Conceptual-based design of an ultrabroadband microwave metamaterial absorber Sichao Qua,1, Yuxiao Houa,1, and Ping Shenga,2 aDepartment of Physics, Hong Kong University of Science and Technology, Hong Kong, China Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved August 4, 2021 (received for review June 07, 2021) By introducing metallic ring structural dipole resonances in the micro- In this work, the basic building block in our design is just a wave regime, we have designed and realized a metamaterial absorber metallic ring that is fabricated by the printed circuit board (PCB) with hierarchical structures that can display an averaged −19.4 dB technology. The final integrated sample can exhibit near-perfect reflection loss (∼99% absorption) from 3 to 40 GHz. The measured absorption from 3 to 40 GHz, covering the whole high-frequency performance is independent of the polarizations of the incident wave 5G bands (32). Another crucial measure of an absorber is its thick- at normal incidence, while absorption at oblique incidence remains ness. While a thick enough absorber can absorb everything over considerably effective up to 45°. We provide a conceptual basis for all frequency regimes, it is nevertheless impractical in terms of our absorber design based on the capacitive-coupled electrical di- applications. For a passive absorber, there is a common standard pole resonances in the lateral plane, coupled to the standing wave set by the causality-dictated minimum sample thickness that is along the incident wave direction. To realize broadband imped- associated with any given reflection spectrum (33). In our case, ance matching, resistive dissipation of the metallic ring is optimally the overall thickness of the absorber is 14.2 mm, only 5% over tuned by using the approach of dispersion engineering. To further the minimum thickness dictated by the causality limit. This the- extend the absorption spectrum to an ultrabroadband range, we oretical limit also plays a crucial role in the impedance design of employ a double-layer self-similar structure in conjunction with the the acoustic (34–38) and microwave transmission line network sys- absorption of the diffracted waves at the higher end of the fre- tems (6, 39, 40). The causal optimality of broadband metamaterial quency spectrum. The overall thickness of the final sample is 14.2 absorber has been reported in acoustic systems (35, 36). However, to mm, only 5% over the theoretical minimum thickness dictated by the the best of our knowledge, the electromagnetic counterpart has causality limit. not yet been experimentally realized, which therefore serves as APPLIED PHYSICAL SCIENCES one of our motivating targets. Although metallic ring structures metamaterials | microwave absorption | high-impedance resonances | have been extensively studied in the literature (8, 21, 41, 42), it dipole resonances | hierarchical structures has not been found that they can serve as the basis for broadband microwave absorption. We show that the crucial underlying physics he advent of fifth generation (5G) technology implies an order lies in the generation of two high-impedance resonances from the Tof magnitude, or more, microwave power permeating the 5G- interaction of the electric dipole resonance of the metallic ring active space, owing to the usage of multiple high-frequency mi- with its image resonance, and the resulting impedance matching crowave bands (1), a significant increase in transmission stations, attained by adding resistive loss to the metallic ring. The latter is and the fact that the microwave power varies quadratically as a usually denoted dispersion engineering (21). function of frequency. Associated with this microwave power Here, we iterate the following four conceptual-based elements increase, there is a need for balancing the necessary monitoring that underpin our absorber design and its excellent performance. activities and privacy as well as for remediating health concerns (2–4) arising from the long-term exposure to a much higher Significance microwave power environment. A suitable microwave absorption structure can play an important role in resolving such issues. Mean- While microwave absorption is a widely pursued topic, a conceptual- while, there also exists extensive demands for electromagnetic ab- based design can offer a theoretical basis for generalization sorption, with applications in electromagnetic compatibility (5), and improvements. We offer a design recipe for ultrabroadband radar cross-section reduction (6), energy harvesting (7), etc. The absorption based on the use of electrical dipole resonance in a recent development of metamaterial absorbers, based on designed metallic ring to generate, via interaction with its image