Low Observations Research Articles

An Ultra-Thin, Triple-Band, Incident Angle-Insensitive Perfect Metamaterial Absorber.

We created an ultra-thin, triple-band incident angle-insensitive perfect metamaterial absorber (MMA) with a metallic patch and a continuous metal ground isolated by a central dielectric substrate. The top metallic patch, placed across the edges of the 0.58 mm thickness Rogers RO4003C (lossy) substrate, forms the bulk of the projected absorber's ultra-thin layer. Nonetheless, absorption is exceedingly strong, covering C-band, X-band and K-band and reaching levels of 97.8%, 99.9%, and 99.9%, respectively, under normal and even oblique (0° to 45°) incident conditions. In chosen ranges of frequency of 6.24, 10.608, and 18.624 GHz for both TM and TE mode, the displayed Q-factors were 62.4, 17.68, and 26.61, respectively. We correspondingly calculated the RAB (relative absorption bandwidth) to evaluate absorption performance. An equivalent circuit proved its performance capabilities, indicating that it would produce a high-quality MMA from ADS software. Furthermore, the absorber's performance has been verified in free space on a sample being tested using a different array of unit cells. Moreover, the proposed structures with HFSS simulators to display the MMA's absolute absorption at each absorption peak are somewhat inconsistent with the results of the CST simulator. Because of its superior performance, the ultra-thin absorber is suited for a wide range of applications, including satellite applications such as radar systems, stealth technology, imaging, and electromagnetic interference reduction.

An Ultra-Broadband and Highly-Efficient Metamaterial Absorber with Stand-Up Gradient Impedance Graphene Films

Metamaterial absorbers (MMAs) that absorb electromagnetic waves among an ultra-broad frequency band have attracted great attention in military and civilian applications. In this paper, an ultra-broadband and highly-efficient MMA is presented. The unit cell of the proposed MMA was constructed with two cross-placed stand-up gradient impedance graphene films, which play a key role in improving impedance matching. Considering the trade-off between absorbing performance and processing complexity, in our design, we adopted the stand-up graphene films that have a gradient with three orders of magnitude in total. The simulated results of the proposed absorber show an ultra-broadband absorption (absorptivity > 90%) from 1.8 GHz to 66.7 GHz and a highly-efficient absorption (absorptivity > 97%) in the range of 2-21.7 GHz and 39.6-57 GHz. The field analysis was adopted to explain the mechanism of the proposed absorber. To validate this design, a prototype of 20 × 20 units was processed and assembled. The graphene films were processed with graphene conductive ink using screen print technology. The measured results are in good agreement with the simulated ones. The proposed absorber may find potential applications in the field of stealth technologies and electromagnetic interference.

An ionic liquid-based ultra-broadband absorber with temperature stability, polarization insensitivity, and wide incident angle

In this paper, an ultra-broadband absorber based on ionic liquids is proposed. It is an absorber that is a fully enclosed resin container, and the container is filled with an ionic liquid 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][N(CN)2]) with a circular cross-section. Simulation results show that the single-ring absorber can achieve more than 90% absorption in the microwave range of 12.25–50 GHz, and the relative bandwidth reaches 121.29%. Based on the theories of equivalent medium and impedance matching, the mechanism of broadband absorption is explored by analyzing the electromagnetic field distribution and power loss density of the absorber. In addition, the absorber exhibits good thermal stability in the range of 20 °C–100 °C, and has the characteristics of polarization insensitivity and wide incident angle. The proposed metamaterial absorber can be fabricated at a low cost, is environment-friendly, and may find potential applications in the fields of electromagnetic energy harvesting and stealth technology.

Dynamically switchable multifunctional terahertz absorber based on graphene and vanadium dioxide hybrid metamaterials

A multifunctional terahertz absorber based on a hybrid configuration of vanadium dioxide ( V O 2 ) and graphene is proposed, which is enabled to dynamically switch between ultrabroadband and double-narrowband absorption characteristics using the phase-shifting property of V O 2 . When V O 2 is in the metallic phase and the Fermi energy level of graphene is 0.01 eV, ultrabroadband absorption from 3.04 THz to 8.78 THz can be achieved. The absorption can be continuously adjusted from 0.7% to 99.9% by changing the conductivity of the V O 2 through temperature. When V O 2 is in the insulating phase and Fermi energy level of graphene is 0.4 eV, the proposed absorber exhibits a double-narrowband absorption characteristic, achieving 99.8% and 97.3% absorption at 1.79 THz and 4.62 THz, respectively. Since graphene is an electronically controlled material, it is able to dynamically modulate the resonant frequency and absorption intensity by changing the Fermi energy level of graphene. In addition, the physical mechanism of the multifunctional absorber is revealed using electric field distribution and the impedance-matching theory. The proposed absorber has potential applications in optical switching, image processing, and stealth technology.

