Objective Metamaterial electromagnetic absorbers play an important role in many aspects, such as electromagnetic compatibility, radar stealth, and sensing. Owing to the limited applications of narrow-band absorbers, researchers combine resonant unit cells with different sizes in a plane or stack them layer by layer to obtain a wide bandwidth absorption. Most of these structures are designed with metal as the main material; this not only increases the complexity of design and production, but also loses the absorption of the metal, and these defects greatly limit the development of electromagnetic absorbers. Recently, all-dielectric metamaterial absorbers have become a new research focus because of their potential in improving wide-band impedance matching, and water has gradually been applied to the all-dielectric design of electric broadband absorbers. Simultaneously, water has large permittivity and excellent dispersion characteristics in the microwave band and its fluidity and transparency can satisfy the bending and transparency characteristics of materials. Therefore, the design of broadband absorbers based on water materials provides an excellent possibility for the design of ultrabroadband electromagnetic absorbers.
Methods A water-based metamaterial absorbing structural unit was first designed based on the basic principle of the metamaterial absorbers. The structure mainly includes three parts: a metal bottom plate, a resin shell, and a circular water column. Moreover, the overall structure comprises a unit cell that periodically expands in the x and y directions. Furthermore, we used computer simulation technology software to simulate the scattering parameters of the absorbing structure and calculated the absorptivity by the related formula in origin. To further analyze the formation of the wide absorption band, electric and magnetic fields and power loss density at the resonance points were analyzed. Based on the above, the structure was analyzed by changing the incident and polarization angles to explore its absorption performance. Finally, according to the Debye formula, the dielectric constant of water at different temperatures was calculated, and the absorption rate of the wave-absorbing structure was simulated to analyze the performance of the designed absorber when the temperature changed.
Results and Discussions A water-based metamaterial absorber is designed based on the principle of equivalent medium and impedance matching (Fig. 1). The results show that the frequency range where the absorber's absorptivity of the designed water-based absorber exceeds 90% is 8.2--78.32 GHz (Fig. 3), whereas the absorption rate is relatively low at low frequencies (Fig. 4). The equivalent complex impedance of the absorbing structure is simulated, and the result shows that its real and imaginary parts are always kept at about 1 and 0, respectively (Fig. 5). To analyze the formation mechanism of the absorption peaks, the electric field, magnetic field, and power loss density at five resonance frequencies were simulated. Different local resonance effects occured at these frequencies in the absorber, and most of the power losses occured in the water layer (Fig. 6). The absorption rates of the metamaterial absorber at different incident angles were analyzed in transverse-electric (TE) and transverse-magnetic (TM) polarizations, respectively. The results show that the proposed metamaterial absorber can maintain an excellent absorption characteristic when the incident angle changes (Fig. 7). Then, the absorptivity of the structure was simulated at different polarization angles. The results show that the structure has excellent polarization-independent characteristics for electromagnetic waves (Fig. 8). Finally, the absorption performances of the water-based metamaterial absorber at different temperatures were analyzed. The results prove that except in the frequency band near 12 and 22.8 GHz, the absorption rate of the absorber is almost insensitive to temperature changes, and remains above 90% throughout the designed waveband (Fig. 9).
Conclusions In this paper, a water-based ultrabroadband metamaterial absorbing structure is proposed. According to the electromagnetic characteristics of water in the microwave band, the structure realizes an excellent absorption performance of over 90% for incident electromagnetic waves in the 8.2--78.32 GHz frequency band. When the incident angle of electromagnetic waves changes in the TE and TM modes, respectively, the structure has good absorption characteristics. When the polarization angle of the electromagnetic wave is changed, the results show that the structure has excellent polarization-independent characteristics. The absorptivity of the absorber is also analyzed when the temperature changes and the results show that as the temperature increases, absorption effect decreases slightly around 12 and 22.8 GHz, and absorption rates of other frequency bands stay above 90%. The water-based metamaterial absorber has not only the wide absorption-frequency band and high absorptivity but also the characteristics of insensitivity to the incident angle, polarization-independence, and thermal stability. Simultaneously, the structure has low complexity, good transparency, and excellent bending characteristics, which is essential to the application and development of new dielectric wave absorbers.
