Broadband tunable laser and infrared camouflage by wavelength-selective scattering metamaterial with radiative thermal management.

Metamaterial-based multispectral (including infrared and multiple lasers) camouflage compatible with non-atmospheric window radiative cooling is effective for low observability against multiple detection means. However, simultaneously achieving low reflectance in a non-atmospheric window band and broadband laser scattering, especially for a broadband tunable long-wave infrared laser, remains challenging. This Letter proposes a wavelength-selective scattering metamaterial (WSSM) that realizes effective camouflage for mid-wave infrared (MWIR), long-wave infrared (LWIR), broadband tunable LWIR and near-infrared (NIR) lasers. Moreover, the WSSM achieves radiative cooling in a non-atmospheric window (5-8 µm). The simulated emissivity is 0.19/0.20 in MWIR and LWIR bands, while it is 0.54 in a non-atmospheric window band that ensures radiative cooling. The WSSM also achieves low specular reflectance (4.35%) in 8-12 µm for broadband tunable laser camouflage, together with low reflectance at 1.06 µm and 1.55 µm. The thermal simulation is also conducted, demonstrating that the WSSM has a surface temperature decrement of 12.6°C compared to the conventional low-emissivity reference at the heated temperature of 400°C due to selective emission. The radiation temperatures have a reduction of 37%/64% than the real surface temperature in MWIR and LWIR bands. This work achieves the multispectral compatible camouflage by regulating specular reflection and scattering, providing a novel, to the best of our knowledge, approach for manipulating electromagnetic waves.

Biomimetic multilayer film simulating solar spectrum reflection characteristics of natural vegetations for optical camouflage.

The camouflage for developed hyperspectral detection technology, which can accurately distinguish the spectrum between object and background, has emerged as an important unsolved challenge. In this study, a biomimetic film (Ge/ZnS multilayer structure) for optical camouflage of hyperspectral and laser with color simulation has been proposed and experimentally demonstrated. By taking advantage of the wavelength selective property of Ge/ZnS multilayer through film interference, the biomimetic film which can simulate the reflection spectral characteristics of vegetation background and eliminate laser signal has been realized based on inverse design. The selective narrowband absorption can manipulate the contrary condition for hyperspectral camouflage (high reflectance in 0.8-1.3 µm) and laser camouflage (low reflectance at 1.06 µm) in the same waveband. The planarized biomimetic multilayer film presents several distinct advantages: (1) elaborate simulation of vegetation reflectance spectrum for hyperspectral camouflage (the spectral similarity coefficient of 92.1%), and efficient absorption at 1.06 µm for laser camouflage (reflectance of 17.8%); (2) tunable color chrominance of various vegetation types for visual camouflage; (3) thermally robust camouflage performance (up to 250 °C) due to temperature endurable property of Ge and ZnS. The hyperspectral-laser camouflage film expands the design strategy of optical camouflage application.

Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation.

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO2) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO2 between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO2, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Conceptual radar trap model realized via polarization conversion metasurface.

General metasurfaces (MSs) can realize low observability of radar by manipulating the polarization mode and transmission direction of the electromagnetic (EM) waves. Here, we propose the radar trap model to realize EM wave imprisonment. This three-layer model is composed of the transmission polarization converter, the connected dielectric substrate and the reflection polarization converter. Using Jones calculation as a guide, we optimized the geometric parameters of the upper and lower layers to realize specific polarization conversion functions. The middle layer is regarded as the support and matching layer. On this basis, the combined radar trap model can realize the imprisonment of EM waves between upper and lower layers, which is attributed to the cooperative effect of asymmetric transmission and polarization conversion. We further verified the feasibility and correctness of our investigations through two kinds of model designs based on linear and circular polarization conversion mechanisms. Good agreements are observed between simulation and experiment. Even though the design presents a narrow operating bandwidth, it still provides novel ideas for developing radar stealth technology.

