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This work introduces a wideband metamaterial (MM) absorber designed to operate effectively across a wide reception angle and be polarization-insensitive within ISM band (2.4 GHz) applications. The proposed absorber unit cell comprises four copper sectors loaded with lumped resistors and a full copper ground plane hosted onto two FR4 substrates. Furthermore, an air layer suspended between the ground plane and a FR4 substrate is applied to achieve wideband absorption. In addition, the simulation results show that particular design factors, such as lumped resistors and unit cell geometry, can be optimized to improve the efficiency of the absorber. The simulations demonstrate that the proposed absorber achieves a wideband absorption, exceeding 90%, over a broad frequency range from 1.94 GHz to 2.98 GHz. The designed absorber was fabricated and tested, and the simulation and measurement results were agreed well.

Multi-band terahertz (THz) absorber based on a non-symmetric double-sided graphene comb resonator array is designed and simulated by the finite element method (FEM) in CST Software. Then, an equivalent circuit model (ECM) based on admittance with a fast MATLAB code is proposed to analyze the absorber in the THz region. The admittance-based ECM approach could be used for any metamaterial absorber containing one layer of resonators sandwiched between two dielectric slabs and backed by a metal layer consisting of a layer of resonators with a thickness much smaller than the minimum wavelength in the considered wavelength range. The proposed absorber is dynamically tunable with a one-layered resonator array. It has strong linear dichroism (LD) response of 98% and the frequency range of 0.7-5 THz with absorption $>$ 96%: two absorption bands for TE mode and three for TM mode. The proposed absorber can be used in polarization-sensitive devices and systems in the THz region. The ECM model of the metastructure was derived to provide an efficient approach to analyzing the performance of the absorber. The FEM simulation results are in good agreement with the ECM ones.

An all-dielectric transparent metamaterial absorber with encapsulated water is demonstrated in this paper. Because the proposed absorber is realized using only water and polydimethylsiloxane (PDMS) without any conductive patterns, optical transparency is achieved. In addition, the high dielectric loss of water renders it a good candidate for an electromagnetic absorber. The absorptivity is increased by encapsulating the water within the PDMS, and the absorptivity of the proposed absorber is numerically compared with that of the PDMS and water with the same size. When the proposed absorber is realized using two layers, 92.5% absorptivity is achieved at 10.8 GHz, and the absorptivity exceeds 90% in the range of 10.45 to 11.20 GHz, which corresponds to 6.9% bandwidth. Therefore, the proposed absorber has advantages of high transparency, low cost, wide absorption bandwidth, and eco-friendly fabrication.

This paper proposes a switchable metasurface for broadband and polarization-insensitive absorption, whose absorption spectrum can be fluidically switched using liquid metal. The proposed metasurface consists of a top metallic pattern, a flexible printed circuit board substrate, a polydimethylsiloxane substrate, and a bottom metallic ground plane. The proposed unitcell is inspired by a four-circular-sector structure. To maintain the symmetrical geometry and increase the switching range, a symmetrical microfluidic channel is designed with a circular ring capillary and four isolated patches. To measure the absorptivity of the proposed metasurface, 10 × 10 unitcells are fabricated with 20 inlets and 20 outlets. When the microfluidic channels of the metasurface are empty, the fabricated metasurface achieves an absorptivity of higher than 90%, from 6.23 to 12.14 GHz. When eutectic gallium-indium alloy is injected into the microfluidic channels, the fabricated metasurface achieves higher than 90% absorptivity, from 5.44 to 6.12 GHz. Thus, for both cases, the absorptivity is constant at different incident polarization angles.

In this paper, we propose a metamaterial absorber using 3-D printing technology. To miniaturize the footprint size of a unit cell, we introduce a 3-D printed stair-like Jerusalem cross (JC) structure. It is demonstrated that the absorption frequency can be decreased by building up one and two stairs from the planar JC. In this paper, the unit cell footprint size of the three-stair JC is reduced by 41% compared with the unit cell footprint size of the planar JC. The proposed metamaterial absorber with 9×9 unit cells is fabricated using a 3-D printer. Poly lactic acid is used as a dielectric material for 3-D printing, and a conductive pattern is realized by applying the silver paste to the 3-D printed dielectric structure. The simulated and measured absorptivity are 99.9% and 99.8%, respectively, at 5.18 GHz. Due to the symmetric unit cell structure, the absorptivity does not change, although the incident polarization gets rotated.