-IR radiation of black body with −73 • C, 27 • C, 87 • C occurs at 8, 10, and 15 µm, respectively (Moghimi et al., 2018), mid-IR transmittance spectra covering a wider range (3-21 µm) ...
On-face electronics (On-faceE) are urged to sense “anytime, anywhere” for enabling human senses to globally dominate the Internet of Things. However, with the limited lifetime of batteries and the miniaturization of the On-faceE, On-faceE are confronting two challenges: sustainable operation at the temperature (T) < 0 °C and frost/ice accretion security hazard. Of note, the face and On-faceE themselves continuously generate heat and are more reliable than solar-/motion-based heat supplies. However, such self-generated heat often occurs as waste heat, lost into the environment. Here, a film pioneering the intelligent conservation and utilization of this waste self-generated heat for enabling On-faceE to work even at -30 °C for 8 h is developed, through making a versatile cellulose hydrogel and enabling the super-hygroscopic gel to capture moisture at the T < 0 °C. It creates a ΔT up to 60 °C between inside T and ambient T (-30 °C) after sealing On-faceE and, importantly, intelligently stops conserving heat when the working T > 30 °C and demonstrates outstanding heat-conserving stability and sustainability, thus solving the two challenges without the expense of energy for heating and positioning it as an energy-saving “smart” T-regulating candidate. This work could address the energy shortage and heating costs for conquering low-T limitations for globalizing On-faceE.
... Metamaterial absorbers are an important developing branch of MMs and have attracted substantial research attention due to their various applications, which include radiative cooling [11] thermal energy devices [12] color filters [13] selective thermal emitters [14] and bio-sensing [15]. Moghimi et al. proposed an infrared broadband absorber for an infrared stealth device [16]. Wu et al. presented an Earth's atmospheric transparency window absorber for radiative cooling [17]. ...
... We also demonstrate that the epsilon ε xx was near zero at λ = 42.879μm and the epsilon ε yy is near zero at λ = 75.874μm. Numerical calculations showed that the average absorption reached 93.4% from 30 to 90 μm (3.3-10 THz), which to the best of our knowledge offers better absorption performance than any previously reported absorber operating in the IR to THz region [16,17,[40][41][42][43]56,57] as shown in Tables 1 and 2. Insight into the material's physical structure was gained using electric field distributions and absorbed power distribution. Different from the physical mechanism of previous met-al/dielectric sawtooth absorbers, which is owing to slow-light effect, the absorption of our proposed absorber is attributed to the localized surface plasmon resonance (LSPR), VPP. ...
We propose a biaxial hyperbolic metamaterials(HMMs) sawtooth absorber using anisotropic black phosphorus (BP) and demonstrate that it can achieve a topological transition from an elliptical dispersion to a hyperbolic dispersion. We also show the material to be an epsilon-near-zero HMM and validate an effective medium theory. Due to BP’s anisotropic permittivity and light confinement, our absorber achieves an average absorption of up to 93.4%, omnidirectionality (up to 70°), and polarization sensitivity from 30 to 90 μm. We gain physical insights about the high absorption using electric field distributions and absorbed power distribution. Unlike the physical mechanism of common multilayer metal/dielectric sawtooth absorbers, which is related to slow-light effect, the absorption of our proposed absorber is attributed to the localized surface plasmon resonance, plasmon polaritons. Our proposed HMM can be applied in the infrared to terahertz region and we conclude by providing practical guidelines for future research on biaxial hyperbolic metamaterials and anisotropic two-dimensional materials.
... 2016年, 武汉理工 大学Li等 [30] 提出了用于0.2-7.6 GHz频段的铁 氧体-超构材料复合吸波材料. 2017年, 新加坡南 洋理工大学Mou和Shen [31] 设计了一种非福斯特 图 2 吸收式隐身举例 (a)中红外波段吸收型表面遮罩 [32] , 若被隐身物体放置遮罩下方, 则反射波会被吸收而不会进入任何探 测器; (b)红外隐身/幻觉对热成像图像的影响 [33] Fig. 2. Examples of absorption cloaking. (a) A mid-infrared absorption cloaking sheet [32] . ...
... 2017年, 新加坡南 洋理工大学Mou和Shen [31] 设计了一种非福斯特 图 2 吸收式隐身举例 (a)中红外波段吸收型表面遮罩 [32] , 若被隐身物体放置遮罩下方, 则反射波会被吸收而不会进入任何探 测器; (b)红外隐身/幻觉对热成像图像的影响 [33] Fig. 2. Examples of absorption cloaking. (a) A mid-infrared absorption cloaking sheet [32] . Most of the reflected wave will be absorbed without entering any detector. ...
With the development of science and technology, the invisibility has gradually moved from a simple and plain visual deception trick to a precise and systematic modern technology system. By designing appropriate electromagnetic parameters, the novel electromagnetic wave cloaking technology is able to control the propagation and scattering of electromagnetic wave, thereby reducing the detectability of the cloaked object. The electromagnetic parameters of these novel cloaking devices can be realized by using the artificially designed nanostructures, or by combining the medium that already exists in nature. In this review, according to a detailed introduction of the research progress of novel electromagnetic wave cloaking, we discuss the difficulties and challenges in this field, and give an outlook on the future development.
... Additionally, reducing the surface temperature through radiative heat dissipation in non-atmospheric windows (undetected bands) can enhance the IR camouflage performance by mitigating heat load 8,9 . Considerable efforts have been directed toward developing thermal camouflage using various techniques in the MWIR and LWIR bands, including metallic/dielectric structures [10][11][12][13][14][15][16][17][18][19][20][21] , electrochromic [22][23][24][25][26][27][28][29] and thermochromic [30][31][32][33] materials. Additionally, progress has been made in thermal camouflage through simultaneous radiative heat dissipation in the 5-8 μm non-atmospheric window using nano-structures (e.g., photonic crystals 34,35 , metal-insulator-metal metasurfaces [36][37][38][39][40][41][42] , Fabry-Perot cavities 43,44 , anti-reflection layers [43][44][45] , and porous nanostructures 46 ). ...
Advanced multispectral detection technologies have emerged as a significant threat to objects, necessitating the use of multiband camouflage. However, achieving effective camouflage and thermal management across the entire infrared spectrum, especially the short-wave infrared (SWIR) band, remains challenging. This paper proposes a multilayer wavelength-selective emitter that achieves effective camouflage across the entire infrared spectrum, including the near-infrared (NIR), SWIR, mid-wave infrared (MWIR), and long-wave infrared (LWIR) bands, as well as the visible (VIS) band. Furthermore, the emitter enables radiative heat dissipation in two non-atmospheric windows (2.5–3 μm and 5–8 μm). The emitter’s properties are characterized by low emittance of 0.270/0.042/0.218 in the SWIR/MWIR/LWIR bands, and low reflectance of 0.129/0.281 in the VIS/NIR bands. Moreover, the high emittance of 0.742/0.473 in the two non-atmospheric windows ensures efficient radiative heat dissipation, which results in a temperature decrement of 14.4 °C compared to the Cr reference at 2000 W m ⁻² input power density. This work highlights the role of solar radiance in camouflage, and provides a comprehensive guideline for developing multiband camouflage compatible with radiative heat dissipation, from the visible to LWIR.
... In terms of practical applications, an SSA should possess both high solar spectral absorptance and low thermal emissivity in the infrared region, in accordance with Planck's law of blackbody radiation. The spectral density of thermal radiation from a blackbody absorber can be defined by [41]: ...
The selective broadband absorption of solar radiation plays a crucial role in applying solar energy. However, despite being a decade-old technology, the rapid and precise designs of selective absorbers spanning from the solar spectrum to the infrared region remain a significant challenge. This work develops a high-performance design paradigm that combines deep learning and multi-objective double annealing algorithms to optimize multilayer nanostructures for maximizing solar spectral absorption and minimum infrared radiation. Based on deep learning design, we experimentally fabricate the designed absorber and demonstrate its photothermal effect under sunlight. The absorber exhibits exceptional absorption in the solar spectrum (calculated/measured = 0.98/0.94) and low average emissivity in the infrared region (calculated/measured = 0.08/0.19). This absorber has the potential to result in annual energy savings of up to 1743 kW h/m ² in areas with abundant solar radiation resources. Our study opens a powerful design method to study solar-thermal energy harvesting and manipulation, which will facilitate for their broad applications in other engineering applications.
... Consequently, thermal camouflage can be fooled by thermal imagers, which compare the differences in temperature between the object and the background to distinguish them. A number of nanophotonic designs have been proposed based on nanostructure [6][7][8] or metal-semiconductor multilayer [9][10][11][12][13] film for controlling thermal radiation with low surface emittance across the entire IR spectrum. In addition, wavelength-selective emitters have been applied to mitigate heat instability without affecting IR camouflage in the non-atmospheric window (5-8 µm) via radiative cooling [12,14]. ...
