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

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

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

Thermal camouflaging metamaterials

Refractive index of biological tissues: Review, measurement techniques, and applications.

In this paper, we present an anisotropic metamaterial (AMM) composed of a sub-wavelength metal grating sandwiched with bi-layered double-arrow-shaped (DAS) structure array, which can achieve high-efficiency circular polarization (CP) conversion via giant asymmetric transmission (AT) in terahertz (THz) region. Numerical simulation results indicate that near complete CP conversion with cross-polarization transmission coefficients can reach 0.91 and 0.93, which can be observed at 0.31 and 0.55 THz, respectively. Based on the combination of polarization conversion effects and Fabry-Perot-like cavity-enhanced effect of AMMs, the AT parameter for CP wave can reach a maximum of 0.83 at 0.30 THz, and 0.87 at 0.56 THz, respectively. With appropriate geometric parameters design of each unit-cell, embedded in the proposed AMM, the cross-polarization transmission coefficient and AT parameter for CP waves can be increased to the maximal values of 0.98 and 0.9, respectively. The proposed AMM shows great potential applications in high performance dual-band CP convertor and isolator in THz region.

Dual-band and high-efficiency circular polarization conversion via asymmetric transmission with anisotropic metamaterial in the terahertz region

Nanoparticles offer unique properties as photocatalysts with large surface areas. Under irradiation with light, the associated near-fields can induce, enhance, and control molecular adsorbate reactions on the nanoscale. So far, however, there is no simple method available to spatially resolve the near-field induced reaction yield on the surface of nanoparticles. Here we close this gap by introducing reaction nanoscopy based on three-dimensional momentum-resolved photoionization. The technique is demonstrated for the spatially selective proton generation in few-cycle laser-induced dissociative ionization of ethanol and water on SiO2 nanoparticles, resolving a pronounced variation across the particle surface. The results are modeled and reproduced qualitatively by electrostatic and quasi-classical mean-field Mie Monte-Carlo (M3C) calculations. Reaction nanoscopy is suited for a wide range of isolated nanosystems and can provide spatially resolved ultrafast reaction dynamics on nanoparticles, clusters, and droplets. Understanding light-matter interaction is important for the control of energy and charge transfer at the fundamental level. Here the authors spatially resolve proton generation in laser-induced dissociative ionization of ethanol and water on SiO2 nanoparticles and discuss the role of surface charge distribution.

/pdf/few-cycle-laser-driven-reaction-nanoscopy-on-aerosolized-1wfhcg726y.pdf

Few-cycle laser driven reaction nanoscopy on aerosolized silica nanoparticles.

The present paper describes a hyper-spectral polarization imaging system based on a non-collinear, acousto-optic tunable filter (AOTF) and a linear polarizer. The paper begins with an analysis of the equivalent relationship of the AOTF to describe the principle of polarization detection of this system. Appropriate parameters of hardware components are assigned later. This system, being electronically controllable and tunable, can not only ensure the precision of imaging but also obtain hyper-spectral polarization signatures. The prototype has two optional working modes, namely, hyper-spectral polarization imaging and hyper-spectral intensity imaging. Moreover, parameters of this system – such as diffraction efficiency, spectral resolution and modulation precision – are tested using a halogen tungsten lamp and an optical fibre spectrometer. The results indicate that this instrument is compact, vibration-insensitive, robust and precisely controllable. The system designed in this paper has further ap...

Design and Characterization of an AOTF Hyper-Spectral Polarization Imaging System

Investigation of terahertz waves propagating through far infrared/CO 2 laser stealth-compatible coating based on one-dimensional photonic crystal

On the basis of the principle of polarization detection, a hyper-spectral polarization imaging system, which is based on linear polarizer and acousto-optic tunable filter (AOTF), was designed to detect and recognize camouflage target intelligently and rapidly in this paper Our design has neither moving parts nor modulation, and has fast and electronically tuning property, so a quick scan of spectrum at 400 nm~1000 nm can be realized electronically At the same time, it not only could obtain the intensity image, hyper-spectral information, but also could acquire polarization signatures of the scene Then the spectral polarization experiment about aluminous plane which covered with the bottle green, shallow green and khaki camouflage pigments within meadow were conducted at specifically wavelength by the instrument Finally, the polarization information of the man-made targets and natural background in the scene, and the fusion image based on HIS color space were deduced through processing the experiment data The experimental result demonstrates that the polarization characteristics of camouflage pigments were different from that of natural background As the contrast of target and background could be enhanced by polarization information, the camouflage target could be identified effectively from the image according to polarization information On the other hand, the camouflage target is more obvious in the fused image Therefore, the proposed method and the system in this paper are reasonable and effective Consequently, the hyper-spectral polarization detection technique which relative to the classical intensity detection is of significance to improve the accuracy of recognition of camouflage targets in mixed background under proper detection condition

Recognition of camouflage targets with hyper-spectral polarization imaging system

Based on the characteristics of terahertz wave, the ability of THz for uncovering target covered by camouflage coating is studied. Firstly, the paper introduces the camouflage coating classification and reflectivity. Secondly, a coating reflection characteristic calculation model is established on conditions of smooth surface with methods of matrix optics. Finally, according to the calculation model, the reflectance of infrared coating and laser coating in different wavebands including 337μm terahertz wave was calculated. According to the calculating result, it’s indicated that THz has better ability to penetrate through coating than IR ; meanwhile, it’s also indicated that THz has the ability for uncovering target on some conditions.

Anti-camouflage capacity study of THz

The invention relates to an imager integrating hyperspectral and polarization hyperspectral detectability. The imager is composed of a front-end optical collimating system, an electrically-controlled rotating polarizer, an AOTF filter, a rear-end imaging optical system and an imaging detector. When the electrically-controlled rotating polarizer at the front end of the AOTF filter operates only in a vertical or horizontal state, the imager is in a hyperspectral imaging detection mode and has performance equivalent to that of a common hyperspectral imager. When the electrically-controlled rotating polarizer at the front end of the AOTF filter operates at three or more different angles for detection, the imager is in a hyperspectral polarization imaging detection mode, has polarization information acquiring ability that the common hyperspectral imager does not have, and can achieve recognition of different objects with the same spectrum. The two operating modes can be automatically switched through electrical control, and the imager can be used for conducting hyperspectral imaging detection or hyperspectral polarization imaging detection according to usage occasions and needs.

Imager integrating hyperspectral and polarization hyperspectral detectability

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