Electromagnetic metamaterials are artificial materials engineered to gain their properties from structure rather than composition, using metallic inclusions (such as wire- or ring-type resonators) smaller than the wavelength of an illuminated electromagnetic wave to achieve effective macroscopic behavior. Although initial interest in metamaterials arose due to their ability to exhibit exotic electromagnetic effects (such as negative index of refraction), which are impossible to achieve with natural materials, they are excellent candidates for electromagnetic wave absorbers. The metamaterial perfect absorbers (MPAs) have utilized a three-layer structure (two metallic layers separated by a dielectric substrate). Magnetic coupling is achieved via anti-parallel currents in the conductor on the top layer and the ground conductor on the bottom layer. Since the electric and magnetic response is influenced by the dimension of the resonators, it is possible to take theoretical and experimental approaches to achieve two or more resonances by scaling multiple structures.
This study aims to overcome the narrow-band limit of the typical MPAs by using the multi-resonance of split cut wires (SCWs) or patterned grids on grounded dielectric substrate in the frequency region of GHz, THz, and UHF. First, in the GHz and THz regions, the scaling effect of a single SCW on magnetic resonance is investigated, and then wide-band or multi-band absorbance is investigated with multiple arrangements of SCWs of different dimensions. In UHF region, where the various types of commercial RF-ID are operated, the multi-resonance behavior of patterned grid is investigated, focusing on the design of thin absorbers at 800 MHz and 2.45 GHz.
Computational tools (HFSS 14.0) are used to model the interaction between electromagnetic waves and metamaterials for an accurate estimation of the MPA’s performance. All simulation output is the reflection coefficient. Another parameter of interest is the surface current, which is used to show the resonating behavior of the metallic portions of the MPAs. The samples for measurement of reflection loss in GHz region were fabricated by printed circuit board (PCB) method in which a certain thickness of copper is deposited on both sides of a photo-sensitized board (FR-4). The reflection loss was measured using the free space measurement system which is composed of a pair of spot-focusing horn lens antennas for C-band (5.8-8 GHz), a specimen holder, and network analyzer.
In the metamaterials of SCWs, magnetic resonance resulting from anti-parallel currents at cut wire portions is observed at the frequency of minimum reflection loss in GHz region. The variation of resonance frequency with SCW length is well consistent with the circuit theory of L-C resonator. Multi-band or broad-band power absorption is obtained in the metamaterials with multiple arrangement of SCWs of different length on top layer. In particular, the multi-resonance of SCWs is greatly dependent on the side spacing between SCWs. With increasing the side spacing, the second absorption peak increases and moves slightly to a lower frequency, which is due to the reduction in coupling strength between SCWs.
The similar simulation results of multi-resonance effects of SCWs are reproduced in the THz metamaterial absorbers, where the dimensions of unit cell and SCWs are reduced to a level of micrometer. Dual-band absorption can be obtained by arrangement of two SCWs of different length on top layer of the grounded polyimide film substrate with thickness of 7 μm. which is due to multiple magnetic resonance by scaling of SCWs. With increasing the side spacing between SCWs, a more enhanced absorption peak is observed at the first resonance frequency that is shifted to a lower frequency.
The grid-patterned metamaterial absorbers show dual-band absorption peaks at 880 MHz and 2.45 GHz with a small substrate thickness (about 3.7 mm), which can be usefully applied to electromagnetic compatibility (EMC) in RF-ID system. The UHF metamaterial absorbers also exhibit a good oblique incidence performance. At 880 MHz, in particular, a high level of absorption (above 10 dB) is maintained up to 60 degrees of incidence angle for both TE and TM polarization.