Transparent transmission-selective radar-infrared bi-stealth structure

Multi-spectral compatible metasurface with low infrared emissivity, independent microwave complex-amplitude control, and high visible transparency

With the rapid development of communication technology and detection technology, it is difficult for devices operating in a single spectrum to meet the application requirements of device integration and miniaturization, resulting in the exploration of multi-spectrum compatible devices. However, the functional design of different spectra is often contradictory and difficult to be compatible. In this work, a transparent slit circular metasurface with a high filling ratio is proposed to achieve the compatibility of microwave, infrared and visible light. In the microwave, based on the Pancharatnam-Berry phase theory, the continuous amplitude and binary phase can be customized only by rotating the slit angle to achieve an Airy beam function at 8-12 GHz. In the infrared, the mean infrared emissivity is reduced to 0.3 at 3-14 µm by maintaining high conductive filling ratio, and in visible light, based on the transparency of materials, the mean transmittance can achieve 50% at 400-800 nm. All the results can verify the multi-spectral compatibility performance, which can also verify the validity of our design method. Importantly, the multi-spectral compatible metasurface contributes an option for multifunctional integration, which can be further applied in communication, camouflage, and other fields.

Multifunctional Ultrathin Metasurface with a Low Radar Cross Section and Variable Infrared Emissivity

The development of stealth devices that are compatible with both infrared (IR) and radar systems remains a significant challenge, as the material properties required for effective IR and radar stealth are often contradictory. In this work, based on an IR electrochromic device (IR-ECD), concepts of metamaterial manipulating electromagnetic waves are applied to develop a multifunctional ultrathin metasurface with a low radar cross section (RCS) and variable infrared emissivity. This paper presents a linear-to-linear polarization conversion metasurface (PCM) designed by hollowing the IR-ECD. In this way, the IR-ECD based on polyaniline (PANI) can also modulate the reflection waves in the microwave band without affecting its features in the infrared region. Thus, the proposed metasurface integrates both microwave stealth and variable infrared emissivity through a single layer. The measured results show that a 10 dB RCS reduction is achieved in the band of 8.46-9.5 GHz, and the infrared emissivity can be adjusted from 0.870 to 0.513 in the infrared stealth band of 8-14 μm. Due to the ultrathin thickness (only 0.081λ0 at 9 GHz), low RCS in the X-band, and variable infrared emissivity, the designed multifunctional stealth metasurface has promising applications on military platforms with various surrounding environments.

Low infrared emissivity and broadband wide-angle microwave absorption integrated bi-functional camouflage metamaterial with a hexagonal patch based metasurface superstrate

This work proposed and demonstrated a bi-functional metamaterial to implement the multispectral camouflage in infrared and microwave bands. Aiming at integrating broadband, wide-angle and low infrared emissivity into one structure, the bi-functional structure is made up of three metasurface layers with different functions. Specifically, a metasurface superstrate based on hexagonal metallic patch was deployed to achieve a low infrared emissivity and a high transmittance of microwave simultaneously. In the framework of equivalent circuit model, the bi-functional structure was designed and optimized. A dielectric transition layer was introduced into the structure to obtain better microwave absorption performances. A sample of such structure was prepared based on optimized geometric parameters and tested. The simulated and measured results indicate that the novel hexagonal patch metasurface superstrate significantly reduces infrared emissivity and the measured emissivity of the structure is about 0.144 in 8-14µm infrared band. Meanwhile, the multilayered structure has a broadband absorption band from 2.32 GHz to 24.8 GHz with 7  mm thickness and is equipped with good angular stability under oblique incidence. In general, the method and specific design proposed in this work will benefit utilizing metasurface to implement bi-functional microwave and infrared camouflage materials with outstanding performances, which are promising for extensive applications.

Actively tunable rasorber with broadband RCS reduction and low infrared emissivity

In this paper, an actively tunable rasorber with broadband RCS reduction and low infrared emissivity is proposed. The rasorber can achieve flexible control of the peak of the transmission frequency and make the platform invisible in multiple spectrum. Based on the combination of varactor diodes and bandpass frequency-selective surface (FSS), the transmission window can be continuously tuned from 1.8 to 4.5 GHz. The designed rasorber has more than 10 dB RCS reduction from 5.4 to 14.1 GHz. Furthermore, an infrared low emissivity layer based on ITO resistance film is added above the rasorber, and the average infrared emissivity of the measured surface is 0.33. The experimental and simulation results are in good agreement. This work is expected to be applied to frequency hopping secure communication and ultra-wideband, multi-spectrum stealth.

