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In order to extend the performance of radar absorbing materials, it is necessary to design new structures with wideband properties and large angles of incidence which are also as thin as possible. The objective of this work, realized within the framework of the SAFAS project (self-complementary surface with low signature) is, then, the development of an ultra-wideband microwave absorber of low thickness. The design of such material requires a multilayered structure composed with dielectric layers, metasurfaces, and wide-angle impedance matching layers. This solution has been realized with on-the-shelf materials, and measured to validate the concept. At normal incidence, the bandwidth ratio, defined for a magnitude of the reflection coefficient below −10 dB, is 4.7:1 for an absorber with a total thickness of 11.5 mm, which corresponds to λ/7 at the lowest operating frequency. For an incidence of 60°, this bandwidth ratio is reduced to 3.8:1, but the device remains ultra-wideband.

In this study, we have demonstrated the fabrication of perforated absorbers on two substrates, i.e., ITO/PET and Twill weave cloth. Perforation is required to enable the use of absorbers in the application where air breathability, ventilation and thermal equilibrium are necessary. For perforations, holes were machined in all the layers of the absorber. To ascertain the effect of perforations on both the absorbers’ performance, simulation, using ANSYS HFSS software, was carried out. In the ITO/PET-based absorber, it was found that there were no significant effects of the variation of hole radius on the absorption. However, for textile-based absorber, the hole radius had a significant impact on the absorption. The proposed ITO/PET-based fabricated MMA can absorb radiation in the frequency band from 7.64 GHz to 16.6 GHz, whereas the textile-based absorber can absorb more than 90% of the frequency band corresponding to 6.61 GHz to 17.91 GHz. The measured absorptions are found to be in good agreement with the simulated results. Furthermore, perforation gives two mechanical advantages to the absorber: first, it reduces the absorber’s weight by 25% and 35%, respectively, in the case of ITO/PET- and textile-based absorber, and second, it increases the bendability of the absorber. Through experiments, we found that the perforated sample bends by an extra 22 ∘ and 24 ∘ , respectively, for ITO/PET- and TWC-based absorber when placed as a cantilever. Theoretically, it was calculated that there would be a four-time increase in the absorber’s bendability due to perforations.

Electromagnetic metamaterials are artificial subwavelength composites with periodic structures, which can interact strongly with the incident light to achieve effective control of the light field. Metamaterial absorbers can achieve nearly 100% perfect absorption of incident light at a specific frequency, so they are widely used in sensors, optical switches, communication, and other fields. Based on the development history of metamaterials, this paper discusses the research background and significance of metamaterial perfect absorbers. Some perfect absorption mechanisms, such as impedance matching and coherent perfect absorption, are discussed. According to the functional division, the narrowband, dual frequency, multi-frequency, broadband, and tunable metamaterial perfect absorbers are briefly described.

There are more than thousands of concepts for harvesting wave energy, and wave energy converters (WECs) are diverse in operating principles, design geometries and deployment manners, leading to misconvergence in WEC technologies. Among numerous WEC devices, the point absorber wave energy converter (PAWEC) concept is one of the simplest, most broad-based and most promising concepts that has been investigated intensively all over the world. However, there are only a few reviews focusing on PAWECs, and the dynamical advancement of PAWECs merits an up-to-date review. This review aims to provide a critical overview of the state of the art in PAWEC development, comparing and contrasting various PAWEC devices and discussing recent research and development efforts and perspectives of PAWECs in terms of prototyping, hydrodynamic modelling, power take-off mechanism and control.

A bistable nonlinear energy sink conceived to mitigate the vibrations of host structural systems is considered in this paper. The hosting structure consists of two coupled symmetric linear oscillators (LOs), and the nonlinear energy sink (NES) is connected to one of them. The peculiar nonlinear dynamics of the resulting three-degree-of-freedom system is analytically described by means of its slow invariant manifold derived from a suitable rescaling, coupled with a harmonic balance procedure, applied to the governing equations transformed in modal coordinates. On the basis of the first-order reduced model, the absorber is tuned and optimized to mitigate both modes for a broad range of impulsive load magnitudes applied to the LOs. On the one hand, for low-amplitude, in-well, oscillations, the parameters governing the bistable NES are tuned in order to make it functioning as a linear tuned mass damper (TMD); on the other, for high-amplitude, cross-well, oscillations, the absorber is optimized on the basis of the invariant manifolds features. The analytically predicted performance of the resulting tuned bistable nonlinear energy sink (TBNES) is numerically validated in terms of dissipation time; the absorption capabilities are eventually compared with either a TMD and a purely cubic NES. It is shown that, for a wide range of impulse amplitudes, the TBNES allows the most efficient absorption even for the detuned mode, where a single TMD cannot be effective.

In this article, a metamaterial multi-band perfect absorber with a Pi-inverted-Pi resonator is designed and investigated. It is observed that the meta-surface with a typical metal–insulator-metal structure showed nearly perfect absorption in almost five narrow bands with the lowest 93.6% and the highest 99.7% absorption level, according to the full-wave simulation results. The tunability of the Pi-inverted-Pi resonator has been examined based on the geometrical parameters. The origin of the five resonance bands has been analyzed based on the electric field and charge distributions occurring at the metal–dielectric interface. These resonance peaks are located at 1817.52 nm, 1968.38 nm, 2721.31 nm, 3765.6 nm, and 5740.63 nm wavelengths. The polarization dependence of the structure is investigated under TE and TM light, and our findings reveal that the Pi-inverted-Pi structure is partially polarization-dependent, which is a beneficial property for optical filtering. Compared to similar multi-band absorbers in the literature to the best of our knowledge, our proposed system offers a narrow band with high Q-factors of 118.7, 53, 32, 33, and 32 at each resonance wavelength. Therefore, our proposed design can be used in optical filters, Raman spectroscopy, bio-sensing, and many other applications, especially for the mid-infrared region’s spectroscopic sensing and filtering applications.