Design and Analysis of Metamaterial Absorber using Split Ring Resonator for Dual Band Terahertz Applications

There are numerous applications for microwave absorbers in the L, S, C, X, and Ku bands. Creating a terahertz absorber, on the other hand, has proven difficult. This paper presents a metamaterial absorber operates at terahertz frequencies. It has a square shaped outer ring and a circle shaped inner ring unit cell split ring resonator. The substrate is dielectric material and the ground is metallic. According to the simulation results, the unit cell resonates at two frequencies that is at 1.3027THz and 1.7853THz with absorptivity is greater than 90% at normal incidence .The main cause of high absorbance is due to strong electromagnetic field. The proposed structure in this paper is useful for terahertz imaging, detection of malignant tumors and stealth technology.

High Signal-to-Noise Ratio MEMS Noise Listener for Ship Noise Detection

Ship noise observation is of great significance to marine environment research and national defense security. Acoustic stealth technology makes a variety of ship noise significantly reduced, which is a new challenge for marine noise monitoring. However, there are few high spatial gain detection methods for low-noise ship monitoring. Therefore, a high Signal-to-Noise Ratio (SNR) MEMS noise listener for ship noise detection is developed in this paper. The listener achieves considerable gain by suppressing isotropic noise in the ocean. The working principle and posterior end signal processing method of the listener are introduced in detail. A gain of 10 dB over the sound pressure detector is obtained by detecting the standard sound source. In addition, the traffic vessel noise monitoring experiment verifies that the listener can detect the ship noise. The results show that the listener has a very broad application prospect in the field of low-noise ship observation.

Investigations of physicochemical and microwave absorption performance of NiCo2S4/AgBr nanocomposite in X-band frequency

The advantages of the microwave materials' low weight and exceptional absorbing power are crucial for addressing the EM problems in stealth technology. The structure and absorber thickness optimization is suggested by the current work. Herein, we increased the absorption properties of the NiCo2S4/AgBr (NCAG) nanocomposite by modifying the filler loading in the composite matrix. XRD, VSM, Raman, and FESEM were employed to find the structural and morphological properties of the NCAG composites. At 9.7 GHz, the thinnest layer of 1.4 mm was reported to have a -24 dB minimum reflection loss. The improved absorption bandwidth, however, is 3.8 GHz. The H-plane radiation patterns of an NCAG sample with a 25 wt.% filler loading were determined using the free-space approach (thickness of 1.4 mm). The RL resonance peaks at 10.9 GHz and 11.4 GHz demonstrate the 25 wt% and 35 wt% samples' strong high-frequency absorption responses. The key factors in the improvement of the reflection loss and absorption efficiency were excellent conductive response, better matching properties, and electromagnetic losses.

3D metamaterial ultra-wideband absorber for curved surface

This paper proposes a three-dimensional metamaterial absorber based on a resistive film patch array to develop a low-cost, lightweight absorber for curved surfaces. An excellent absorption over a large frequency band is achieved through two different yet controllable mechanisms; in the first mechanism, a considerable attenuation in the wave power is achieved via graphite resistive films. The absorption is then intensified through magnetic dipoles created by the surface currents, leading to absorption peaks. The simulation results of the absorber show that a broadband absorption greater than 85% is achieved over 35–400 GHz for both TE and TM polarization waves at normal incidence. The structure has more than 167% and 80% absorption bandwidth above 85% and 90%, respectively. It is shown that the proposed metamaterial absorber is independent of incident wave polarization. In addition, the structure is insensitive to incident angles up to 60° for TE mode and full range angle 90° for TM mode. To describe the physical mechanism of the absorber, E-field, power loss density and surface current distributions on the structure are calculated and shown. Moreover, the oblique incidence absorption efficiency is also explained. This absorber paves the way for practical applications, such as sensing, imaging and stealth technology. In addition, the proposed structure can be extended to terahertz, infrared and optical regions.