Camouflage technology has attracted growing interest in many thermal applications. In particular, 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. Here, we proposed a coating to demonstrate high-temperature IR camouflage with efficient thermal management. This coating is a combination of hyperbolic metamaterial (HMM), gradient epsilon near zero (G-ENZ) material, and polymer. HMM makes the coating transparent in the visible range (300–700 nm) and highly reflective in the IR region, so it can serve as a thermal camouflage in the IR. G-ENZ and polymer support BE mode (at higher angles ∼50° to 90° in the 11–14 µm atmospheric window) and vibrational absorption band (in 5–8 µm non-atmospheric for all angles), respectively. So it is possible to achieve efficient thermal management through radiative cooling. We calculate the temperature of the object's surface, considering the emissivity characteristics of the coating for different heating temperatures. A combination of silica aerogel and coating can significantly reduce the surface temperature from 2000 K to 750 K. The proposed coating can also be used in the visible transparent radiative cooling due to high transmission in the visible, high reflection in the near-IR (NIR), and highly directional emissivity in the atmospheric window at higher angles, and can therefore potentially be used as a smart window in buildings and vehicles. Finally, we discuss one more potential future application of such a multifunctional coating in water condensation and purification.
... Where emissivity is the only parameter related to material, wavelength, and temperature, and the emissivity of a particular wavelength in thermal equilibrium is numerically equal to the absorption rate. Therefore, the control of thermal radiation can be achieved by dynamically regulating optical absorption using infrared electrochromic devices, which have potential applications in adaptive infrared camouflage and thermal control of spacecraft [13][14][15][16][17]. The schematics of these applications are shown in Fig. (1). ...
Background:
Electrochromic materials can dynamically change their optical properties (such as transmittance, absorbance, and reflectance.) under the action of an applied voltage, and their research and application in the visible band have been widely concerned. In recent years, with the continuous development of electrochromic technology, the related research has been gradually extended to the infrared region.
Objective:
This invited review aims to provide an overview of the current status of several inorganic infrared electrochromic materials, to provide some references for future research, and to promote the research and application of electrochromic technology in the infrared region.
Methods:
This review summarizes various research results in the field of infrared electrochromic, which includes a detailed literature review and patent search. Starting from the key performance parameters and device structure characteristics of infrared electrochromic devices (ECDs), the research and progress of several types of inorganic infrared electrochromic materials, including metal oxides, plasma nanocrystals, and carbon nanomaterials, are mainly presented, and feasible optimization directions are also discussed.
Conclusion:
We believe that the potential of these materials for civilian and military applications, for example, infrared electrochromic smart windows, infrared stealth/disguise, and thermal control of spacecraft, can be fully exploited by optimizing the materials and their devices to improve their performance.
... Two IR and one visible light camera in each module capture IR and visible light emitted or reflected from surfaces to generate 3D geometry and colored textures. The major obstacle to defeating the IR cameras is to conceal the spontaneous emission of objects over a broadband IR spectrum, most importantly two transmission windows of 3-5 μm and 8-14 μm, within which the IR radiation easily passes through the atmosphere [7]. Coating objects with materials whose IR radiation properties are outside these transmission windows should make these objects invisible to the MOVE4D systems' cameras. ...
In this white paper, the opportunities and current obstacles to use the Move 4D markerless motion tracking system (Instituto de Biomecánica de Valencia- IBV), for capturing human product interactions is discussed. In particular, this whitepaper focuses on scanning cyclists to map and improve biomechanical efficiency and aerodynamics. If successful, it could lay the technological foundation to extend current CFD (computational fluid dynamics) analyses, with flow simulations executed with a dynamically moving body and bicycle based on a high resolution and high framerate 4D mesh. Furthermore, the homologous mesh export function could allow bike fitters to use digitally standardized measurement points for biomechanical assessments. In contrast, marker-based motion tracking systems require hand placement of markers that are susceptible to faults such as placement position errors, low battery while recording, and falling off the skin. A markerless system could also improve flexibility for researchers to find new relevant measurement points due to the flexibility of digital markers. Current obstacles are revealed during an exploratory observation: 1) recording a professional time trail rider. A primary obstacle in capturing human product interactions is that the system does not provide a usable homologous mesh, due to the system's inability to separate the object, in this case, the bicycle, from the cyclist. Two follow-up observations explore the effect of coating objects, used in humanproduct interactions, with material that do not emit infrared (IR) in broadbands of 3–5 μm and 8–14 μm: 2) exploring coatings that avoid detection by IR cameras, and 3) recording a human product interaction with an object coated in a material that avoids detection by IR cameras. Results shows that these coatings on objects used in a human product interaction enables the system to capture the actor’s full dynamic movement, without the object being captured. This way a dynamically moving, watertight, and accurate homologous 4D mesh can be created of the actor.
... As different from the ideas mentioned above, hiding the thermal characteristics of objects in the broad infrared spectrum is also an essential means of achieving infrared stealth. For example, Jiang et al. pointed out that ultra-thin and lightweight sheets are used to absorb infrared radiation in broad bands [72]. Theoretically, vertically aligned carbon nanotube (VACNT) arrays are the blackest substance known to be measured, exhibiting super high absorptivity (over 99.95 %), and have been shown to have many applications. ...
Carbon materials have revolutionized the field of stealth technology with their intriguing properties, such as low weight, large specific surface area, high mechanical strength and good electrical conductivity. The advancement of infrared stealth is particularly critical in order to counter the threat of infrared detection and the deadly strike of precision guidance. The employment of carbon black, carbon nanotubes or graphene offers attractive opportunities to synthesize lightweight, versatile and intelligent infrared stealth materials. This review first introduces brief background information about infrared stealth materials, followed by the analysis of infrared stealth mechanisms. Importantly, we aim to summarize and critically evaluate the hitherto efforts in the design and preparation of carbon-based composites to facilitate the development of infrared stealth waves. The future prospects of carbon-based composites for next-generation infrared stealth materials are presented.
... But the textile fabric made from cotton and polyester has high emissivity and could easily be detected by IR cameras. To overcome this technical problem, various types of special yarns have been used and different approaches were taken to synthesize the novel finishes treating the surface of the fabric to make it less or not visible to thermal cameras (Rijavec and Bukosek 2009;Zubair 2021;Jew et al. 2018;Moghimi, Lin, and Jiang 2018;Rubežiene et al. 2013;Zhu et al. 2020). ...
In this study, graphene oxide (GO) was prepared with modified Hummer’s method while reduce graphene oxide (rGO) was prepared in the presence of hydrazine hydrate and then coated on polyester cotton (PC) blend camouflage pattern print fabric using the pad-dry-cure method. The as-synthesized GO and rGO were characterized by dynamic light scattering (DLS) analysis, X-ray diffraction (XRD), and UV-visible Spectrophotometry. The surface morphology and the thermal camouflage property of the coated fabric were investigated by scanning electron microscopy (SEM) and infrared thermal imaging test, respectively. The average particle size of GO and rGO were 99.8 nm and 0.4 nm, respectively. The fabrics treated with GO and rGO apparently reduced the emitting temperature of the human body and made it similar to the surrounding environment. The increase in the concentration of coating materials (GO and rGO) resulted in the enhancement of camouflage property. Interestingly, the camouflage property of the coated fabric was very good even after several washing cycles. Based on these scientific outcomes, the coating of PC fabric with GO and rGO is a novel and successful approach in the development of thermal camouflage technical textiles which can be potentially applied on pilot-scale experiments to establish its industrial-scale feasibility.
... Typically, objects with the same temperature usually show different infrared images on the detector due to the differences in infrared emissivity. 1 And the infrared emissivities of HMCS@f-Fe 2 O 3 -x/PEI films were measured in the 3−5 and 8−14 μm infrared wavebands, as infrared radiation is essentially opaque outside of the atmospheric infrared windows. 61,62 As a matter of fact, films with thicknesses of 30 ± 5 μm coated on aluminum (Al) plates were used as test samples ( Figure S10). Notably, the size of f-Fe 2 O 3 particles and the filling ratios of HMCS@f-Fe 2 O 3 -x nanoparticles in the PEI matrix (10, 20, 30 wt %) had no regular effect on infrared emissivities in 3−5 and 8−14 μm (Figure 9). ...
The design and development of radar--infrared compatible stealth materials are challenging in the field of broadband absorption due to the contradiction of stealth requirements and mechanisms in different frequency bands. However, hollow structures show great promise for multispectral stealth because they can lengthen the attenuation path of electromagnetic waves (EMWs) for microwave absorption, interrupt the continuity of heat-transport channels, and lower the thermal conductivity to realize infrared stealth. Here, a new morphological fabrication strategy has been developed to efficiently prepare compatible stealth nanomaterials. In a specific hydrothermal process, the confined growth of flake α-Fe2O3 (f-Fe2O3) outside of hollow mesoporous carbon spheres (HMCS) is achieved using NH3·H2O as a shape-controlled reagent. The introduction of f-Fe2O3 helps to lower infrared emissivity and improve high-frequency impedance matching, which depends on the stable dielectric property of the specific flake shape. Moreover, the size of f-Fe2O3 can be regulated by changing the constituent proportion in the hydrothermal suspension to obtain excellent performance. The minimum reflection loss (RL) of the HMCS@f-Fe2O3-6 composite is -34.16 dB at 2.4 mm, and the effective absorption bandwidth (EAB) reaches 4.8 GHz. Furthermore, the lowest emissivities of the HMCS@f-Fe2O3-6-20 wt %/polyetherimide (PEI) film in the 3-5 and 8-14 μm infrared wavebands are 0.212 and 0.508, respectively. These discoveries may pave the way for the development of radar-infrared compatible stealth materials from the perspective of microstructural design.
