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An ultrathin and flexible metamaterial absorber (MA) with triple absorption peaks is presented in this paper. The proposed absorber has been designed in such a way that three absorption peaks are located at 8.5, 13.5, and 17 GHz (X and Ku bands) with absorption of 99.9%, 99.5%, and 99.9%, respectively. The proposed structure is only 0.4 mm thick, which is approximately 1/88, 1/55, and 1/44 for the respective free space wavelengths of absorption frequency in various bands. The MA is also insensitive due to its symmetric geometry. In addition, the proposed structure exhibits minimum 86% absorption (TE incidence) within 60° angle of incidence. For TM incidence, the proposed absorber exhibits more than 99% absorptivity up to 60° incidence. Surface current and electric field distributions were investigated to analyze the mechanism governing absorption. Parameter analyses were performed for absorption optimization. Moreover, the performance of the MA was experimentally demonstrated in free space on a sample under test with 20 × 30 unit cells fabricated on a flexible dielectric. Under normal incidence, the fabricated MA exhibits near perfect absorption at each absorption peak for all polarization angles, and the experimental results were found to be consistent with simulation results. Due to its advantages of high-efficiency absorption over a broad range of incidence angles, the proposed absorber can be used in energy harvesting and electromagnetic shielding.

Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed.

This study proposes and experimentally demonstrates enhanced broadband absorption with twisted multilayer metal–dielectric stacking. Compared with the traditional metal–dielectric pyramid, the resonance frequencies of the third-order magnetic resonances in the twisted quadrangular frustum redshifted obviously. Hence, the proposed structure enables an ultra-broadband absorption by combining the third-order magnetic resonances with the fundamental mode. The broadband absorption is insensitive to the incident wave polarization, whereas the twisted angle of the stacking plays an important role in deciding the absorption bandwidth. The sample was fabricated via the multi-material hybrid micro-droplet jetting modeling (MHMJM) technology to verify the enhanced absorbing performance. The measured results suggest that the proposed strategy provides a potential path to realize broadband electromagnetic wave absorption. Moreover, it is possible to extend the twisted metamaterial to the terahertz and infrared frequencies using the advanced nano fabrication techniques.

This paper proposes a wideband and polarization-insensitive metamaterial absorber (MA) based on tractable conductive plastic, which is compatible with an additive manufacturing technology. We provide the design, fabrication, and measurement result of the proposed absorber and investigate its absorption principle. The performance characteristics of the structure are demonstrated numerically and experimentally. The simulation results indicate that the absorption of this absorber is greater than 90% in the frequency range of 16.3–54.3 GHz, corresponding to the relative absorption bandwidth of 108%, where a high absorption rate is achieved. Most importantly, this additive manufactured structure provides a new way for the design and fabrication of wideband MAs.

Purpose 3D printing based on additive manufacturing has advantages in manufacturing products with high geometrical complexity. However, there are many limitations to print plastic products with the existing commercial 3D printers. The polymer materials processing industry needs new devices which can satisfy the trend of processing individual units and small batch sizes of plastic parts. Design/methodology/approach In this study, a freeform fabrication system with the method of polymer melt droplet deposition is proposed. The performance of this system under different conditions was studied by changing the operating parameters. Furthermore, the dimensional uniformity of droplets and their deposition process are analyzed, and a plastic sample was fabricated with this system as an example. Findings The results show a clear correlation between the processing parameters and the droplet diameter. In the experiment for examining the dimensional uniformity of the droplet, the droplets become spindles, and there appears a melt filament between the droplets. The variation of the droplet’s diameters is within 5 per cent. Furthermore, a successfully processed rectangular plastic sample verified the feasibility of this technology for the printing of plastic products. Originality/value A freeform fabrication system with polymer melt droplet deposition is proposed, which can process a wide variety of materials in the form of standard granulates like injection molding or extrusion. Based on the principle of droplet deposition, multi-component or colorful materials can be printed.

In this work, an ultra-broadband and polarization-insensitive metamaterial absorber (MA) with high optical transparency is presented. Indium-tin-oxide (ITO) films are used as frequency selective surfaces in multiple conductive layers and also as the reflective backplane. Numerical simulation analysis demonstrates that the absorptivity of the proposed absorber exceeds 90% in the frequency range from 10 to 75.5 GHz, corresponding to a relative absorption bandwidth of 153%. The proposed structure has a total thickness of 2.9 mm, which is approximately 1/10 of the free space wavelength of its lowest operating frequency. The absorber also has good polarization insensitivity due to its symmetric geometry. Moreover, for TE-polarized waves, the proposed absorber can maintain an absorptivity greater than 70% for frequencies ranging from 10 to 80 GHz as the incident angle is increased to 45°. For TM-polarized waves, it provides absorptivity of more than 80% at incident angles up to 60°. Surface current distribution on each conductive ITO layer was simulated to analyze the absorption mechanism of the MA. A prototype composed of 16 × 16 unit cells was fabricated and the experimental results show a good agreement with the numerical simulations. Due to its ultra-wideband absorption and wide-incident-angle stability, the proposed MA has potential value in electromagnetic applications such as optically transparent EM shielding and microwave radiation protection.