Effective strategy for visible-infrared compatible camouflage: surface graphical one-dimensional photonic crystal.

Herein, a novel design concept of a surface graphical photonic crystal (SGPC) has been proposed as an effective strategy to achieve angle-insensitive visible-infrared compatible camouflage. The SGPC, designed as a quasi-periodic Ge/ZnS photonic crystal following an arithmetic sequence in the physical thickness for each period, possesses a functionalized ZnS surface consisting of lithography-fabricated mosaic patterns with various etching depths. Our experiment data demonstrate the excellent infrared camouflage capability of the SGPC with a high average reflectance of 92.7% (surface emissivity ϵ=0.07) in 8-14 µm, and related simulations further reveal the satisfying angle-insensitive reflection characteristic with a maximum effective relative photonic bandgap δBW=91.3% in 0°≤θ≤60°. Besides, the irregular mosaic patterns with various etching depths constitute a colorful digital camouflage on the surface of the SGPC, realizing an outstanding optical camouflage capacity without distinct angle dependency (dominant wavelength shift δλd=|λθ-λ0|/λ0≤2.33%).

Improved broadband spectral selectivity of absorbers/emitters for solar thermophotovoltaics based on 2D photonic crystal heterostructures.

We propose a novel heterostructure based on 2D photonic crystals as broadband selective absorbers/emitters for solar thermophotovoltaics. Alternating hafnium oxide (HfO2) and titanium oxide (TiO2) filled cylinder cavities with tetragonal lattices are embedded into antireflection-coated tungsten (W) film. The simulated results show that the designed structures can obtain high solar collection efficiency ηc of 87.9% as an absorber and great spectral emission efficiency ηe of 81.6% as an emitter. Meanwhile, high average absorptivity of 84.5% under 45° oblique incidence exhibits good performance of wide-angle absorption. This study provides a new way to acquire broadband spectral selective absorbers/emitters.

Perfect dual-band circular polarizer based on twisted split-ring structure asymmetric chiral metamaterial.

A near-perfect dual-band circular polarizer based on bilayer twisted, single split-ring resonator structure asymmetric chiral metamaterial was proposed and investigated. The simple bilayer structure with a 90° twisted angle allows for equalizing the orthogonal components of the electric field at the output interface with a 90° phase difference for a y-polarized wave propagating along the backward (-z) direction. It is found that right- and left-hand circular polarization are realized in transmissions at 7.8 and 10.1 GHz, respectively. Experiments agree well with numerical simulations, which exhibit that the polarization extinction ratio is more than 30 dB at the resonant frequencies. Further, the simple design also can be operated at the terahertz range by scaling down the geometrical parameters of the unit cell.

Electromagnetic manifestation of chirality in layer-by-layer chiral metamaterials.

The three-dimensional (3D) and two-dimensional (2D) chiral metamaterials (CMMs) have been proved to exhibit circular dichroism and circular conversion dichroism, respectively. The layer-by-layer chiral metamaterials, as a category of 3D CMMs, are expected to show the same properties as bulk 3D structures (e.g. helices). However, in this paper, we demonstrated that the layer-by-layer CMMs exhibit circular dichroism and circular conversion dichroism simultaneously by using both theoretical and experimental methods. This work showed that asymmetric transmissions of circular polarizations can also be observed in layer-by-layer CMMs. Moreover, we provided some necessary requirements for the existing of asymmetric transmissions in layer-by-layer CMMs.

Similar structures, different characteristics: circular dichroism of metallic helix arrays with single-, double-, and triple-helical structures.

We fabricated three-dimensional metallic helix arrays with single-, double-, and triple-helical structures. The transmission performances with the normal incident angle were measured in the microwave frequency of 12-18 GHz. For the single- and double-helical structures, giant circular dichroism with fairly wide bands is observed in the transmission spectra. However, the triple-helical structure does not exhibit circular dichroism. Based on the phenomenon of circular dichroism, the single- and double-helical structures can be used as broadband circular polarizers in the microwave region, but triple-helical ones cannot. The experiments have a good agreement with our simulation results, which were studied by the finite-difference time domain method.