Single-channel blind source separation of underwater acoustic signals using improved NMF and FastICA

When automatic monitoring buoys receive mixed acoustic signals from multiple underwater acoustic targets, the statistical blind source separation (BSS) task is used to separate the signals and identify vessel features, which is overly complex and needs improvement, especially noting that noise cancellation and stealth technologies are advancing rapidly. To fill this gap in capability, an improved non-negative matrix factorization (NMF) based BSS algorithm is built on a FastICA machine learning backbone. With this tool, the spatial and spectral correlation of underwater acoustic signals is introduced into the NMF algorithm improved by to resolve the non-convex and feature correlation problems commonly encountered by contemporary NMF algorithms. Moreover, the improved modulation feature adaptability of the NMF increases the local expressivity and independence of the decomposed base matrix, which is proven to meet the requirements of FastICA and used to improve the BSS effect of the FastICA. Simulated and empirical results show that compared with state-of-the-art FastICA and NMF based BSS algorithms, our novel approach obtains better signal-to-noise reduction and separation accuracy while maintaining superior target signal recognition features.

Flexible human-applicable infrared camouflage materials with temperature and emissivity tunability

Infrared stealth technology plays a vital role in development of defense industry and new military equipment. Infrared stealth materials with adjustable temperature and emissivity have potential applications in adaptive thermal camouflage. We used thermoplastic polyurethane (TPU) encapsulated with ionic liquid as the intermediate layer and graphene as the outer layers to prepare the infrared thermal camouflage composite material with adjustable temperature and emissivity. The ionic liquids with wide phase change transition temperature range from −40 to 37 °C could absorb heat from human body and keep the actual temperature of the surface of the composite film constant, due to their high latent heat during phase change process. Moreover, the infrared emissivity of graphene outer layer could be controlled by the intercalation of ionic liquids via applied electric field, which further achieves the thermal camouflage of the composite film. Under the continuous heating of heat source close to the body temperature, the obtained composite film shows a tunable thermal camouflage temperature range of 33 to 28 °C through the detection of an infrared camera, and the emissivity decreases 0.26. The obtained composite film has excellent mechanical properties could effectively realize the on-body infrared stealth, which provides constructive suggestions for the application of adaptive thermal camouflage such as new military equipment.

Broadband electrically tunable linear polarization converter based on a graphene metasurface.

In this study, a broadband tunable reflective graphene-based linear polarization converter (GLPC) is proposed based on the graphene-ionic liquid-ITO structure (GIIS) integrated with a periodic double split ring resonator (DSRR) in the millimeter-wave regime. The tuning characteristic of the designed GLPC is analyzed using full-wave simulations and the equivalent circuit model method (ECM), which is based on multi-section transmission lines. There is a good agreement between ECM and simulation results. A comprehensive physical mechanism for the proposed broadband GLPC is then achieved by analyzing the surface current distributions. After manufacturing, the GLPC prototype's co- and cross-polarized reflection coefficients were measured using various bias voltages. The reflectivity can be controlled from -4.5 to -20 dB by changing the bias voltage in the range of +1.1 to -3.3 V. The designed GLPC can provide a tunable polarization conversion within the frequency range of 15.5∼35 GHz and shows a more than 75% conversion efficiency. The results of the simulation and the measurement are also in good agreement. The designed GLPC has potential applications in radar cross-section reduction, antenna design, and stealth technology by reconfiguring its polarized reflection characteristic dynamically.

Recent Research Progress in Electrochromic Materials for Intelligent Stealth(Visible and Infrared): A review

Stealth technology is critical for the existence and the employment of the modern combat weapons systems. Electrochromic materials, which are known as a kind of novel intelligent stealth materials with multi-coloring effect under dynamic control, shows reversible color and emittance change under applied electric filed. Current emerging EC technologies utilizing electrochromism being widely used in intelligent stealth to produce more enhanced effects than traditional static options. This review mainly discusses recent advancement in the electrochromic materials with focus on its applications in the intelligent stealth technologies. Firstly, the principle and classification of electrochromic materials are presented along with summary of recent developments in electrochromism. Secondly, the potential applications of inorganic and organic electrochromism in visible and infrared stealth bands are highlighted. Finally, the challenges and further development approaches are emphasized; it will be intended to underline the future perspectives in realizing intelligent stealth for different applications.