We propose a dual-band metamaterial absorber (MA) based on a stereo resonant structure. The unit cell of the proposed MA is composed of double metallic rings with a rampart on each ring into a dielectric substrate. In contrast to the conventional plane resonator, our proposed metamaterial exhibits wider incident angle stability. The power loss density distribution shows that the standing ramparts play a key role in enhancing the electromagnetic wave absorption. Experimental results verify the wide-incident-angle stability of the absorber. Our proposed strategy paves a new way on developing MAs with wide incident angle stabilities.

Fused depositing modeling (FDM) is a fast, efficient process among 3D printing techniques. In this paper, we report the fabrication of the 3D printed flexible circuits based on graphene. Modified two-step in-situ reduced method is used to synthesize reduced graphene oxide (r-GO), whose conductivity can reach to 600 S/cm. Polylactic acid (PLA) and r-GO are mixed by melt blending. The SEM images show that the r-GO can be homogenous dispersed in the PLA. The 3D print-used composites filaments with the diameter of 1.75 mm are fabricated through melt extrusion. The conductivity of the composite filaments from 3D printer can reach to 4.76 S/cm (6 wt% r-GO). The orientation of r-GO occurs during the extrusion process, which contributing to increase the conductivity of the filaments. The composite also exhibit superior mechanical property. The printed 2D and 3D flexible circuits have strong interface bonding force between the layers. The filaments from 3D printer can replace the copper wire because of the high conductivity. This arbitrary 3D graphene-based structure printing technic may open a new prospect in electronic and energy storage fields.

In this paper, we investigate a novel efficient metasurface-based wideband cross-polarization converter. The device rotates the y-polarized electromagnetic (EM) wave into an x-polarized EM-wave for a wide range of wavelengths (9.8 GHz–18.9 GHz) and vice versa. The unit cell of the proposed converter is very small and consists of four metal/dielectric layers. The asymmetric resonators represent a solid base to allow higher conversion efficiencies, while the slightly varying magnetic and electric resonances between neighboring layers produce a broad operating frequency range. Furthermore, only additive manufacturing technology was used, which offers a low-cost and simple solution to produce the complicated multilayer device. Both measured and simulated results are in good agreement confirming the excellent performance. This study can help both the affordable design and fabrication of multilayer wideband polarization converters in the future.

Meta-hologram is a hologram constructed with metasurface which is composed of subwavelength optical antennas. Most of the meta-holograms are phase-only modulated. The phase distributions in these holograms are obtained by phase retrieval algorithm, which requires a lot of time and computing resources. Here, we design and demonstrate a meta-hologram with both phase and amplitude modulations for three-dimensional (3D) display by a simple method of inverse Fresnel-Kirchhoff integral in terahertz (THz) waveband. In this work, a meta-hologram with 8-level phase and 8-level amplitude modulations is realized by 64 subwavelength metallic C-shaped slot antennas. A THz holographic imaging system is utilized to detect the reconstructed images on different z planes along the direction of THz wave propagation. After the meta-hologram, three letters are clearly reconstructed in the preset positions. The meta-hologram with both phase and amplitude modulations can offer great potential applications in cartoon, virtual reality, and other applications bases on 3D figures display.

In this paper, a tunable polarization-dependent terahertz (THz) metamaterial absorber based on liquid crystal (LC) is presented. The measurement results show that absorption peak is at 239.5 GHz for a TE-polarized wave and 306.6 GHz for a TM-polarized wave, without exerting the bias voltage on the LC layer. An increase in bias voltage affects the orientation of LC molecules and causes redshifted resonant frequencies. By adjusting the bias voltage from 0 to 10 V, frequency tunabilities of 4.7% and 4.1% for TE- and TM-polarized waves, respectively, were experimentally demonstrated. Surface current and power loss distribution was analyzed to explain the physical mechanism of the absorber, while the absorption dependence on geometrical parameters and incident angles was also studied in detail. According to the obtained results, the proposed absorber is shown here to be capable of achieving tunable polarization-dependent absorption, and to have potential application in terahertz polarization imaging, terahertz sensing, and polarization multiplexing.