A Dual-Frequency Reflectionless Absorber Based on the Subwavelength Helical Structures

In this letter, a dual-frequency reflectionless absorber based on the subwavelength helical structures is presented. The proposed structure is composed of two sets of subwavelength helical structures with different sizes, designed to achieve dual-frequency absorption performance. For the subwavelength helical structures, their electric and magnetic responses are simultaneously excited under the same resonant mode. To eliminate the bianisotropy of a single subwavelength helical structure and achieve the dual-polarized performance, eight different subwavelength helical units are used to construct an absorber unit, among which four units achieve dual polarization through rotation, and the other four units are mirror images of them to form a combined arrangement of left-handed and right-handed structures. Consequently, the proposed structure demonstrates a narrow peak of nearly full absorption, while remaining nearly invisible in reflection across a wide frequency range. Specifically, the overall structure exhibits two absorption bands with absorptive peaks above 90% at 2.31 and 2.8 GHz, respectively, while the out-of-band reflection is kept very low. Moreover, the absorber achieves an ultralow profile of
$ 0.023\,{\lambda }_{L}$
and exhibits dual-polarization characteristics. A prototype of the proposed absorber is fabricated and measured. A good agreement between the results of the experiment and simulation can verify the effectiveness of the presented idea.