... IR camouflage, especially for the long wavelength infrared (LWIR) band (8)(9)(10)(11)(12)(13)(14) μm) -the most used IR detection band, usually aims at reducing the IR radiation from the target to match that from the surrounding environment. It has been realized by tuning the target's emissivity through various approaches, including paints [13,14], metal-based surfaces [15,16], graphene [17,18], phase change materials [19,20], epsilon-near-zero (ENZ) materials [21], photonic crystals [22][23][24][25][26], plasmonic structures [27][28][29], metasurfaces [30,31], metamaterials [32,33], and low-emissivity Lambertian surfaces [34] or by tuning its temperature through different thermal regulation techniques, including thermal insulation [35], Peltier plates [36], and transformation thermotics [37][38][39][40]. For visible camouflage, the modification of reflectivity is the key so that the color of the target matches the surrounding background, which can be achieved by using pigments [41], photonic crystals [42], metasurfaces [43], optical films [44], etc. ...
Multispectral camouflage, especially for the infrared-microwave range, is an essential technology for the safety of facilities, vehicles, and humans. So far, it has been realized mainly by high infrared specular reflection and high microwave absorption. However, external infrared sources can expose the target through specular reflection; also, the heat production from microwave absorption can increase the infrared radiation. This work proposes a multispectral camouflage scheme based on hierarchical visible-infrared-microwave scattering surfaces to address these issues. The proposed device exhibits: (1) low infrared emissivity (ε
8–14 μm = 0.17) and low infrared specular reflectivity (R
s
8–14 μm = 0.13), maintaining low infrared radiation and capability to overcome the presence of an external infrared source simultaneously; (2) high scattering in microwave range, with −10 dB radar cross section reduction bandwidth of 8–13 GHz, simultaneously achieving microwave camouflage and reducing the heat production; (3) tunability of color for visible camouflage. This work proposes a method to control scattering over visible-infrared-microwave bands, thereby introducing a new design paradigm for modern camouflage technology.
... In window materials, reducing the reflection of incident light on the material surface and improving its transmission is crucial, since higher transmittance can bring higher precision and better stability. In optoelectronic devices, the enhancements of absorption with anti-reflection structures decrease the losses of optical power, resulting in high energy conversion efficiencies [2][3][4][5][6]. Therefore, it is crucial to modulate and manage incident light by fabricating anti-reflection surfaces. ...
The anti-reflection properties of hard material surfaces are of great significance in the fields of infrared imaging, optoelectronic devices, and aerospace. Femtosecond laser processing has drawn a lot of attentions in the field of optics as an innovative, efficient, and green micro-nano processing method. The anti-reflection surface prepared on hard materials by femtosecond laser processing technology has good anti-reflection properties under a broad spectrum with all angles, effectively suppresses reflection, and improves light transmittance/absorption. In this review, the recent advances on femtosecond laser processing of anti-reflection surfaces on hard materials are summarized. The principle of anti-reflection structure and the selection of anti-reflection materials in different applications are elaborated upon. Finally, the limitations and challenges of the current anti-reflection surface are discussed, and the future development trend of the anti-reflection surface are prospected.
... Controlling the surface emissivity of materials is a common method for infrared stealth of high-temperature heat sources [3][4][5][6][7][8], in which low-emissivity metal/metal-like materials have been used to cover the surface of heat sources or special micro-nano structures have been constructed to deceive infrared detectors. For example, Jiang et al., integrated nanostructured metal dielectrics with miniature infrared emitters and transferred them to a thin flexible substrate to yield an infrared stealth counterpart with the reduction of the reflectance and transmittance of infrared wavelengths in the range of 2.5-15.5 μm [9]. ...
Infrared stealth technology capable of effectively rendering targets invisible to thermal infrared detectors plays an important role in defense industry and modern military. Therefore, developing novel infrared stealth materials has aroused widespread concern. Phase change materials (PCMs) with thermal storage and management capabilities can modulate temperature to hide infrared radiation, providing a plausible alternative to the rational design of infrared stealth materials. Herein, flexible phase change hydrogels with excellent mechanical and thermophysical properties are fabricated by integrating the hydrogel substrate with phase change microspheres. Owing to the interfacial effect originating from the introduction of the microspheres, the thermal conductivity and the infrared transmittance of the resulting phase change hydrogel are reduced to 0.17 W/(mK) and 0.3%, respectively, enabling the phase change hydrogel to achieve an intriguing infrared stealth function for targets in the mid-/low-temperature range (-20-60 °C). In addition to conforming to arbitrary targets owing to its excellent flexibility and adaptability, the phase change hydrogel can be facilely coated on common fabrics to yield phase change wearable devices, attaining the on-body infrared stealth. This work offers a constructive guidance to prepare high-performance flexible PCMs for emerging infrared stealth applications.
... other capabilities. These passive systems include scattering and absorbing screens that block radiation from an object and equilibrate with the environment, and thus can mimic ambient radiation [6]; enclosures with position-dependent effective thermal conductance that enables the replication of a background temperature distribution onto the enclosure exterior [7]; and coatings with non-trivial temperature-dependent emissivities that can make the emitted thermal radiation of an object not depend on temperature [8]. This last class of infrared-concealment systems-passive coatings based on temperature-dependent emissivityis the focus of this paper. ...
Both the magnitude and spectrum of the blackbody-radiation distribution change with temperature. Here, we designed the temperature-dependent spectral emissivity of a coating to counteract all the changes in the blackbody-radiation distribution over a certain temperature range, enabled by the nonhysteretic insulator-to-metal phase transition of SmNiO3. At each wavelength within the long-wave infrared atmospheric-transparency window, the thermal radiance of our coating remains nearly constant over a temperature range of at least 20 {\deg}C. Our approach can conceal thermal gradients and transient temperature changes from infrared imaging systems, including those that discriminate by wavelength, such as multispectral and hyperspectral cameras.
... In this line, microwave stealth is one prominent approach to hiding a specifically spectrum band and has been extensively investigated on recent years. 2 Radar absorbing and shielding technology have attracted a growing interest due to the recent advances in electronic warfare and detection capabilities, leaving specially infantry forces vulnerable to detection across the microwave spectrum range. Advanced battlefield and ground surveillance radar (BSR/GSR) are readily available in military markets that are highly effective, portable, and automated for large area monitoring. ...
This article proposes an analysis of the effect of combat textile cloth on human radar cross section (RCS) for microwave camouflage applications. Based on numerical simulations for the definition of the geometric profile of the human body and the measurement of electromagnetic parameters of the fabric, made by cotton and polyester, the work reveals that the uniform has a direct influence on the absorption of electromagnetic waves from a hypothetical ground surveillance radar (9.375 GHz). The results of 360° measuring human RCS showed that it can be attenuated, on average, by approximately 8 dB due to the effect of the material.
... To control the emissivity of the objects, aluminum [10], VO 2 [11] and graphene [12] were used as the surface coating material. Besides, semiconductor [13], radiative metasurface [14], multilevel thermal emission control films [15] and nanostructured sheets [16] were pasted on the surface of camouflage objects by researchers. However, emissivity control is only applicable for objects that are higher than the ambient temperature [17]. ...
Thermal infrared camouflage technology has attracted increasing attention with the progress of infrared surveillance technologies. Inspired by honeycombs, a thermal infrared camouflage film with honeycomb sandwich structure based on fluid and temperature control is proposed to cloak objects and display false targets in the thermal infrared spectrum. The structure and manufacturing method of the film are introduced, and the fluid and temperature control system is established and tested to explore the thermal infrared characteristics of the film composed of different thermal units. The camouflage performance of the film is intuitively displayed through the temperature frequency histograms of the infrared images photographed by an infrared camera. The temperature control accuracy of the thermal infrared camouflage system and the dynamic camouflage response of the film are evaluated by the temperature distribution map of each pixel in infrared images and the salient degree values. Furthermore, the influential factors on the camouflage responding time are investigated, including the heating and cooling time of the liquid, the flow rate of pumps inside the driving module, and the energy loss along the liquid lines. An application of displaying false target is also introduced and tested. The results show that the camouflage film has a well camouflage effect and a wide adjustment temperature range, showing promise for a broad range of potential applications, such as counter-surveillance, thermal control and heat shielding.
... Perfect absorbers (PA) based on metamaterials have been the subject of intensive research in recent years owing to their various potential applications in infrared stealth, daytime radiative cooling, solar energy, and thermal imaging, among others [1][2][3][4][5][6][7][8][9]. In 2008, Landy et al. reported the first metamaterial PA functioning in the gigahertz (GHz) range [10]. ...