Reconfigurable Terahertz Spatial Deflection Varifocal Metamirror

A traditional optical lens usually has a fixed focus, and its focus controlling relies on a bulky lens component, which makes integration difficult. In this study, we propose a kind of terahertz spatial varifocal metamirror with a consistent metal-graphene unit structure whose focus can be flexibly adjusted. The focus deflection angle can be theoretically defined by superimposing certain encoded sequence on it according to Fourier convolution theorem. The configurable metamirror allows for the deflection of the focus position in space. The proposed configuration approach presents a design concept and offers potential advancements in the field of developing novel terahertz devices.

Three-Layered Thin Films for Simultaneous Infrared Camouflage and Radiative Cooling

With the rapid advancements in aerospace technology and infrared detection technology, there are increasing needs for materials with simultaneous infrared camouflage and radiative cooling capabilities. In this study, a three-layered Ge/Ag/Si thin film structure on a titanium alloy TC4 substrate (a widely used skin material for spacecraft) is designed and optimized to achieve such spectral compatibility by combining the transfer matrix method and the genetic algorithm. The structure exhibits a low average emissivity of 0.11 in the atmospheric windows of 3-5 μm and 8-14 μm for infrared camouflage and a high average emissivity of 0.69 in 5-8 μm for radiative cooling. Furthermore, the designed metasurface shows a high degree of robustness regarding the polarization and incidence angle of the incoming electromagnetic wave. The underlying mechanisms allowing for the spectral compatibility of the metasurface can be elucidated as follows: the top Ge layer selectively transmits electromagnetic waves ranging from 5-8 μm while it reflects those in the ranges of 3-5 μm and 8-14 μm. The transmitted electromagnetic waves from the Ge layer are first absorbed by the Ag layer and then localized in the Fabry-Perot resonance cavity formed by Ag layer, Si layer and TC4 substrate. Ag and TC4 make further intrinsic absorptions during the multiple reflections of the localized electromagnetic waves.

Multi-functional terahertz metamaterials based on nano-imprinting

This paper reports a multi-functional terahertz (THz) metamaterial based on a nano-imprinting method. The metamaterial is composed of four layers: 4 L resonant layer, dielectric layer, frequency selective layer, and dielectric layer. The 4 L resonant structure can achieve broadband absorption, while the frequency selective layer can achieve transmission of specific band. The nano-imprinting method combines electroplating of nickel mold and printing of silver nano-particle ink. Using this method, the multilayer metamaterial structures can be fabricated on ultrathin flexible substrates to achieve visible light transparency. For verification, a THz metamaterial with broadband absorption in low frequency and efficient transmission in high frequency is designed and printed. The sample's thickness is about 200 µm and area is 65 × 65 mm². Moreover, a fiber-based multi-mode terahertz time-domain spectroscopy system was built to test its transmission and reflection spectra. The results are consistent with the expectations.

Flexible and transparent metadevices for terahertz, microwave, and infrared multispectral stealth based on modularization design

Multispectral stealth technology including terahertz (THz) band will play an increasingly important role in modern military and civil applications. Here, based on the concept of modularization design, two kinds of flexible and transparent metadevices were fabricated for multispectral stealth, covering the visible, infrared (IR), THz, and microwave bands. First, three basic functional blocks for IR, THz, and microwave stealth are designed and fabricated by using flexible and transparent films. And then, via modular assembling, that is, by adding or removing some stealth functional blocks or constituent layers, two multispectral stealth metadevices are readily achieved. Metadevice 1 exhibits THz-microwave dual-band broadband absorption, with average measured absorptivity of 85% in 0.3-1.2 THz and higher than 90% in 9.1-25.1 GHz, suitable for THz-microwave bi-stealth. Metadevice 2 is for IR and microwave bi-stealth, with measured absorptivity higher than 90% in 9.7-27.3 GHz and low emissivity around 0.31 in 8-14 µm. Both metadevices are optically transparent and able to maintain good stealth ability under curved and conformal conditions. Our work offers an alternative approach for designing and fabricating flexible transparent metadevices for multispectral stealth, especially for applications in nonplanar surfaces.

