Ultra-wideband and Polarization-Insensitive Perfect Absorber Using Multilayer Metamaterials, Lumped Resistors, and Strong Coupling Effects
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We theoretically and experimentally proposed a new structure of ultra-wideband and thin perfect metamaterial absorber loaded with lumped resistances. The thin absorber was composed of four dielectric layers, the metallic double split ring resonators (MDSRR) microstructures and a set of lumped resistors. The mechanism of the ultra-wideband absorption was analyzed and parametric study was also carried out to achieve ultra-wideband operation. The features of ultra-wideband, polarization-insensitivity, and angle-immune absorption were systematically characterized by the angular absorption spectrum, the near electric-field, the surface current distributions and dielectric and ohmic losses. Numerical results show that the proposed metamaterial absorber achieved perfect absorption with absorptivity larger than 80% at the normal incidences within 4.52~25.42 GHz (an absolute bandwidth of 20.9GHz), corresponding to a fractional bandwidth of 139.6%. For verification, a thin metamaterial absorber was implemented using the common printed circuit board method and then measured in a microwave anechoic chamber. Numerical and experimental results agreed well with each other and verified the desired polarization-insensitive ultra-wideband perfect absorption.
KeywordsMetamaterial absorbers Polarization Subwavelength structures Ultra-wideband
Metallic double split ring resonators
Periodic boundary conditions
Perfect metamaterial absorber
Split ring resonator-I
Split ring resonator-II
As an artificially engineered material, metamaterial has attracted significant interest because it exhibited fantastic electromagnetic properties unusual or difficult to obtain over the last decade [1, 2, 3]. With the rapid development, metamaterial with dynamical mass anisotropy has been applied to develop acoustic cloaks, hyperlenses, perfect absorbers, gradient index lenses [4, 5, 6, 7], metalense, optofluidic barrier, polarization convertor, etc. [8, 9, 10, 11, 12, 13, 14, 15, 16]. In particular, the perfect metamaterial absorber (PMA) with ultrathin profile and near-unity absorption was firstly proposed by Landy et al. . Relative to conventional absorbers, metamaterial absorber, which offers great benefits of thin profile, further miniaturization, increased effectiveness, and wider adaptability, has become promising applications of metamaterials. Later, researchers make several efforts on PMA to achieve wide incident angle absorption [17, 18, 19], multi-band absorption [20, 21], polarization-insensitive absorption [22, 23, 24], and the tunable absorption [25, 26]. However, absorbers with narrow bandwidth limit their applications in practice. Hence, it is necessary to design the ultra-broadband, polarization-insensitive, and thin metamaterial absorber.
To increase the absorption bandwidth, several methods such as by using the multi-resonance mechanism [27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38], the fractal structures , the multilayer [40, 41, 42, 43, 44], the magnetic medium [45, 46], and loading the lumped elements [47, 48, 49] have been proposed in the design of gigahertz and terahertz metamaterial absorbers. For instance, a broadband polarization-insensitive perfect absorber exhibiting a bandwidth of 9.25 GHz has been designed in a single layer based on the double octagonal-ring metamaterials and lumped resistances . Additionally, a gigahertz perfect metamaterial-inspired absorber was proposed which was composed of three-layer substrates, double split-serration-rings, and a metal ground . Although a relative bandwidth of 93.5% was obtained, the absorption bandwidth is still insufficient for applications, such as electromagnetic protection, stealth, and electronic warfare.
Different from the previous metamaterial absorbers, we proposed a thin and ultra-wideband perfect metamaterial absorber by combining the resonant and resistive absorptions using strong coupling effects. The absorber was composed of four dielectric layers, two metallic double split ring resonators (MDSRR) and several lumped resistors. The characteristics of polarization-insensitive and wide-incident absorption had been verified both numerically and experimentally. This perfect metamaterial absorber is promising for many practical applications such as radar cross scatter reduction, stealth, and electromagnetic protection in different flight platform.
It is found that the absorption is close to 100%, when the real part and imaginary part of the effective impedance are respectively close to 377 Ω and 0. The absorptivity is enhanced because of the different resonant modes. Generally, the excellent absorption could be obtained as the effective permittivity was equal to effective permeability. So the broadband absorption would be achieved by modulating the effective parameters.
The ultra-wideband metamaterial absorber was simulated by employing the commercial software, Ansoft High Frequency Structure Simulator (HFSS 18.0), which was based on the finite-element analysis method. In the calculation, a plane electromagnetic wave with the electric field along the direction of x-axis was used as the incidences, which was perpendicularly irradiated to the resonance structure along the direction of the z-axis (shown in Fig. 1). The frequency range from 1.0 to 30 GHz of the incidences had been used in simulation. The size of the incidences should be slightly larger than that of the presented period of the structure; at the same time, enough simulation times and the suitable boundaries (periodic boundaries in directions of x- and y-axis and perfectly matched layers in direction of z-axis) should be utilized for ensuring the accuracy of calculation results.
Results and Discussion
A parametric study was carried out by ANSYS HFSS Solver. In this study, it was main objective to achieve ultra-broadband absorption. According to this goal, some parameters of the lumped resistances R1,2 and R3,4 in the inner and outer split rings, the cell length P of the PMA, the length s of the splits for F-MDSRR and S-MDSRR, the thickness d1 of the antireflection coating substrate, and the thickness d2 were selected in the study.
From Figs. 2 and 3, it could be seen that the absorption bandwidth of the proposed PMA was sensitive to the thicknesses of d1 and d2, and the values of lumped resistances. Moreover, the splits in the F-MDSRR and S-MDSRR were necessary for achieving the wideband absorption in our design. Hence, the thicknesses and the lumped resistances needed to be optimized for ultra-broadband absorption.
Fabrication and Measurement
In conclusion, we have proposed, designed, and fabricated an ultra-wideband perfect metamaterial absorber with polarized-insensitivity and wide-incident absorption. The angular absorption spectrum, surface current, and near electric-field distributions were explored to validate the excellent characteristics of the proposed perfect metamaterial absorber with strong coupling effects. The fabricated metamaterial absorber device was fabricated, measured, and analyzed. The experimental results indicated that the ultra-broadband absorption from 4.48 to 25.46 GHz could be achieved with absorptivity larger than 80% with normal incidences for x-polarization and y-polarization. For the oblique incidences with the incident angle of 45°, the perfect metamaterial absorber exhibited the relative bandwidth of 136% with absorptivity larger than 60% for different polarized incidences. This perfect metamaterial absorber device with the innovation is promising for many practical applications such as radar cross scatter reduction and electromagnetic protection in different flight platform.
This work was supported in part by the National Natural Science Foundation of China under Grant Nos. 61501494, 61471389, 61671464, and 61701523, in part by the Natural Science Foundational Research Fund of Shaanxi Province under Grant No. 2017JM6025, in part by the Young Talent fund of University Association for Science and Technology in Shaanxi, China (No.20170107), in part by the Key Program of National Natural Science Foundation of Shaanxi Province under grant No.2017KJX-24, National Defense Foundation of China (2201078), and Aviation Science Foundation of China under No. 20161996009. They also thank the reviewers for their valuable comments.
Availability of Data and Materials
All date are fully available without restriction.
SJL conceived the research and wrote the manuscript. PXW, YLZ, and JFH conducted the simulations and analyses. HXX, XYC, and LMX supervised the whole work. CZ and HHY completed the whole measurement and assisted in processing the data and figures. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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