The absorption and reflection of electromagnetic waves by various particles in the earth’s atmosphere allow the passage of only certain electromagnetic wavelengths to reach the ground, called Earth’s atmosphere transparent window. In this study, perfect absorption was theoretically obtained in the range of near-and mid-infrared earth’s atmospheric transparency window using a simple absorber with metal-dielectric-metal structure. The numerical simulations showed the average absorption to reach 96.2wavelengths from 2000 to 6000 nm. Also, the broadband absorption was noticed and attributed to combined physical mechanisms, such as anti-reflection effect, localized surface plasmon polariton, propagating surface plasmon polarization, Fabry–Pirot cavity and slow light mode. Meanwhile, the proposed absorber displayed simple-structure, low-cost, wide-angle, and polarization-independent. In sum, the proposed absorber might be useful for future applications related to atmospheric transparency window, such as remote sensing, energy harvesting, infrared detection, and stray light elimination.
... Therefore, reducing object radiation can be achieved by cooling or reducing emissivity. The main means to achieve stealth is to reduce the emissivity which causes the cost of cooling to be high [19]. There is a serious problem that the low emissivity of the whole band will affect the heat exchange process, making it difficult for heat to diffuse out in time. ...
In this paper, a similar Fabry-Perot cavity structure utilizing a multilayer film structure consisting of an ultrathin metal film is demonstrated for absorbing the infrared ray. This structure has low emissivity in the atmospheric window (3–5 and 8–14 μm) and high emissivity in the nonatmospheric window (5–8 μm). These properties improved the stealth performance which causes the high emissivity in 5–8 μm to radiate more energy to reduce its temperature. Based on this, the periodic microstructures were added to the surface of the materials that enhanced the absorption of terahertz wave (0.1–2.7 THz). The absorber based on multilayer film has a simple structure and low manufacturing cost. This work may provide a new strategy for infrared and terahertz compatible stealth technology.
... The protection of the moving objects or objects with constantly changing background using camouflage techniques requires active approach using adaptive tehniques [25][26][27][28] and development of the new materials [29][30][31] . The application of the IR smoke 32 could be considered as active camouflage technique contributing to the object hiding or cloaking. ...
Rapid advancements in EO/IR imaging technology boosted by developments in focal plane array technology led to significant increase in performance, availability and accordingly in application in the various systems. As a consequence the more efforts in the area of possible countermeasure development are necessary. Starting from EO/IR generalized image forming process and related influences on the imager performances as key part of imager performance, using knowledge generated from well-established electronic countermeasure science and known results in EO/IR countermeasure application, EO/IR countermeasures classification is proposed. Using this classification the currently known countermeasures are analyzed trying to identify existing challenges for future efforts. Also the recent advanced in image processing techniques, i.e. application of artificial intelligence for automatic target recognition, could be used as
EO/IR imaging counter-counter measure and should be considered separately.
... Notably, the "black" SiNW arrays act as an excellent absorber in the visible and UV spectral range [18,19], while the "white" samples exhibit a high level of the diffuse optical reflection in the near IR spectral region [17]. These optical properties of SiNWs are proposed to be used in a stealth sheet technology [20]. ...
In the present paper, we report on the investigation of thermal annealing (TA) effect on structural and optical properties of crystalline silicon nanowires produced by metal-assisted chemical etching approach. In particular, the impact of TA on nanowire length, relative volume and size distribution of voids is described in terms of Lifshitz-Slyozov-Wagner theory considering the TA induced Oswald ripening in the SiNW arrays. It was also found that TA leads to a decrease of the SiNWs total reflection in the wide UV-VIS-IR spectral range. The reported effects can be used for tuning of crystalline SiNWs arrays in view of their further applications in photonics related fields.
... B λ (λ, T) = 2hc 2 λ 5 1 e hc λkT −1 (1) where λ is the wavelength, T is the absolute temperature of the black body absorber, h is Planck constant, and k is the Boltzmann constant [18]. Maximum radiation for absolute temperatures, T = 1337 and 2180 K, will occur at 1.33 and 2.17 µm, respectively. ...
Here, we have theoretically proposed an ideal structure of selective solar absorber with multilayer planar films, which can absorb the incident light throughout the entire solar spectrum (300–2500 nm) and over a wide angular range, whatever the polarization angle of 0°~90°. The efficiency of the proposed absorber is proven by the Finite-Difference Time Domain (FDTD) simulation. The average absorption rate over the solar spectrum is up to 96.6%. The planar design is extremely easy to fabricate and modify, and this structure does not require lithographic processes to finish the absorbers. Improvements of the solar absorber on the basis of planar multilayer-film structures is attributed to multiple asymmetric highly lossy Fabry–Perot resonators. Because of having many virtues, such as using different refractory and non-noble metals, having angle and polarization independence, and having ideal absorption for entire solar spectrum, our proposed absorbers are promising candidates for practical industrial production of the solar-energy harvesting.
Processing transition metal carbides/nitrides (MXenes) inks into large-area functional coatings expects promising potential for electromagnetic interference (EMI) shielding and infrared stealth. However, the coating performances, especially for scalable fabrication techniques, are greatly constrained by the flake size and stacking manner of MXene. Herein, the large-area production of highly densified and oriented MXene coatings is demonstrated by engineering interfacial interactions of small MXene flakes with catecholamine molecules. The catecholamine molecules can micro-crosslink MXene nanosheets, significantly improving the ink's rheological properties. It favors the shear-induced sheet arrangement and inhibition of structural defects in the blade coating process, making it possible to achieve high orientation and densification of MXene assembly by either large-area coating or patterned printing. Interestingly, the MXene/catecholamine coating exhibits high conductivity of up to 12 247 S cm-1 and ultrahigh specific EMI shielding effectiveness of 2.0 ×10 5 dB cm2 g-1 , obviously superior to most of the reported MXene materials. Furthermore, the regularly assembled structure also endows the MXene coatings with low infrared emissivities for infrared stealth applications. Therefore, MXene/catecholamine coatings with ultraefficient EMI shielding and low infrared emissivity prove the feasibility of applications in aerospace, military, and wearable devices.
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.
Although the effective “stealth” of space vehicles is important, current camouflage designs are inadequate in meeting all application requirements. Here, a multilayer wavelength-selective emitter is demonstrated. It can realize visible light and dual-band mid-infrared camouflage with thermal control management in two application scenarios, with better effect and stronger radiation cooling capability, which can significantly improve the stealth and survivability of space vehicles in different environments. The selective emitter demonstrated in this paper has the advantages of simple structure, scalability, and ease of large-area fabrication, and has made a major breakthrough in driving multiband stealth technology from simulation research to physical verification and even practical application.
The research of metamaterial shows great potential in the field of solar energy harvesting. In the past decade, the design of broadband solar metamaterial absorber (SMA) has attracted a surge of interest. The conventional design typically requires brute-force optimizations with a huge sampling space of structure parameters. Very recently, deep learning (DL) has provided a promising way in metamaterial design, but its application on SMA development is barely reported due to the complicated features of broadband spectrum. Here, this work develops the DL model based on metamaterial spectrum transformer (MST) for the powerful design of high-performance SMAs. The MST divides the optical spectrum of metamaterial into N patches, which overcomes the severe problem of overfitting in traditional DL and boosts the learning capability significantly. A flexible design tool based on free customer definition is developed to facilitate the real-time on-demand design of metamaterials with various optical functions. The scheme is applied to the design and fabrication of SMAs with graded-refractive-index nanostructures. They demonstrate the high average absorptance of 94% in a broad solar spectrum and exhibit exceptional advantages over many state-of-the-art counterparts. The outdoor testing implies the high-efficiency energy collection of about 1061 kW h m-2 from solar radiation annually. This work paves a way for the rapid smart design of SMA, and will also provide a real-time developing tool for many other metamaterials and metadevices.
Ultrablack surfaces with stable and omnidirectional light absorption over a wide spectral range are fundamentally crucial for applications concerning strict optical requirements from high-end optical to solar-heat conversion devices. Inspired by nature, we report a needle-like array structure (NAS) prepared by spraying and self-assembling the magnetic composite ink under an external magnetic field. With high structure regularity and small feature size, the NAS presents extremely low hemispherical reflectance (≤0.3%) over a wide spectral range of 300-2000 nm and stable omnidirectional absorption (incident angle insensitivity up to 70°), which could be one of the darkest surfaces ever reported. The exciting light absorption performance can be attributed to the synergistic effects of (1) structural absorption caused by multiple scattering between array units and (2) strong forward scattering and high light absorptivity of magnetic particles. The NAS exhibits outstanding photothermal conversion for solar harvesting, self-cleaning performance, good flexibility, and thermal-aging resistance, offering an appealing alternative to construct ultrablack surfaces for practical applications.
Phase change materials (PCMs) have been particularly concerned as infrared stealth functional materials due to their superior thermal management capability. However, traditional PCMs usually behave rigid solid or flowing liquid states with fixed transition temperature, greatly limiting their application especially in multi‐band stealth and multiple scenes. Herein, an intrinsically flexible self‐healing phase change film used for synchronous visual/infrared stealth for the first time is designed and constructed. The phase change film possesses a solid–solid phase transition behavior with adjustable transition temperature (from 38.8 to 51.1 °C) and enthalpy (from 79.7 to 116.7 J g⁻¹), long‐term cycling stability (500 cycles), and outstanding flexible and self‐healing performance. Remarkably, the phase change film can be customized with different colors and various configurations to exhibit attractive visual stealth functions in multiple scenes. Additionally, owing to phase transition property, this phase change film can possess a thermal management capability and behave infrared stealth performance for objectives at various temperatures. Combining the above unique functions, the intrinsically flexible self‐healing phase change film developed in this work may show great potential for applications in the synchronous visual/infrared stealth across a wide range of scenarios and temperatures.