Ultra-wideband flexible radar-infrared bi-stealth absorber based on a patterned graphene

In this work, an ultra-wideband flexible radar absorber with low infrared emissivity for a radar-infrared bi-stealth application utilizing multilayer patterned graphene is proposed. The proposed absorber consists of three layers of graphene films with different patterns, flexible substrates, lightweight foam, and a ground layer. The flexible graphene films, rather than the conventional lumped resistors, are adopted as omnidirectional resistors to achieve dual polarization and flexibility. On the top of the absorber, an infrared shielding layer (IRSL) consists of patterned Indium tin oxide (ITO) separated by a thin foam layer. Due to the low-pass characteristics and the high filling ratio of the top ITO layer, the infrared emissivity of the whole structure is reduced effectively while the radar absorption property is slightly affected. As a result, the 90% absorption band is from 1.96 GHz to 20.72 GHz (fractional bandwidth 165.4%), with a low infrared emissivity of about 0.35. Besides, a miniaturized unit is achieved with the period of 0.079 λl at the lowest absorption frequency, and the oblique angle incidence response is up to 45° for TE mode and 60° for TM mode. A plane and a bending prototype are fabricated and measured, respectively. The screen-printing technology is adopted to print the graphene resistive films, and the measurement results agree well with the simulation.

Multispectral meta-film design: simultaneous realization of wideband microwave absorption, low infrared emissivity, and visible transparency

There is a huge challenge to target multispectral compatible designs to satisfy the conflicting parametric requirements according to specific engineering requirements. In this work, a novel design method of multispectral compatible integration based on a lossy capacitive multispectral meta-film (MMF) is proposed. The simple guidelines from the impedance matching conditions of MMF derived from the transmission line model were employed to guide and analyze the broadband microwave absorption behavior. An autonomous optimization platform was constructed to simultaneously realize the customization of low infrared emissivity, as well as the widest microwave absorption bandwidth while ensuring maximum visible transparency. Following the guidance of the design method, a flexible structure with a low infrared emissivity of 0.534, wideband microwave absorption from 8.9 to 16.4 GHz covering X, Ku, and high visible transmission of 70.18% and ultra-thin thickness of 2.3 mm was finally obtained. The experimental results and simulation results were in high agreement, indicating the MMF has great application potential in multispectral stealth on optical windows, further demonstrating the versatility and effectiveness of the design method.

Scalable-Manufactured Metamaterials for Simultaneous Visible Transmission, Infrared Reflection, and Microwave Absorption

Scalable manufacturing of metamaterials with multispectral manipulation capabilities remains highly challenging, which was generally circumvented by integrating several single-spectral metamaterials, potentially leading to complex processes, large thicknesses, and limited fabrication size. We experimentally demonstrate a standalone and scalable-manufactured multispectral metamaterial featuring simultaneous visible transmission, infrared reflection, and microwave absorption. The prepared multispectral metamaterial with an area of 255 cm2 exhibits a visible transmittance of 74.5% at wavelengths of 400-700 nm (the highest 80.2% at 510 nm), a thermal emissivity of 0.08 at the infrared (IR) wavelengths of 2.5-20 μm (the lowest 0.03 at 19.5 μm), and a microwave absorptance of 63.4% at frequencies of 8.2-12.4 GHz (the near-perfect 97.4% at 11.5 GHz) on average with a deep-subwavelength thickness of λ/47. The deep-subwavelength multispectral metamaterial consists of a submillimeter-thick polyethylene terephthalate dielectric spacer sandwiched by a patterned ultrathin metal and a metal mesh back-reflector with ultralow sheet resistances. Unlike the conventional optically transparent microwave absorbers made from indium tin oxides, the surface plasmonic modes can be excited within the submillimeter-thick multispectral metamaterial, bringing about the gap plasmon polaritons-induced microwave attenuation, together with the excellent visible transparency and high IR reflection/low IR emissivity. This work may inspire the designs and practical production of standalone multispectral metamaterials and benefit the protection against ubiquitous IR and microwave reconnaissance without impeding visual observation.

Full-space omnidirectional cloak by subwavelength metal channels filled with homogeneous dielectrics

Cloaks can greatly reduce the scattering cross-section of hidden objects through various mechanisms, thereby making them invisible to outside observers. Among them, the full-space omnidirectional cloak based on transformation optic with full parameters are difficult to realize without metamaterials and often needs to be simplified before realization, while most cloaks with simplified parameters have limited working direction and cannot achieve omnidirectional cloaking effect. In this study, a full-space omnidirectional cloak is designed based on transformation optics and optic-null medium, which only needed natural materials without metamaterials. The designed omnidirectional cloak is realized by subwavelength metal channels filled with isotropic dielectrics whose refractive indices range from 1 to 2, which is homogeneous in each channel. The numerical simulation results verify good scattering suppression effect of the designed cloak for various detecting waves.