Construction of hollow core-shelled nitrogen-doped carbon-coated yttrium aluminum garnet composites toward efficient microwave absorption.

High-performance microwave absorbing materials (MAMs) play a vital role in electromagnetic (EM) pollution protection. Multi-interfacial heterogeneous structure design has become a mainstream direction for designing and fabricating excellent MAMs. Herein, multi-interfacial hollow core-shelled yttrium aluminum garnet@nitrogen-doped carbon (YAG@NC) composites were synthesized by coprecipitation, thermal treatment, self-polymerization and carbonization processes. Thermal treatment temperatures were used to regulate the defect level and interfaces in carbon materials. Defects of NC and multiple interfaces favor dielectric polarization, and the hollow cavity endows the MAMs with lightweight characteristics and ideal impedance matching. The results indicated that YAG@NC composites possess excellent microwave absorption properties with an effective absorption bandwidth (EAB) of 5.5 GHz at an absorber thickness of only 1.95 mm. The radar cross section (RCS) reduction of YAG@NC composites was verified by CST simulation in the far field, and the strongest RCS reduction value was up to 32.64 dBm2 with a scattering angle of 0°. This work paves the way for designing multicomponent microstructure dielectric loss absorbers with broadband and strong microwave absorption.

Hollow Beaded Fe3C/N-Doped Carbon Fibers toward Broadband Microwave Absorption.

Microwave-absorbing materials have attracted enormous attention for electromagnetic (EM) pollution. Herein, hollow beaded Fe3C/N-doped carbon fibers (Fe3C/NCFs) were synthesized through convenient electrospinning and subsequent thermal treatment. The special hollow morphology of the samples is conducive to achieve lightweight and broadband microwave absorption properties. The thermal treatment temperatures exhibit a significant impact on conductivity and EM properties. The broadest effective absorption bandwidth (EAB) is 5.28 GHz at 2.16 mm when the thermal treatment temperature is 700 °C, and the EAB can cover 13.13 GHz with a tunable absorber thickness from 1.0 to 3.5 mm when the thermal treatment temperature is 750 °C. The excellent microwave absorption properties of the samples are due to the synergistic effect of impedance matching and strong EM energy attenuation abilities. Hence, the magnetic hollow beaded Fe3C/NCFs are expected to be an attractive candidate material as a lightweight and efficient microwave absorber in the future.

Synthesis of nitrogen-doped graphene wrapped SnO2 hollow spheres as high-performance microwave absorbers.

Nitrogen-doped graphene (NG)/SnO2 hollow sphere hybrids were synthesized in this work. The chemical composition, crystal structure and morphology have been characterized by FT-IR spectra, XRD, Raman spectra, XPS, SEM and TEM in detail. Reflection loss (RL) values of NG/SnO2 hollow sphere hybrids less than -10 dB and -20 dB are found in the wide frequency range of 4.5-18 GHz and 5-16.2 GHz within 1.3-3.5 mm, and a minimum RL of -50.3 dB is achieved at 8.6 GHz with the matching thickness of only 2.3 mm. The results indicate that the NG/SnO2 hollow sphere hybrids with high-performance microwave absorption properties have a promising future in decreasing electromagnetic wave irradiation and interference.

Metal-Based Graphical SiO2/Ag/ZnS/Ag Hetero-Structure for Visible-Infrared Compatible Camouflage.

A brand-new approach to realizing visible-infrared compatible camouflage is proposed based on a metal-based graphical hetero-structure (MGHS) SiO2/Ag/ZnS/Ag. For different thicknesses (20, 40, and 60 nm) of color-controlling sub-layer, high-contract and large-span structure colors (yellow, navy, and cyan) were observed due to reintroducing constructive interference with a matching intensity of reflected waves. Ultra-low infrared emissivity values of 0.04, 0.05, and 0.04 (with high average reflectance values of 95.46%, 95.31%, and 95.09%) were obtained at 3⁻14 µm. In addition, the well-performing trisecting-circle structure further indicates that it is feasible to design on-demand compatible camouflage patterns using the easily-prepared MGHS.