reso- structures with subwavelength thickness, has injected new mo- nance, two high-impedance resonances. Impedance matching mentum to this subject, with potential applications (8) to broad over the frequency range in between the two resonances is frequency bands ranging from microwave to terahertz (9–11), in- obtained by adding resistance to the metallic ring. To extend frared (12–14), and visible light (15, 16). Due to microwave’slong the absorption to an ultrabroadband spectrum, we employ a wavelength and high penetrability through solids, its absorption double-layer self-similar structure in conjunction with absorp- has always been the most challenging. tion of the diffracted waves at the higher frequency end. The An ideal microwave absorber should absorb over a wide fre- resulting absorber pushes the overall performance close to the quency bandwidth, with simple geometry for both experimental causality limit over a large absorption bandwidth. implementation and mass production. However, recent metamaterial- based absorbers can only display near-perfect absorption at ei- Author contributions: P.S. designed research; S.Q., Y.H., and P.S. performed research; S.Q., ther one frequency (13, 17–19) or several discretized frequencies Y.H., and P.S. analyzed data; and S.Q. and P.S. wrote the paper. (9, 20) due to the inherent resonance-based mechanism of met- The authors declare no competing interest. amaterials and their attendant dispersive characteristics (21). In This article is a PNAS Direct Submission. order to extend the absorption frequency spectrum, many efforts This open access article is distributed under Creative Commons Attribution-NonCommercial- have been devoted to either increasing the dissipation (22–26), NoDerivatives License 4.0 (CC BY-NC-ND). for example, by using resistive sheets or loading with lumped 1S.Q. and Y.H. contributed equally to this work. elements, or superposing resonant units (27–31), for example, by 2To whom correspondence may be addressed. Email: [email protected]. exploiting multilayer patch absorbers. Yet, the broadband per- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ formance has still been limited, and the geometry of such struc- doi:10.1073/pnas.2110490118/-/DCSupplemental. tures has evolved toward ever-more complexity and sophistication. Published September 3, 2021. PNAS 2021 Vol. 118 No. 36 e2110490118 https://doi.org/10.1073/pnas.2110490118 | 1of7 Downloaded by guest on September 25, 2021 First is the use of electrical dipole resonance and the interaction its original value w1 = 16 mm and compare the predicted fre- with its image that insures an appreciable magnetic permeability quency fd = c/λ = c/[2(w1+h1)] with the resonance peaks given by with the generation of two high-impedance resonances in the mi- the simulations. Good agreements are obtained as seen in Fig. 1D. crowave regime. Second is the use of dispersion engineering via the We note that the resonance also occurs with enhanced electric tuning of the dissipative resistance to achieve broadband imped- field in the gap between two nearby rings, which means that the ance matching. Third is the use of self-similar hierarchical structure resonance is of a collective response. The fact that it is an electric to extend the absorption spectrum to the ultrabroadband regime. dipole resonance can be further confirmed by the symmetrical Fourth is the absorption of the diffraction orders in the higher radiating electric field pattern and the anti-symmetrical magnetic frequency regime by using foam patches. Below, we detail each of field pattern (Fig. 1 E and F). Owing to the scattered radiation, the conceptual elements together with their structural realizations. the width of the resonance peak is noted to be fairly broad. Results Interaction with the Image Mode and the High-Impedance Resonances. Metallic Ring. The basic element of our absorber is a rectangular In Fig. 2A, we show the effect of introducing a metallic boundary metallic ring, each separated from its neighbor by a capacitive at the bottom side of the sample. By approximating the metallic gap (Fig. 1A, two of the half rings are plotted to show the periodic boundary as a perfect electric conductor (PEC) plane, its effect configuration). The rings are embedded on PCB stipes [FR4 with can be determined through the interaction of the current loop relative permittivity e1 = 4.3(1 + 0.025i)], which are structured to with its image. At specified frequencies, the retardation effect form a two-dimensional checkerboard array (Fig. 1B) to attain can lead to high-impedance resonances as detailed below. Con- polarization-independent properties. The lattice constant (i.e., sider the current densities J1 and J2 in Fig. 2A, equally distant =