Optically transparent infrared selective emitter for visible-infrared compatible camouflage

Visible-infrared compatible camouflage is significant to enhance the equipment survivability through counteracting the modern detecting and surveillance systems. However, there are still great challenges in simultaneously achieving multispectral camouflage with high transmittance in visible, low emissivity in the atmospheric windows and high emissivity in the non-atmospheric window, which can be attributed to the mutual influence and restriction within these characteristics. Here, we proposed an optically transparent infrared selective emitter (OTISE) composed of three Ag-ZnO-Ag disk sub-cells with anti-reflection layers, which can synchronously improve the visible transmittance and widen absorption bandwidth in the non-atmospheric window by enhancing and merging resonance response of multi-resonators. Test results reveal that low emissivity in infrared atmospheric windows, high emissivity in the 5-8 µm non-atmospheric window and high optical transparency have been obtained. In addition, the radiative flux of OTISE in 3-5 µm and 8-14 µm are respectively 34.2% and 9.3% of that of blackbody and the energy dissipation of OTISE is 117% of that of chromium film. Meanwhile, it keeps good optical transparency due to the ultrathin Ag film. This work provides a novel strategy to design the optically transparent selective emissive materials, implying a promising application potential in visible and infrared camouflage technology.

Configuration of Multifunctional Polyimide/Graphene/Fe3O4 Hybrid Aerogel-Based Phase-Change Composite Films for Electromagnetic and Infrared Bi-Stealth

Electromagnetic (EM) and infrared (IR) stealth play an important role in the development of military technology and the defense industry. This study focused on developing a new type of multifunctional composite film based on polyimide (PI)/graphene/Fe₃O₄ hybrid aerogel and polyethylene glycol (PEG) as a phase change material (PCM) for EM and IR bi-stealth applications. The composite films were successfully fabricated by constructing a series of PI-based hybrid aerogels containing different contents of graphene nanosheets and Fe₃O₄ nanoparticles through prepolymerizaton, film casting, freeze-drying, and thermal imidization, followed by loading molten PEG through vacuum impregnation. The construction of PI/graphene/Fe₃O₄ hybrid aerogel films provides a robust, flexible, and microwave-absorption-functionalized support material for PEG. The resultant multifunctional composite films not only exhibit high microwave absorption effectiveness across a broad frequency range, but also show a good ability to implement thermal management and temperature regulation under a high latent-heat capacity of over 158 J/g. Most of all, the multifunctional composite films present a wideband absorption capability at 7.0–16.5 GHz and a minimum reflection loss of −38.5 dB. This results in excellent EM and IR bi-stealth performance through the effective wideband microwave absorption of graphene/Fe₃O₄ component and the thermal buffer of PEG. This study offers a new strategy for the design and development of high-performance and lightweight EM–IR bi-stealth materials to meet the requirement of stealth and camouflage applications in military equipment and defense engineering.

Broadband surface wave coupler with low infrared emission and microwave reflection

Metasurfaces possess excellent capabilities to flexibly manipulate electromagnetic waves in multiple frequency domains, which show great potential application in multispectral stealth. Herein, a broadband surface waves coupler based on the design of thin Pancharatnam-Berry (PB) phase gradient metasurfaces (PGMs) of thickness 0.12λ₀ is proposed to reduce infrared emission and microwave reflection simultaneously. Low infrared emission results from the high filling ratio of the indium-tin-oxide (ITO) on the surface, and low microwave reflection results from the conversion from propagating waves to surface waves. Intriguingly, this design is also capable of acting as a simple circular polarized (CP) discriminator because orthogonal CP waves are coupled into surface waves propagating along opposite directions. A proof-of-concept prototype is simulated and measured to validate the effectiveness of our methodology. The results indicate that the broadband surface waves coupler shows low infrared emissivity less than 0.28 from 3 to 14 µm and has microwave reflection reduction larger than 10 dB in 7.3-9.5 GHz. The exceptional performances of the proposed broadband surface waves coupler make us believe that our design offers an alternative strategy for multispectral stealth and multifunctional application.