Microwave absorption characteristics of cobalt doped zinc spinel ferrites in Ku-band (12.4–18 GHz)

This research study targets to fabricate microwave absorbers with narrow sample thickness, strong absorption power and broad bandwidth simultaneously. The zinc-cobalt spinel ferrites with compositional formula Zn1-xCoxFe2O4 (x = 0.00, 0.25, 0.50, 0.75 and 1.00) are successfully synthesized via sol–gel citrate route by sintering at 1000 °C. Vector network analyzer is employed to determine dielectric permittivity εr=ε′-jε" and magnetic permeability μr=μ′-jμ" in Ku-band. Enhancement in dielectric losses from 0.3051 to 0.4066 and magnetic losses from 0.0087 to 0.5911 with increasing cobalt concentration from x = 0.00 to x = 1.00 at 12.4 GHz frequency helps in achieving good absorption. Normalized wave impedance and quarter wavelength model are used to maximize absorption or minimize reflection. Significant absorption is achieved by tuning thickness of the sample from 1.8 mm to 3 mm. The composition x = 0.75 exhibits superior absorbing properties by showing reflection loss as low as −31.45 dB along with −10 dB bandwidth of 5.6 GHz at a matching thickness of 2.2 mm. The fabricated ferrite samples are proved to be promising applicants for absorption of electromagnetic interference (EMI) in radar and stealth technology.

Field Synthesis With Azimuthally Varying, Cascaded, Cylindrical Metasurfaces Using a Wave Matrix Approach

In recent years, there has been extensive research on planar metasurfaces capable of arbitrarily controlling scattered fields. However, rigorous studies on conformal metasurfaces, such as those that are cylindrical, have been few in number likely due to their more complex geometry and corresponding analysis. Here, wave propagation in cascaded cylindrical structures consisting of layers of dielectric spacers and azimuthally varying metasurfaces (subwavelength patterned metallic claddings) is investigated. A wave matrix approach, which incorporates the advantages of both ABCD (transmission) matrices and scattering matrices ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> matrices), is adopted. Wave matrices are used to model the higher-order coupling between metasurface layers, overcoming the fabrication difficulties associated with previous works. The proposed framework provides an efficient approach to synthesize the inhomogeneous sheet admittances that realize desired cylindrical field transformations. Design examples are reported to illustrate the power and potential applications of the proposed method in antenna design and stealth technology.

RI-CDI-FDTD method and program implementation for electromagnetic characteristics simulation of lossy Debye dispersive medium

Dispersive media refer to a class of natural substances, including living organisms, composite materials, plasma and water, with diverse applications in areas such as biomedicine, microwave sensing, electromagnetic protection, and stealth technology. In the pursuit of investigating the electromagnetic properties of these media, time-domain numerical methods, including finite difference in time domain (FDTD), finite element method, and time domain boundary integral equation method, have been widely utilized. Time-domain numerical methods are preferred to their frequency-domain counterparts owing to their ability to handle nonlinear and wideband problems, as well as various material properties. The FDTD method, in particular, is a highly adaptable, robust, and easy-to-use numerical method that directly solves the Maxwell equations while also simulating the reflection, transmission, and scattering of electromagnetic waves in complex dispersion media. Nonetheless, the traditional FDTD method suffers low computational efficiency arising from the Courant-Friedrichs-Lewy (CFL) stability condition. To solve the problem of low computational efficiency, a new method, the complying divergence implicit finite-difference time-domain (CDI-FDTD) method with a one-step leapfrog scheme, is introduced for lossy Debye dispersive media. The Maxwell equations in the frequency domain form a starting point, and the Fourier transform is utilized to transform the electromagnetic field components from the frequency domain to the time domain. To approximate the integral terms arising from the frequency-to-time domain transformation, a recursive integration (RI) method is employed. Subsequently, the time-domain Maxwell equations and auxiliary variables are discretized with a one-step leapfrog implicit scheme. The iterative formula of the RI-CDI-FDTD algorithm for lossy Debye dispersive media is then derived. The RI-CDI-FDTD method does not change the formulas of the traditional CDI-FDTD method while only requiring to add auxiliary variables for updating field components to the dispersive medium region. The numerical implementation is straightforward, and the electromagnetic modeling is flexible. Moreover, the unconditional stability of the RI-CDI-FDTD algorithm is proven by using the von Neumann method. Finally, some numerical examples are presented to demonstrate the effectiveness and efficiency of the proposed method. In conclusion, our work contributes a crucial numerical simulation tool to accurately modeling complex dispersive media while providing a systemic stability analysis method for time-domain numerical methods.

Active thermally tunable and highly sensitive terahertz smart windows based on the combination of a metamaterial and phase change material.