The modulation of thermal radiation in the infrared region is a highly anticipated method to achieve infrared sensing and camouflage. Here, a multiband metamaterial emitter based on the Al / SiO 2 / Al nanosandwich structure is proposed to provide new ideas for effective infrared and laser-compatible camouflage. By virtue of the intrinsic absorption and magnetic resonance property of lossy materials, the thermal radiation in the infrared region can be rationally modulated. The fabricated samples generally present low emissivity ( ε 3 – 5 μm = 0.21 , ε 8 – 14 μm = 0.19 ) in the atmospheric windows to evade infrared detection as well as high emissivity ( ε 5 – 8 μm = 0.43 ) in the undetected band for energy dissipation. Additionally, the laser camouflage is also realized by introducing a strong absorption at 10.6 μm through the nonlocalized plasmon resonance of the SiO 2 layer. Moreover, the fabricated emitter shows promising prospects in thermal management due to the good radiative cooling property that is comparable to the metallic Al material. This work demonstrates a multiband emitter based on the metasurface structure with compatible infrared-laser camouflage as well as radiative cooling properties, which is expected to pave new routes for the design of thermal radiation devices.
High-strength nonmetallic materials with low infrared (IR) emission are rare in nature, yet highly anticipated especially in military and aerospace fields for thermal camouflage, IR stealth, energy-saving heating. Here, we reported a high-strength (422 MPa) nonmetallic film with very low IR emissivity (12%), realized by constructing alternating multilayered structures consisting of successive MXene functionalized outer layers and continuous GO reinforced inner layers. This nonmetallic film is capable of competing with typical stainless steel (415 MPa, 15.5%), and exhibits remarkable thermal camouflage performance (ΔT = 335 °C), ultrahigh Joule heating capability (350 °C at 2 V), excellent solar-to-thermal conversion efficiency (70.2%), and ultrahigh specific electromagnetic interference shielding effectiveness (83 429 dB cm-1). Impressively, these functionalities can be maintained well after prolonged outdoor aging, and even after undergoing harsh application conditions including strong acid/alkali and boiling water immersion, and cryogenic (-196 °C) temperature.
Super broadband optical absorbers with ultrathin films have been keenly pursued for a long time. Although highly lossy materials with sharp light attenuation have the potential to become super absorbers, a large percent of light from free space is inevitably reflected back for the distinct impedance mismatch. Here, a simple strategy, of which reducing the thickness of highly-lossy thin films to minish reflectance and simultaneously folding the ultrathin films to make light multiple pass through, is proposed to obtain super broadband mid-infrared absorbers with ultrathin films. Along this line, the absorbers were prepared by depositing Al-doped ZnO film on scaffolds consisted of alumina spherical shells, whose substrates were opaque. When the thickness of Al-doped ZnO is 43 nm and the layer number of scaffolds is three, a maximum average absorptance was achieved as 97.6% over the wavelength range from 3 to 15 μm. Applying this strategy on polished Al foil, excellent infrared camouflage performance on human-body background was demonstrated. Featured by the strong broadband optical absorption with ultrathin films, flexible access to multiple substrates and low-cost procedures, this approach has the potential in widespread applications of infrared thermal emitters and optoelectronic devices.
Materials with significant infrared (IR) stealth effect have been of great importance for camouflage of military targets and thermal management of special environments. In this paper, we design a dual-working module IR stealth fabric with electroless silver plated fabric component and heat-absorbing component derived from a phase change material (PCM) coating. The aim of electroless silver plating is to reduce the IR emissivity to 0.692 (1-22 µm), 0.687 (8-14 µm) and 0.655 (3-5 µm), and the PCM coating provides a phase change latent heat of 128.5 J/g to further lower temperature. Temperature measurements show that the PCM coating can create a maximum actual temperature difference of 21.6°C with untreated fabric on a 65°C hot plate and maintain the surface actual temperature below 38℃ for 300 s. IR camera images show that the as-prepared IR stealth fabric can significantly shield or interfere with the IR heat signature of the target compared to the untreated fabric. In addition, related performance tests show that it can meet the fastness requirements for practical use. Benefiting from the modulation of the dual working modules and the good fastness, this IR stealth fabric has great potential for practical applications.
Thermal infrared camouflage as a kind of counter-surveillance technique has attracted much attention owing to the rapid development of infrared surveillance technology. Various artificial optical structures have been developed for infrared camouflage applications under cold ambient environment (low thermal radiation), but the realization of infrared camouflage under a hot environment (high thermal radiation) is also highly desirable and has been rarely reported. Here, a lithography-free, ultra-thin, high performance long-wavelength infrared (LWIR) selective emitter for thermal infrared camouflage in a high radiation environment is proposed and experimentally demonstrated. Experimental results show that our designed selective emitter exhibits average emissivity higher than 90% over the LWIR range from 8 to 14 µm and low emissivity less than 35% outside this window. Numerical simulations were performed to optimize the geometrical structures and reveal that such a selective emission effect is attributed to the combination of multiple hybrid plasmonic resonances. LWIR thermal images show that the selective emitter can perfectly blend into the high radiation backgrounds. Furthermore, it is found that the sample displays angle-independent emission properties, indicating that our emitter offers great potential for application in evading large-angle detection.
Infrared electrochromic devices (IR-ECDs) that dynamically regulate the IR optical properties of the objects have gained increasing attention in a wide range of applications such as controllable thermal management and adaptive military camouflage. In this review, detailed summaries and discussions of the recent advances and progress in IR-ECDs are presented. We demonstrate the fundamentals of IR-ECDs, including the working principle, special device configuration as distinguished from the conventional ECDs, and the main performance parameters of the device. The latest research progress on the construction and applications of IR-ECDs is summarized in detail according to the classification of inorganic EC transition metal oxides and organic EC conducting polymers. Some prospects and challenges for the further development of the IR-ECDs are also proposed. This review is anticipated to provide guidance for constructing advanced IR-ECDs towards real world applications in both civilian and military.
The multispectral electromagnetic (EM) absorber based on the frequency selective surface (FSS) has been applied with interest in the field of stealth technology due to its ability to avoid detectors or guided weapons at multiple wavelengths simultaneously. The multispectral absorption performance is generally accomplished by stacking different types of FSS absorbers in multiple layers. Herein, a thin and flexible multispectral single‐layer FSS (MSLF) absorber is presented using a micro‐holed, macroscale FSS combined with an infrared (IR) absorbing ground. The FSSs are simplified to MSLF by changing the absorbing layered microstructure to a transmitting microstructure. Dual‐band millimeter waves (MMWs) are absorbed by the macroscale resonance cavity, while the target IR waves are absorbed by the IR absorbing ground after penetrating the micro‐hole array pattern. Thermal emission is reduced significantly due to its low emissivity at IR bands other than the target IR. It is confirmed analytically and experimentally that patterns of various sizes do not exhibit functional crosstalk. The demonstrated multispectral absorption and low thermal emission make this a very promising material for IR−MMW selective bi‐stealth. Furthermore, the proposed structure can be applied to existing macroscale patterns in order to increase their applicability by providing additional selectivity without any functional interference. A thin and flexible multispectral single‐layer frequency selective surface absorber is presented using a micro‐holed, macroscale frequency selective surface combined with an absorbing ground. Dual‐band millimeter waves (MMWs) and target near‐infrared (IR) signals are absorbed while the thermal emission is reduced significantly. The demonstrated multispectral absorption and low thermal emission make this a very promising material for IR−MMW selective bi‐stealth.
Infrared camouflage is crucial for high-temperature objects to avoid detection, and spontaneous infrared radiation is also an important way for high-temperature objects to dissipate heat. Therefore, selective infrared emission has become significant for the coating design of surfaces such as aircraft, which require low emission in the atmospheric window band (3–5 µm and 8–14 µm) and high emission outside it (5–8 µm). This Letter employs a simple multilayer film structure to achieve selective regulation of the material emission spectrum. Combining the transfer matrix method and genetic algorithm, a multilayer film structure containing 12 layers of three high-temperature-resistant materials ( S i O 2 , T i O 2 and Ge) has been designed. It shows fairly low emissivity in two main bands of infrared detection ( ε 3 ∼ 5 µ m = 0.14 , ε 8 ∼ 14 µ m = 0.21 ) and high emissivity outside them ( ε 5 ∼ 8 µ m = 0.86 ), and this infrared selectivity can be well maintained with the incident angle rising from 0 to 60 deg. The Poynting vector distribution in the material at different incident wavelengths is analyzed to further explore the interference mechanism to achieve spectral selective emission. The significance of this work lies in the construction of a relatively simple coating design while ensuring efficient infrared camouflage and thermal management performance.