Simultaneous reduction of microwave reflection and infrared emission enabled by a phase gradient metasurface

Metasurfaces have shown promising applications in radar-infrared compatible stealth because of its superior electromagnetic wave control capabilities, but, to date, the majority of designs still suffer from the defects of large thickness, limited working bandwidth, relatively high infrared emissivity and so on. Here, an exotic phase gradient metasurface (PGM) is proposed to achieve low microwave reflection and low infrared emission concurrently, which has a small thickness of about 0.10λ₀. The microwave reflection reduction larger than 10 dB in 14-20 GHz is attributed to the anomalous reflection for arbitrary LP incident waves, and the infrared emissivity less than 0.28 from 3 to 14 µm is due to the indium-tin-oxide (ITO) with low infrared emissivity and high filling ratio. Also, the designed PGM can also realize beam deflection for orthogonal CP waves because of the meta-atoms' isotropic characteristics. Our methodology is fully verified by numerous simulations and experiments and may open a new avenue for radar-infrared compatible stealth research.

Ultra-wideband flexible transparent metamaterial with wide-angle microwave absorption and low infrared emissivity

Optically transparent metamaterials with the performance of infrared radar compatible stealth have been designed and manufactured on the basis of the continuous in-depth research on single-band stealth technology. In this paper, metamaterials are designed through theoretical calculations and modeling simulations. The designed structure can achieve higher than 90% broadband (8.7-32 GHz) absorption at wide-angle (45 degrees), emissivity of 0.3 in infrared atmospheric window, and optical transparency. In addition, the material can be bent, which greatly expands its application scenarios. The experimental results are consistent with the theoretical calculation and simulation results.

Genetic-algorithm-empowered metasurface design: simultaneous realization of high microwave frequency-selection and low infrared surface-emissivity

With the improvement of equipment integration, it is difficult to meet the increasing functional requirements with the function of a single spectrum. In this work, a multispectral functional metasurface (MFM) is designed to achieve multispectral compatibility between microwave and infrared using multi-optimization. For microwaves, a frequency selective surface (FSS) is designed to achieve frequency selectivity. And for infrared, a twice genetic algorithm (GA) is employed to further increase the metallic filling ratio, thus reducing the infrared emissivity while maintaining the performance of microwave FSS. In order to verify our design and method, the MFM is fabricated and measured, and all the results are consistent with the theoretical design. The performance of FSS can achieve 3dB bandwidth in 7.2-11.2GHz with low insertion losses and stability, and meanwhile the mean infrared emissivity has been reduced to 0.24 in 3-14μm. In summary, the designed multispectral compatible metasurface has wide application value in radome. What's more, the multi-optimization method for designing the multispectral metasurface can also be extended to other fields.

Thermal infrared and broadband microwave stealth glass windows based on multi-band optimization

With the rapid development of detection technologies, compatible stealth in the infrared and radar ranges has become increasingly essential not only for military application but also for personal privacy protection. In this study, we design a metamaterial window that possesses stealth ability in both the thermal infrared and broadband microwave ranges, using a particle swarm optimization algorithm to realize multi-band optimization. We experimentally verify that the proposed structure can achieve over 90% microwave absorption in the range 5.1 to 19.2 GHz (covering the X and Ku bands), with low infrared emissivity (∼0.15), and also maintain visible transmittance above 60%. Moreover, the window retains good performance up to 200 °C owing to the intrinsic properties of the material. Our multi-band optimization method enables the application of the transparent metamaterial windows in electromagnetic shielding and stealth and can potentially be applied in smart window related industries.

Beyond the Visible: Bioinspired Infrared Adaptive Materials

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.

Experimental demonstration of an ultra-thin radar-infrared bi-stealth rasorber

We propose a radar-infrared bi-stealth rasorber that not only provides broad microwave absorptivity and low infrared emissivity but also possesses a microwave transmission window at low frequency. It is composed of three functional layers, which are carefully designed to independently control the infrared emission, microwave absorption, and transmission, respectively. The structure exhibits broadband (8.1-19.3 GHz) and high-efficiency (>90%) absorption. A transmission window appears at low frequency with a transmission peak of 80% at 2.68 GHz. The thermal emissivity of the structure is about 0.27 in the atmosphere window, which is close to that of metal. Moreover, the total thickness of the proposed structure is only 3.713 mm. The low-infrared-emissivity, high-microwave-absorption and frequency-selective-transmission properties promise it will find potential applications in various stealth fields.