A thermally tunable terahertz window based on the combination of a metamaterial and the phase change material VO2 is proposed. The window is composed of two vanadium oxide films with a SiO2 layer sandwiched between them. The thermochromic phase change properties of VO2 are the key to the functionality of the window. By controlling the temperature around the room temperature of 300 K, our material can be used as a smart window and it is able to regulate both the absorption and transmission of external terahertz waves in response to changes in temperature. The absorbance can be regulated by more than 90% and the transmittance by more than 80%. The switching characteristics of the window are explained by the insulator-metal transition that vanadium oxide undergoes during the heating process, while localized surface plasmon resonance explains the perfect absorption. In addition, the designed window is not only insensitive to polarised waves, but is thermally flexible and maintains excellent performance over a wide angular range of 0° to 40°. This design will have significant potential for applications in stealth technologies, thermal sensing and switching, and terahertz energy harvesting.

The Future Combat Pilot - Asleep at the Wheel?

Technology is a key driver for Air Force capability. There is always debate on the future technology that can advance our future combat aircraft such as artificial intelligence (AI), quantum computing and stealth technology. But what about the human in this human-machine team? There appears to be little debate about the humans in this team.

Research progress of infrared stealth technology of micro-nano optical structure (invited)

Research progress of infrared stealth technology of micro-nano optical structure (invited)

High-performance microwave absorption by optimizing hydrothermal synthesis of BaFe12O19@MnO2 core-shell composites.

Stealth technology advances in radar-absorbing materials (RAMs) continue to grow rapidly. Barium hexaferrite is the best candidate for RAMs applications. Manganese dioxide (MnO2) is a transition metal with high dielectric loss and can be used as a booster for changing polarization and reducing reflection loss. The advantages of BaFe12O19 and MnO2 can be combined in a core-shell BaFe12O19@MnO2 composite to improve the material's performance. MnO2 composition, temperature, hydrothermal holding time, and sample thickness all have an impact on the core-shell structure. In this study, a core-shell BaFe12O19@MnO2 composite is synthesized in two stages: molten salt synthesis to produce BaFe12O19 as the core and hydrothermal synthesis to synthesize MnO2 as the shell. In the hydrothermal synthesis, BaFe12O19 and KMnO4 were mixed in deionized water using different mass ratios of BaFe12O19 to KMnO4 (1 : 0.25, 1 : 0.5, 1 : 0.75, and 1 : 1). The main goal of the analysis was to figure out how well the hydrothermal synthesis method worked at different temperatures (140 °C, 160 °C, and 180 °C) and holding times (9 h, 12 h, and 15 h). The composite material was subjected to characterization using a vector network analyzer, specifically at thicknesses of 1.5 mm, 2 mm, 2.5 mm, and 3 mm. The hydrothermal temperature and composition ratio of BaFe12O19 : MnO2 are the most influential parameters in reducing reflection loss. Accurate control of the parameters makes a BaFe12O19@MnO2 core-shell composite structure with a lot of sheets. The structure is capable of absorbing 99.99% of electromagnetic waves up to a sample thickness of 1.5 mm. The novelty of this study is its ability to achieve maximal absorptions on a sample with minimal thickness through precise parametric control. This characteristic makes it highly suitable for practical applications, such as performing as an anti-radar coating material. BaFe12O19@MnO2 demonstrates performance as a reliable electromagnetic wave absorber material with simple fabrication, producing absorption at C and X band frequencies.

Reconfigurable Plasma Composite Absorber Coupled With Pixelated Frequency Selective Surface Generated by FD-CGAN

Conventional plasma absorbers are challenging to obtain high electron density and sizeable spatial scale for effective absorption while meeting the applied requirements of low profile and low power consumption. Although the frequency selective surface (FSS) has proved to realize a lower profile of plasma absorber with some empirical patterns adequately, the issue of the FSS design matching the dispersion distribution of complicated plasmas is still in suspense. A reverse prediction method referenced as the forecast and design Conditional Generative Adversarial Network (FD-CGAN) is proposed to generate a pixelated FSS between double-layer plasma periodic arrays. The reflection attenuation characteristics examined by experiments show that the addition of the FSS makes the coupling absorption effect surpass that of either pixelated FSS or plasma solely. Measurements in reconfigurable working modes and array arrangements demonstrate that the proposed configuration maximizes absorption effectiveness in the same profile, accompanied by the simulation. An interfacial void model is proposed to assist the design of the composite absorbing structure, together with an equivalent circuit for the hybrid absorber including periodic patterns with stochastic distribution characteristics, which analyze the absorption effect of the composite structure. The study provides a new approach for various microwave applications, including multilayer radar-absorbing structures, plasma-based stealth technology, and reconfigurable filters.