Abstract
Microwave absorbers have important applications in various areas including stealth, camouflage, and antenna. Here, we have designed an ultra-broadband light absorber by integrating two different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has wide but different absorption bands due to broadband slow-light response, into a unit cell. Both the numerical and experimental results demonstrate that in such a design strategy, the low absorption bands between high absorption bands with a single-sized tapered HMM waveguide array can be effectively eliminated, resulting in a largely expanded absorption bandwidth ranging from 2.3 to 40 GHz. The presented ultra-broadband light absorber is also insensitive to polarization and robust against incident angle. Our results offer a further step in developing practical artificial electromagnetic absorbers, which will impact a broad range of applications at microwave frequencies.
Highlights
Waveguide consists of alternating metal and dielectric layers with thickness denoted as tm and td, respectively
For the tapered Hyperbolic metamaterials (HMMs) waveguide array, the absorption frequency range is determined by the top width, bottom width, and the thickness filling ratio of the metal layer
Assuming the width grade of the tapered HMM waveguide array [left upper side of Fig. 1(a)] is sufficiently small, it can be deemed as a series of uniform HMM waveguide array [left down side of Fig. 1(a)]
Summary
The TM0 mode propagates along the tapered waveguide, and localizes at a specific position due to significant reduction of group velocity [see (1) in Fig. 1(d) as an example] It presents more high absorption bands above 5.2 GHz [see (2)–(6) in Fig. 1(c)], which can be attributed to the excitation of slow light of higher order TM modes. Some slow-light modes occur by chance at the bottom of the HMM waveguide array, a portion of EM wave will be reflected back into the air by the metal substrate, forming several low absorption bands (8)–(11) It can be given a further explanation that the slow-light modes cannot confine the light well comparing with the conventional single-layered absorbers[15,16,17].
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