Optical-transparent metasurface for flexible manipulation and analog information modulation

Recently, optically-transparent metasurface based on indium tin oxide (ITO) film has attracted wide attention due to its remarkable optical and electromagnetic characteristics. However, most previous researches on the ITO film mainly focus on the absorption because of its prominent loss-resistance property, but neglecting the further exploration on programmable functions. Here, we present a programmable metasurface based on an optically-transparent ITO glass, on which varactors are integrated to achieve flexible amplitude manipulation range of about 25 dB. More importantly, the presented programmable design can be applied for direct modulation on the carrier incident wave with the desired pre-designed analog wave-form. Within the 10 MHz modulation speed, both programmable amplitude manipulation and analog information modulation are demonstrated in the measurements, showing good agreement with theoretical analysis and simulations. Combining both optical transparency and programmable modulation capability, the presented metasurface will promote the potential applications in wireless communication, internet of things and other smart scenarios.

Optically Transparent Metasurface Absorber Based on Reconfigurable and Flexible Indium Tin Oxide Film

In this paper, we present a flexible, breathable and optically transparent metasurface with ultra-wideband absorption. The designed double layer of indium tin oxide (ITO) films with specific carved structure realizes absorption and electromagnetic (EM) isolation in dual-polarization, as well as good air permeability. Under the illumination of x- and y-polarization incidence, the metasurface has low reflectivity and transmission from about 2 to 18 GHz. By employing ITO film based on polyethylene terephthalate (PET), the presented metasurface also processes the excellent flexibility and optically transparency, which can be utilized for wearable device application. In addition, the dual-layer design enables mechanically-reconfigurable property of the metasurface. The transmission and reflection coefficients in two polarizations show distinct difference when arranging the different relevant positions of two layers of the metasurface. A sample with 14*14 elements is designed, fabricated and measured, showing good agreement with the simulation results. We envision this work has various potentials in the wearable costume which demands both EM absorption and isolation.

Optically transparent coding metasurface with simultaneously low infrared emissivity and microwave scattering reduction

In this paper, an optically transparent coding metasurface structure based on indium tin oxide (ITO) thin films with simultaneously low infrared (IR) emissivity and microwave scattering reduction is proposed. To this end, two ITO coding elements which can reflect 0° and 180° phase responses are firstly designed. Based on these two elements, four coding sequences with different scattering patterns are designed. Three of them can realize anomalous reflections and the fourth can realize random diffusion of normal incident electromagnetic (EM) waves. A prototype of the random diffusion coding metasurface was fabricated and measured. The experimental results show that for normal incident EM waves, at least 10dB backward scattering reduction from 3.8GHz to 6.8GHz can be achieved, and the structure is polarization insensitive. The averaged transmittance of visible light through the coding metasurface reaches up to 72.2%. In addition, due to the high occupation ratio of ITO on the outside of the coding metasurface, a low IR emissivity of about 0.275 is obtained. Good consistency between the experiment and simulation results convincingly verifies the coding metasurface. Due to its multispectral compatibility, the proposed coding metasurface may find potential applications in multi-spectral stealth, camouflage, etc.

Transparent ultrawideband absorber based on simple patterned resistive metasurface with three resonant modes

A transparent low-profile polarization-insensitive metamaterial absorber with ultrawideband microwave absorption is presented. A fractional bandwidth of 125.2% (4.3-18.7 GHz, absorptance > 90%) is achieved using a simple patterned resistive metasurface. The thickness of the absorber is only ∼0.086 times the upper-cutoff wavelength. The experimental results agree with full-wave simulation results. A Cu-metal-mesh ground plane enhances the shielding efficiency and visible transparency. Radar cross-sections (RCS) are reduced across all reflection angles, over frequencies spanning the C, X, and K_u bands. With its visible-wavelength transparency, low profile, polarization insensitivity, excellent absorption, and wideband RCS reduction, the proposed absorber has wide applicability.