Enhanced Microwave Absorption and Surface Wave Attenuation Properties of Co0.5Ni0.5Fe2O4 Fibers/Reduced Graphene Oxide Composites.

Co0.5Ni0.5Fe2O4 fibers with a diameter of about 270 nm and a length of about 10 µm were synthesized by a microemulsion-mediated solvothermal method with subsequent heat treatment. The Co0.5Ni0.5Fe2O4 fibers/reduced graphene oxide (RGO) composite was prepared by a facile in-situ chemical reduction method. The crystalline structures and morphologies were investigated based on X-ray diffraction patterns and scanning electron microscopy. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature. Co0.5Ni0.5Fe2O4 fibers/RGO composites achieve both a wider and stronger absorption and an adjustable surface wave attenuation compared with Co0.5Ni0.5Fe2O4 fibers, indicating the potential for application as advanced microwave absorbers.

Dynamics of two coupled Bose-Einstein Condensate solitons in an optical lattice.

The characteristics of two coupled Bose-Einstein Condensate (BEC) bright solitons trapped in an optical lattice are investigated with the variational approach and direct numerical simulations of the Gross-Pitaevskii equation. It is found that the optical lattice can be controllably used to capture and drag the coupled BEC solitons. Its effect depends on the initial location of the BEC solitons, the lattice amplitude and wave-number, and the amplitude of the coupled BEC solitons. The effective interaction between the two coupled solitons is the attractive effect.

Emittance of a radar absorber coated with an infrared layer in the 3~5microm window.

By use of the Kubelka-Munk theory, the Mie theory and the independent scattering approximation, we obtain the explicit expression of the emittance of an infrared coating attached to a radar absorber with a high emittance, in the 3~5microm window. Taking aluminum particles with spherical shape as the pigments within the coating, we give the dependence of the coating emittance with respect to the particle radius, the thickness of the coating. At a volume fraction of 0.05, we propose the optimum particle radius range of the pigment particles is around 0.35~0.6microm. When the thickness of the coating exceeds 300microm, the decrease of emittance at 4microm wavelength becomes negligible. Too much thickness of IR layer wouldn't contribute to the decrease of emittance. We study the influence of the infrared coating on the performance of the radar absorber, and believe that not too much thick infrared coating consisting of spherical Al particles wouldn't result in a remarkable deterioration of the absorbing ability of the radar absorber.

Numerical study on a designable linear-resonant multiband single-layer fractal frequency-selective surface.

In this paper, the multiband transmission resonant properties of single-layer Minkowski fractal planar frequency-selective surfaces (FSSs) at normal-incidence case were numerically investigated in detail. The self-similar iterative transformational method was proposed to construct the fractal FSSs. Both the finite integration technology and finite element method were used to calculate the magnitudes and phases of transmittances, and then the induced surface current and magnetic energy density distributions were demonstrated to analyze the insight physical picture of the multiband resonant feature. The results indicate that the individual resonant frequencies of our fractal FSSs present a periodical linear-resonant phenomenon and they could be designed easily and precisely just according to the geometrical characteristic of each iteration. With the intriguing resonant properties, the fractal FSS has a potential application in the fields of dual-band or multiband resonance.

Calculation of emittance of a coating layer with the Kubelka-Munk theory and the Mie-scattering model.

The radiative transfer equation in a coating layer with a not too high concentration of pigment particles, which is attached to a substrate, is resolved in terms of Kubelka-Munk theory and a Mie-scattering model. In the case of a coating layer constituted with aluminum spherical particles, the dependence of emittance of the coating layer on particle radius and thickness of the layer are studied. The optimum radius of a pigment particle is suggested as well.