A compact tunable graphene-based broadband metamaterial absorber in the terahertz (THz) frequency band was presented in this paper. The proposed absorber is the classic sandwich structure, which is composed of graphene cross resonators of different sizes placed on the top of a dielectric spacer backed with a gold ground at the bottom. It is shown that the absorber can achieve a broad absorption band from 2.81 THz to 4.67 THz with absorption up to 97 % under the normal incident. Meanwhile, the absorption property can be kept to a moderate level when the incident angle is up to 45° for both TE and TM incident light, and the absorption is polarization insensitive. By increasing the chemical potential of graphene from 0.40 eV to 0.60 eV, the broad absorption band can be tuned to shift blue with slightly changing maximum absorption rate. What's more, the absorption bandwidth can be further extended to over 2.34 THz with absorption larger than 97 % by adding another graphene cross layer. Based on the optimal and novel characteristics, we hope that the designed absorber may have potential applications in THz absorbing, sensing, switching, modulating, stealthy and so on.
The development of ultrathin, flexible, large‐scale, high‐temperature‐tolerant infrared camouflage devices, which are immune to the external environment, has emerged as an important unsolved challenge. This paper proposes an infrared camouflage device based on the Lambertian surface. The proposed device simultaneously exhibits low emissivity (≈0.1), low specular reflectance (≈0.05), and high temperature (290 °C) tolerance over a broad infrared range (0.75–25 µm). Furthermore, the proposed device is ultrathin (≈50 µm), highly flexible, scalable, and can be fabricated at a low cost. The experimental results show that while camouflaging a target (at 65 °C), the proposed Lambertian surface can reduce the peak value of the target‐background contrast by 68.4% (indoor case) and 76.0% (outdoor case) compared to the conventional low‐e (low‐emissivity) smooth surface. The calculated detection range of the proposed low‐e Lambertian surface is 60% less than that of both the low‐e smooth surface and the blackbody. This work proposes a novel method to simultaneously control the radiation and the reflection, thereby introducing a new design paradigm for modern camouflage technology and energy harvesting applications. Infrared camouflage with the immunity to external heat sources is realized by utilizing an ultrathin (≈50 µm), highly flexible, scalable, and high‐temperature‐tolerant (290 °C) low‐emissivity (≈0.1) Lambertian surface, which can reduce the calculated detection range by 60%, compared to a highly emissive target or a low‐emissivity smooth camouflaging device under the influence of an external heat source.
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. Infrared adaptation phenomena are ubiquitous in nature and biological systems. This review provides a state‐of‐the‐art account on emerging bioinspired infrared adaptive materials and their potential applications. The perspectives for future scope, challenges and opportunities on such a multidisciplinary topic are outlined.
Thermal camouflage technologies, which aim at blending the infrared (IR) signature of targets into the background to counter the IR detection, have witnessed increasing development. To achieve thermal camouflage, the rule of thumb is to balance the thermal radiation between the target and the background, and the corresponding conductive strategy is to tune the local temperature field while the radiative strategy is to tune the local emissivity. Following these two basic strategies, the thermal metamaterials and wavelength-selective emissivity engineering to achieve thermal camouflage are first introduced. Then the more advanced dynamic strategies are reviewed that can adapt to the varying environment under the external stimuli, like electricity, light, strain, chemical, wetting, temperature, etc. Particularly the phase-changing and bioinspired materials are presented and reviewed. Finally, critical considerations on the challenges and opportunities of next-generation thermal camouflage technologies are elaborated and four future directions are cast, including temperature-responsive emissivity engineering, soft materials, multispectral camouflage, and detection-feedback system. Overall, a detailed introduction to the working principle, the state-of-the-art progress, and the critical thinking on the future development on thermal camouflage technologies are presented.
Investigations of interactions of highly toxic substances with various materials by methods of theoretical chemistry are of considerable interest for prediction of their behavior, as it is difficult to investigate experimentally. In this contribution, adsorption of two hazardous organophosphate compounds-dimethoate and sarin-on graphene, has been investigated using density functional theory (DFT), slab model and a plane wave basis set. Graphene features were represented through three different surface models in a 3x3 supercell-pristine graphene, graphene with a monovacancy (MV) defect and graphene with an oxydefect. Adsorption energies of both dimethoate and sarin on all three surfaces were calculated for several adsorption geometries. Adsorption energies of investigated molecules on pristine graphene and oxydefect surfaces were of an order of magnitude 0.5-1 eV, while adsorption on monovacancy defect surface was dissociative, and much stronger-more than 3 eV per molecule. Obtained results are in good agreement with previous theoretical predictions considering adsorption of atoms and simple molecules on graphene, implying to the good expected reliability of the proposed model, and representing a step forward in theoretical modeling of adsorption of hazardous molecules on carbonaceous materials.
The proliferation of electronic equipment and wireless communication technology has resulted in great convenience while simultaneously giving rise to the issue of electromagnetic interference (EMI). The deleterious effects of EMI on both unshielded electronics and human health have driven researchers to explore highly efficient electromagnetic (EM) wave absorbing and shielding materials to suppress EM radiation. Most reported powder EM wave absorbing and shielding (EMAS) materials are required to be further mixed with matrices to produce the composite coating layers. The complicated processing procedures restrict the application of these materials. Furthermore, such layers often suffer from poor durability. However, if EMAS materials are functionally and structurally integrated into a macrostructure, these drawbacks may be overcome. More importantly, EMAS materials with macrostructures can be integrated with other functions to satisfy ever-growing application demands in harsh environments. This review article discusses the design principles of advanced EMAS materials in terms of their macrostructures and multifunctions. Representative integrated macrostructures of EMAS materials and devices are introduced in detail. The multifunctionalities of some advanced EMAS materials, such as wearable, hydrophobic, and thermal insulating characteristics, are discussed as well. In the end, the current challenges and future directions for developing multifunctional EMAS materials are discussed.
The implementation of complete infrared (IR) camouflaging/disguise technology requires three-dimensional (3D) thermal camouflage that can respond rapidly to changes in the ambient temperature and operate uniformly at various angles. In this study, a fiber-based IR conductive material was developed that responds sensitively and rapidly to changes in the ambient temperature. Moreover, the material can be woven into various patterns, enabling the IR camouflage of a 3D wearable garment. A full-color IR radiation image is produced by generating three different IR intensities based on the voltage applied to a far-IR fiber. The use of passive camouflage technology enables objects to be hidden from their backgrounds, while active camouflage technology permits objects to be camouflaged against changing surrounding backgrounds or to be recognized as arbitrary objects (e.g., car, animal, or plant). It is expected that this differentiated camouflaging/disguise technology based on IR imaging can be applied to various industrial, military, and new scientific fields.
Cyclooxygenase-2 isozyme is a promising anti-inflammatory drug target, and overexpression of this enzyme is also associated with several cancers and neurodegenerative diseases. The amino-acid sequence and structural similarity between inducible cyclooxygenase-2 and housekeeping cyclooxygenase-1 isoforms present a significant challenge to design selective cyclooxygenase-2 inhibitors. Herein, we describe the use of the cyclooxygenase-2 active site as a reaction vessel for the in situ generation of its own highly specific inhibitors. Multi-component competitive-binding studies confirmed that the cyclooxygenase-2 isozyme can judiciously select most appropriate chemical building blocks from a pool of chemicals to build its own highly potent inhibitor. Herein, with the use of kinetic target-guided synthesis, also termed as in situ click chemistry, we describe the discovery of two highly potent and selective cyclooxygenase-2 isozyme inhibitors. The in vivo anti-inflammatory activity of these two novel small molecules is significantly higher than that of widely used selective cyclooxygenase-2 inhibitors.
Signaling through the Ror2 receptor tyrosine kinase promotes invadopodia formation for tumor invasion. Here, we identify intraflagellar transport 20 (IFT20) as a new target of this signaling in tumors that lack primary cilia, and find that IFT20 mediates the ability of Ror2 signaling to induce the invasiveness of these tumors. We also find that IFT20 regulates the nucleation of Golgi-derived microtubules by affecting the GM130-AKAP450 complex, which promotes Golgi ribbon formation in achieving polarized secretion for cell migration and invasion. Furthermore, IFT20 promotes the efficiency of transport through the Golgi complex. These findings shed new insights into how Ror2 signaling promotes tumor invasiveness, and also advance the understanding of how Golgi structure and transport can be regulated.
The creation of anti-reflective surfaces is reliant on the engineering of the surface textures and patterns to enable efficient trapping or transmission of light. Here we demonstrate anti-reflective layers composed of hierarchical nano/microscale features that are prepared on Si using a combination of wet and dry etching processes, and which are both scalable and affordable. The performance of the structured surfaces was tested through optical measurements of the reflectance, transmittance, and scattering spectra from the visible to mid-infrared wavelength regions, and the results were verified using numerical simulations to identify the performance of the textured anti-reflective layers. The anti-reflective properties of the layers were shown to be dramatically improved by the composite nanostructured surfaces over a broad spectral range, which thus provides a basis for the design rules that are essential for the progress towards effective anti-reflector fabrication. At normal incidence, the hierarchical surfaces achieve reflectances that are 10–80 times lower than that of conventional single-etch nano-microstructures. Portions of the absorbed, transmitted, scattered, and reflected light in the visible-IR spectrum are presented to illustrate the results.