Perfect metamaterial absorbers have attracted researchers’ attention in solar energy harvesting and utilization. An ideal solar absorber should provide high absorption, be ultra-wideband, and be insensitive to polarization and incident angles, which brings challenges to research. In this paper, we proposed and optimized an ultra-wideband solar absorber based on Ti-Al2O3 cross elliptical disk arrays to obtain the ultra-wideband absorption of solar energy. The addition of a cavity greatly improves the energy-absorbing effect in the operating band, which has research value. The absorption spectrum and field distribution were analyzed by the finite difference time domain method. For the physical mechanism, the electric and magnetic field distribution indicates that ultra-wideband absorption is caused by propagation surface plasmon resonance (SPR), localized SPR and Fabry–Perot (F-P) resonance excited between Ti and Al2O3 disks. The results demonstrate that the absorption bandwidth with the absorption rate beyond 90% reaches 1380 nm (385–1765 nm), and the average absorption reaches an astonishing 98.78%. The absorption bandwidth matches the main radiation bandwidth of the solar energy, which is approximately 295–2500 nm according to the data from the literature, and the total thickness of the structure is only 445 nm. Moreover, the ultra-wideband solar absorber is insensitive to the polarization angle and oblique incidence angle. The proposed ultra-wideband solar absorber has research and application value in solar energy harvesting, photothermal conversion and utilization.

Graphene's unique electrical properties make it an ideal material for conductive inks, enabling efficient electromagnetic wave absorption at WiMAX (Worldwide Interoperability for Microwave Access) frequency bands. This paper presents the preparation of graphene-based conductive ink for use in flexible metamaterial absorbers on paper substrates, targeting WiMAX applications. The ink was synthesized to ensure homogeneous dispersion and excellent conductivity, suitable for printing on flexible, environmentally friendly paper substrates. The ink formulation consisted of 6% modified graphene, 0.75% cellulose acetate butyrate binder, and solvent. This ink formulation has a resistance value of approximately 22 Ω cm and sheet resistance of 7.6 Ω/square. The resistance of ink can be altered by changing the binder content for the different absorption ranges. The resulting metamaterial absorber demonstrated superior flexibility, light weight, and effective absorption performance within the WiMAX frequency range. The unit cell consists of a cross-shaped resonator and a thin layer of aluminum behind the ground layer. Cross-shaped layers of graphene ink are printed on Kodak printing paper, making them completely bendable along the surfaces. This flexible absorber is environmentally friendly because it does not waste chemicals and is biodegradable. A full-wave simulation was conducted for the graphene material absorber (GMA) prototype, and it was then fabricated using a two-axis computer numerical control (CNC) printer. The experimental results showed that the absorption of incident angle-insensitive GMA at 5.8 GHz reached 95%.

Theoretical and numerical studies were conducted on plasmonic interactions at a polarization-independent semiconductor–dielectric–semiconductor (SDS) sandwiched layer design and a brief review of the basic theory model was presented. The potential of bull’s eye aperture (BEA) structures as device elements has been well recognized in multi-band structures. In addition, the sub-terahertz (THz) band (below 1 THz frequency regime) is utilized in communications and sensing applications, which are in high demand in modern technology. Therefore, we produced theoretical and numerical studies for a THz-absorbing-metasurface BEA-style design, with N-beam absorption peaks at a sub-THz band, using economical and commercially accessible materials, which have a low cost and an easy fabrication process. Furthermore, we applied the Drude model for the dielectric function of semiconductors due to its ability to describe both free-electron and bound systems simultaneously. Associated with metasurface research and applications, it is essential to facilitate metasurface designs to be of the utmost flexible properties with low cost. Through the aid of electromagnetic (EM) coupling using multiple semiconductor ring resonators (RRs), we could tune the number of absorption peaks between the 0.1 and 1.0 THz frequency regime. By increasing the number of semiconductor rings without altering all other parameters, we found a translation trend of the absorption frequencies. In addition, we validated our spectral response results using EM field distributions and surface currents. Here, we mainly discuss the source of the N-band THz absorber and the underlying physics of the multi-beam absorber designed structures. The proposed microstructure has ultra-high potentials to utilize in high-power THz sources and optical biomedical sensing and detection applications based on opto-electronics technology based on having multi-band absorption responses.

Metamaterial absorbers (MMAs) with dynamic tuning features have attracted great attention recently, but most realizations to date have suffered from a decay in absorptivity as the working frequency shifts. Here, thermally tunable multi-band and ultra-broadband MMAs based on vanadium dioxide (VO2) are proposed, with nearly no reduction in absorption during the tuning process. Simulations demonstrated that the proposed design can be switched between two independently designable multi-band frequency ranges, with the absorptivity being maintained above 99.8%. Moreover, via designing multiple adjacent absorption spectra, an ultra-broadband switchable MMA that maintains high absorptivity during the tuning process is also demonstrated. Raising the ambient temperature from 298 K to 358 K, the broadband absorptive range shifts from 1.194–2.325 THz to 0.398–1.356 THz, while the absorptivity remains above 90%. This method has potential for THz communication, smart filtering, detecting, imaging, and so forth.