A multimode, broadband and all-inkjet-printed absorber using characteristic mode analysis

In this paper, we demonstrate a multimode and broadband absorber that is fabricated directly on PET substrate using a commercial direct-to-garment (DTG) inkjet printer. A design procedure of this kind of absorber is presented. Based on the theory of characteristic mode, the underlying modal behaviors of the absorber structure are firstly analyzed to guide the design of multimode absorber. Two modes on the absorber structure are designed to resonate around 1.83 GHz and 4.28 GHz to cover the working frequency range. Simulation and measurement results show that the multimode absorber with a total thickness of 0.0883λ_L at the lowest operating frequency can achieve broadband microwave absorption with efficiency over 90% in the frequency band of 1.0 ∼ 4.5 GHz (127.3% in fractional bandwidth) through deliberate design. Both the simulated and experimental results demonstrate the validity of the proposed method and indicate that the method can be applied to other microwave and millimeter-wave regions.

Optically transparent metamirror with broadband chiral absorption in the microwave region

Chiral metamirror is one of the recently developed metadevices which can reflect designated circularly polarized waves, mimicking the exoskeleton of iridescent green beetles. Here, an optically transparent metamirror that can absorb microwave chiral photons in a broadband spectrum is demonstrated. A coupled mode theory is adopted to reveal the underlying physics for the improved bandwidth performance. Excellent agreements have been observed between numerical and experimental results, indicating a bandwidth for chiral absorption as high as 2.37 GHz. The optical transparence of the resistive patterns and substrate make the designed metamirrors suitable as microwave coatings in front of optical devices, which may find potential applications in cascaded optical systems working for both microwave and optical signals.

Switchable multifunctional terahertz metasurfaces employing vanadium dioxide

In this paper, we design a type of switchable metasurfaces by employing vanadium dioxide (VO₂), which possess tunable and diversified functionalities in the terahertz (THz) frequencies. The properly designed homogeneous metasurface can be dynamically tuned from a broadband absorber to a reflecting surface due to the insulator-to-metal transition of VO₂. When VO₂ is in its insulating state, the metasurface can efficiently absorb the normally incident THz wave in the frequency range of 0.535-1.3 THz with the average absorption of ~97.2%. Once the VO₂ is heated up and switched to its fully metallic state, the designed metasurface exhibits broadband and efficient reflection (>80%) in the frequency range from 0.5 to 1.3 THz. Capitalizing on such meta-atom design, we further extend the functionalities by introducing phase-gradients when VO₂ is in its fully metallic state and consequently achieve polarization-insensitive beam-steering and polarization-splitting, while maintaining broadband absorption when VO₂ is in insulating state.

High-transmittance double-layer frequency-selective surface based on interlaced multiring metallic mesh

In this work, we proposed an optically transparent double-layer frequency-selective surface (FSS) based on interlaced multiring metallic mesh. By changing the large metal area of a conventional double-layer FSS into triangular-orthogonal distributed basic rings and nested rotated subrings, we achieved an FSS with high optical transmittance and low normalized high-order diffraction intensity while maintaining a flat passband and steep transition band. The results showed that our fabricated FSS had a normalized visible transmittance of 90.31%, stable filtering passband of ∼33.9  GHz, 3 dB bandwidth of 13.4 GHz, and uniform diffraction distribution, which are favorable characteristics for optically transparent FSS applications.

Polarization-selective dual-wavelength gap-surface plasmon metasurfaces

In this paper, we present a general method to realize polarization-selective dual-wavelength gap-surface plasmon metasurfaces (GSPMs), which are composed of strongly anisotropic meta-atoms periodically arranged in a rectangular lattice with two degrees of freedom to independently control the reflection phase and amplitude of orthogonal linear polarizations at two discrete wavelengths. We design and demonstrate dual-wavelength GSPMs as polarization beam splitters and focusing metamirrors operating at 850 and 1550 nm simultaneously. Our work provides a general approach to design multiwavelength, multifunctional metasurfaces with various potential applications.

Transparent coupled membrane metamaterials with simultaneous microwave absorption and sound reduction

Metamaterials offer a novel strategy to control wave propagation in different physical fields ranging from acoustic, electromagnetic, and optical waves to static electric and thermal fields. However, fundamental and practical challenges still need to be overcome for multi-physical manipulation, especially for independent control of acoustic and electromagnetic waves simultaneously. In this paper, we propose and experimentally demonstrate a transparent bifunctional metamaterial in which acoustic and electromagnetic waves could be engineered jointly and individually. Specifically, a transparent composite coupled membrane metamaterial is introduced with indium tin oxide (ITO) patterns coated on the top and bottom membranes, giving rise to simultaneous electromagnetic wave dissipation and sound reduction. Our results could help broaden the current research scope for multiple disciplines and pave the way for the development of multi-functional devices in new applications.