Wrap-around invisibility cloak
An invisibility cloak can be used to conceal an object from view by guiding light around it. Most cloaks developed so far have bulky structures that are difficult to scale up for hiding large objects. To design a thin invisibility cloak that can be wrapped around an object such as a sheet or skin, Ni et al. designed a two-dimensional metamaterial surface. Such flexible, highly reflective materials could be manufactured at large scale to hide large objects.
Science , this issue p. 1310
In this study we report novel silicon nanowire (SiNW) array structures that have near-unity absorption spectrum. The design of the new SiNW arrays is based on radial diversity of nanowires with periodic diamond-like array (DLA) structures. Different array structures are studied with a focus on two array structures: limited and broad diversity DLA structures. Numerical electromagnetic modeling is used to study the light-array interaction and to compute the optical properties of SiNW arrays. The proposed arrays show superior performance over other types of SiNW arrays. Significant enhancement of the array absorption is achieved over the entire solar spectrum of interest with significant reduction of the amount of material. The arrays show performance independent of angle of incidence up to 70 degrees, and polarization. The proposed arrays achieved ultimate efficiency as high as 39% with filling fraction as low as 19%.
Controlling the flow of light at subwavelength scales provides access to functionalities such as negative or zero index of refraction, transformation optics, cloaking, metamaterials and slow light, but diffraction effects severely restrict our ability to control light on such scales. Here we report the photon transport and collimation enhanced by transverse Anderson localization in chip-scale dispersion-engineered anisotropic media. We demonstrate a photonic crystal superlattice structure in which diffraction is nearly completely arrested by cascaded resonant tunnelling through transverse guided resonances. By modifying the geometry of more than 4,000 scatterers in the superlattices we add structural disorder controllably and uncover the mechanism of disorder-induced transverse localization. Arrested spatial divergence is captured in the power-law scaling, along with exponential asymmetric mode profiles and enhanced collimation bandwidths for increasing disorder. With increasing disorder, we observe the crossover from cascaded guided resonances into the transverse localization regime, beyond both the ballistic and diffusive transport of photons.
Significance
Artificial systems that replicate functional attributes of the skins of cephalopods could offer capabilities in visual appearance modulation with potential utility in consumer, industrial, and military applications. Here we demonstrate a complete set of materials, components, fabrication approaches, integration schemes, bioinspired designs, and coordinated operational modes for adaptive optoelectronic camouflage sheets. These devices are capable of producing black-and-white patterns that spontaneously match those of the surroundings, without user input or external measurement. Systematic experimental, computational, and analytical studies of the optical, electrical, thermal, and mechanical properties reveal the fundamental aspects of operation and also provide quantitative design guidelines that are applicable to future embodiments.
Metal-enhanced fluorescence of molecular probes in plasmonic nanostructures offers highly sensitive chemical and biomedical analyses, but a comprehensive theory of the phenomenon is far from being complete. In this study, a systematic theoretical analysis is provided for overall luminescence enhancement/quenching for fluorophores near silver spherical nanoparticles. The approach accounts for local intensity enhancement, radiative and nonradiative rates modification, light polarization, molecule position, and its dipole moment orientation. Numerical modeling has been performed for fluorescein-based labels (e.g., Alexa Fluor 488) widely used in biomedical studies and development. The maximal enhancement exceeding 50 times is predicted for nanoparticle diameter 50 nm, the optimal excitation wavelength being 370 nm. For long-wave excitation, bigger particles are more efficient. The experiments with a fluorescein isothiocyanate conjugate of bovine serum albumin confirmed theoretical predictions. The results provide an extensive and promising estimate for simple and affordable silver-based nanostructures to be used in fluorescent plasmonic sensors.
To cast no shadow in a murky medium
Cloaks can hide objects from view, but what about their shadows? Schittny et al. devised a cloak to remove even the shadow of an object embedded in a murky medium in front of a computer screen (see the Perspective by Smith). They engineered a core-shell structure within which an object could be hidden and tailored the optical properties of the cloak to match that of the medium. Light was routed around the cloak, leaving no trace of the hidden object.
Science , this issue p. 427 ; see also p. 384
Reflection is a natural phenomenon that occurs when light passes the interface between materials with different refractive index. In many applications, such as solar cells or photodetectors, reflection is an unwanted loss process. Many ways to reduce reflection from a substrate have been investigated so far, including dielectric interference coatings, surface texturing, adiabatic index matching and scattering from plasmonic nanoparticles. Here we present an entirely new concept that suppresses the reflection of light from a silicon surface over a broad spectral range. A two-dimensional periodic array of subwavelength silicon nanocylinders designed to possess strongly substrate-coupled Mie resonances yields almost zero total reflectance over the entire spectral range from the ultraviolet to the near-infrared. This new antireflection concept relies on the strong forward scattering that occurs when a scattering structure is placed in close proximity to a high-index substrate with a high optical density of states.
We have designed and realized a three-dimensional invisibility-cloaking structure operating at optical wavelengths based on
transformation optics. Our blueprint uses a woodpile photonic crystal with a tailored polymer filling fraction to hide a bump
in a gold reflector. We fabricated structures and controls by direct laser writing and characterized them by simultaneous
high–numerical-aperture, far-field optical microscopy and spectroscopy. A cloaking operation with a large bandwidth of unpolarized
light from 1.4 to 2.7 micrometers in wavelength is demonstrated for viewing angles up to 60°.
Artificially structured metamaterials have enabled unprecedented flexibility in manipulating electromagnetic waves and producing new functionalities, including the cloak of invisibility based on coordinate transformation(1-3). Unlike other cloaking approaches(4-6), which are typically limited to subwavelength objects, the transformation method allows the design of cloaking devices that render a macroscopic object invisible. In addition, the design is not sensitive to the object that is being cloaked. The first experimental demonstration of such a cloak at microwave frequencies was recently reported(7). We note, however, that that design(7) cannot be implemented for an optical cloak, which is certainly of particular interest because optical frequencies are where the word 'invisibility' is conventionally defined. Here we present the design of a non-magnetic cloak operating at optical frequencies. The principle and structure of the proposed cylindrical cloak are analysed, and the general recipe for the implementation of such a device is provided.
Invisibility devices have captured the human imagination for many years. Recent theories have proposed schemes for cloaking devices using transformation optics and conformal mapping. Metamaterials, with spatially tailored properties, have provided the necessary medium by enabling precise control over the flow of electromagnetic waves. Using metamaterials, the first microwave cloaking has been achieved but the realization of cloaking at optical frequencies, a key step towards achieving actual invisibility, has remained elusive. Here, we report the first experimental demonstration of optical cloaking. The optical 'carpet' cloak is designed using quasi-conformal mapping to conceal an object that is placed under a curved reflecting surface by imitating the reflection of a flat surface. The cloak consists only of isotropic dielectric materials, which enables broadband and low-loss invisibility at a wavelength range of 1,400-1,800 nm.
The ability to render objects invisible using a cloak - not detectable by an external observer - for concealing objects has been a tantalizing goal1-6. Here, we demonstrate a cloak operating in the near infrared at a wavelength of 1550 nm. The cloak conceals a deformation on a flat reflecting surface, under which an object can be hidden. The device has an area of 225 um2 and hides a region of 1.6 um2. It is composed of nanometre size silicon structures with spatially varying densities across the cloak. The density variation is defined using transformation optics to define the effective index distribution of the cloak. Comment: 12 pages, 5 figures
Invisibility and negative refraction are both applications of transformation optics where the material of a device performs a coordinate transformation for electromagnetic fields. The device creates the illusion that light propagates through empty flat space, whereas in physical space, light is bent around a hidden interior or seems to run backward in space or time. All of the previous proposals for invisibility require materials with extreme properties. Here we show that transformation optics of a curved, non-Euclidean space (such as the surface of a virtual sphere) relax these requirements and can lead to invisibility in a broad band of the spectrum.
Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.
Spontaneous emission is not an inherent property of a luminescent material; rather, it arises due to interaction between the material and its local electromagnetic environment. Changing the environment can thus alter the emission rate, with potential applications in sensing, integrated photonics and solar energy conversion. Significant increases in emission rate require an optical resonator that stores light in as small a volume as possible, for as long as possible. This is currently achieved using two main systems: photonic crystal microcavities and plasmonic metal nanoparticles. These two systems have largely been developed independently, but the underlying physical mechanisms are the same. Comparing the two provides insight into emission modification and illustrates some of the subtleties involved in interpreting experimental results.
We introduce a solid material that is itself invisible, possessing identical
electromagnetic properties as air (i.e. not a cloak) at a desired frequency.
Such a material could provide improved mechanical stability, electrical
conduction and heat dissipation to a system, without disturbing incident
electromagnetic radiation. One immediate application would be towards perfect
antenna radomes. Unlike cloaks, such a transparent and self-invisible material
has yet to be demonstrated. Previous research has shown that a single sphere or
cylinder coated with plasmonic or dielectric layers can have a dark-state with
considerably suppressed scattering cross-section, due to the destructive
interference between two resonances in one of its scattering channels.
Nevertheless, a massive collection of these objects will have an accumulated
and detectable disturbance to the original field distribution. Here we overcome
this bottleneck by lining up the dark-state frequencies in different channels.
Specifically, we derive analytically, verify numerically and demonstrate
experimentally that deliberately designed corrugated metallic wires can have
record-low scattering amplitudes, achieved by aligning the nodal frequencies of
the first two scattering channels. This enables an arbitrary assembly of these
wires to be omnidirectionally invisible and the effective constitutive
parameters nearly identical to air. Measured transmission spectra at microwave
frequencies reveal indistinguishable results for all the arrangements of the
3D-printed samples studied.
Spontaneous emission is not an inherent property of a luminescent material; rather, it arises due to interaction between the material and its local electromagnetic environment. Changing the environment can thus alter the emission rate, with potential applications in sensing, integrated photonics and solar energy conversion. Significant increases in emission rate require an optical resonator that stores light in as small a volume as possible, for as long as possible. This is currently achieved using two main systems: photonic crystal microcavities and plasmonic metal nanoparticles. These two systems have largely been developed independently, but the underlying physical mechanisms are the same. Comparing the two provides insight into emission modification and illustrates some of the subtleties involved in interpreting experimental results.
Significance
Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices. In this work, we introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way for high-performance, large-scale integrated photonic circuits.
Thermal camouflage and cloaking can transform an actual heat signature into a pre-controlled one. We demonstrate a viable recipe of controlling and manipulating heat signatures using thermal metamaterials to empower cloaking and camouflage in heat conduction. The thermal signature of the object is thus metamorphosed and perceived as multiple targets with different geometries and compositions, with the original object cloaked.
In nature, cephalopods employ unique dynamic camouflage mechanisms. Herein, we draw inspiration from self-assembled structures found in cephalopods to fabricate tunable biomimetic camouflage coatings. The reflectance of these coatings is dynamically modulated between the visible and infrared regions of the electromagnetic spectrum in situ. Our studies represent a crucial step towards reconfigurable and disposable infrared camouflage for stealth applications.
Infrared absorption enhancement is observed as occurring at silver colloidal particles spread on an infrared transparent substrate. Its applicability to qualitative analysis was tested by comparing the transmission spectra of 9,10-anthraquinone (AQ) on a KRS-5 plate with and without silver colloidal particles. The largest enhancement factor observed was about 100, and the detection limit for AQ appeared to be less than 10 ng/cm2. The scanning electron micrograph indicated that the colloidal silver film consisted of irregularly shaped islands. AQ does not possess a functional group reactive to silver, so that the infrared absorption enhancement could be assumed to occur through the long-range electromagnetic enhancement mechanism.
A number of infrared target simulation methods are presented and a new method is proposed. The proposed method is based on printing a large number of miniature-size resistors in a resistor matrix. Access to each resistor is obtained by selecting the appropriate row and column of the matrix. Application of a potential difference to a resistor raises its temperature and infrared energy is emitted. Owing to the small physical size of the resistors, they have appreciable thermal dynamics, i.e. their heat-up and cooling times are of the order of 40 msec. As a result, it is possible to obtain effects of target movement or changes in target dimensions by switching groups of appropriate resistors on and off. The system is controlled by a micro-computer. The target can be moved either by hand or along programmed pathways. Its thermal image can be projected by means of an appropriate optical system onto a thermal image camera.
Refractive index data and some extinction coefficients are provided for the infrared region for the following materials: silicon, germanium, zinc sulfide, cadmium telluride, zinc selenide, silica, calcium fluoride, magnesium fluoride, aluminum oxide, magnesium oxide, aluminum, gold and silver. The dependence of these optical constants on wavelength, temperature, crystal form, film preparation technique, radiation and other factors is included.
The permittivity and permeability behaviors of composites made from the multiwalled carbon nanotubes with magnetic impurity Ni and the wax have been studied in 3–18 GHz. The unusual permittivity dispersion behaviors have been explained based on the Cole-Cole model and the conductivity contribution model. Permeability is found to have negative imaginary parts within 3–11 GHz. The composites are found to show good microwave absorbing performances (reflection loss <−20 dB): matching thickness is 1.5 mm and absorbing frequency band is 11.6–12.4 GHz, and the absorbing performance can be explained by the “geometrical effect.”
Invisibility is a notion that has long captivated the popular imagination. However, in 2006, invisibility became a practical matter for the scientific community as well, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm, now termed transformation optics. Since the advent of transformation optics and subsequent initial demonstration of the microwave cloak, the field has grown rapidly. However, the complexity of the transformation optics material prescription has continually forced researchers to make simplifying approximations to achieve even a subset of the desired functionality. These approximations place profound limitations on the performance of transformation optics devices in general, and cloaks especially. Here, we design and experimentally characterize a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics.
The statistical properties of the sky-, forest- and cities-background noise for the four atmospheric windows in the intermediate infrared spectral region (2–14 μm) were analysed. There are the two kinds of spectral regions where the statistical properties about the amplitude distribution of the background radiance are quite different: one is the 2–3 μm region, being dominated by the scattering sunlight, the other is the 8–14 μm region, where the thermal radiation from the background is predominant. In the latter, the background noise is well described by the statistical model based on the random pulse process with the Gaussian amplitude and Poisson's width distribution. In the former, both the amplitude and the width of the random pulse follow the Poisson's statistics. The unified statistical model for the background noise including the whole intermediate infrared spectral region was derived. Its validity was confirmed with some experimental results.
Here, we theoretically suggest the possibility of employing a multilayered plasmonic shell as a cloak for reducing the total scattering cross section of a particle, simultaneously at different frequencies in the optical domain. By exploiting the frequency dispersion of plasmonic materials and their inherent negative polarizability, it is shown, theoretically and with numerical simulations, how covering a dielectric or conducting object of a certain size with this multilayered cloak may reduce its "visibility" by several orders of magnitude simultaneously at multiple frequencies.
We discuss here the use of a graphene monolayer to realize the concept of "cloaking by a surface", proposing the thinnest possible mantle cloak with operation in the far-infrared and terahertz (THz) regime. We show that an atomically thin graphene monolayer may drastically suppress the scattering of planar and cylindrical objects and, at the same time, preserve moderately broad bandwidth of operation. In addition, we exploit the large tunability of the graphene conductivity to provide active, dynamically tunable invisibility cloaks and versatile THz switching devices.
We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
Silicon is a high refractive index material. Consequently, silicon nanowires (SiNWs) with diameters on the order of the wavelengths of visible light show strong resonant field enhancement of the incident light, so this type of nanomaterial is a good candidate for all kinds of photonic devices. Surprisingly enough, a thorough experimental and theoretical analysis of both the polarization dependence of the absorption and the scattering behavior of individual SiNWs under defined illumination has not been presented yet. Here, the present paper will contribute by showing optical properties such as scattering and absorption of individual SiNWs experimentally in an optical microscope using bright- and dark-field illumination modes as well as in analytical Mie calculations. Experimental and calculation results are in good agreement, and both reveal a strong correlation of the optical properties of individual SiNWs to their diameters. This finding supports the notion that SiNWs can be used in photonic applications such as for photovoltaics or optical sensors.
Hydrogenated amorphous Si (a-Si:H) is an important solar cell material. Here we demonstrate the fabrication of a-Si:H nanowires (NWs) and nanocones (NCs), using an easily scalable and IC-compatible process. We also investigate the optical properties of these nanostructures. These a-Si:H nanostructures display greatly enhanced absorption over a large range of wavelengths and angles of incidence, due to suppressed reflection. The enhancement effect is particularly strong for a-Si:H NC arrays, which provide nearly perfect impedance matching between a-Si:H and air through a gradual reduction of the effective refractive index. More than 90% of light is absorbed at angles of incidence up to 60 degrees for a-Si:H NC arrays, which is significantly better than NW arrays (70%) and thin films (45%). In addition, the absorption of NC arrays is 88% at the band gap edge of a-Si:H, which is much higher than NW arrays (70%) and thin films (53%). Our experimental data agree very well with simulation. The a-Si:H nanocones function as both absorber and antireflection layers, which offer a promising approach to enhance the solar cell energy conversion efficiency.
Using the freedom of design that metamaterials provide, we show how electromagnetic fields can be redirected at will and propose
a design strategy. The conserved fields—electric displacement field D, magnetic induction field B, and Poynting vector B—are all displaced in a consistent manner. A simple illustration is given of the cloaking of a proscribed volume of space
to exclude completely all electromagnetic fields. Our work has relevance to exotic lens design and to the cloaking of objects
from electromagnetic fields.
A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency
band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was “hidden”
inside a cloak constructed according to the previous theoretical prescription. The cloak was constructed with the use of artificially
structured metamaterials, designed for operation over a band of microwave frequencies. The cloak decreased scattering from
the hidden object while at the same time reducing its shadow, so that the cloak and object combined began to resemble empty
space.
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