Optimized unidirectional and angle-insensitive ultra-broadband absorber based on layered graphene photonic structure

Using a chaotic gray wolf optimization algorithm to drive the transfer matrix method, a nonreciprocal and angleinsensitive absorber with ultra-wide bandwidth absorption is realized in a layered photonic structure comprised of graphene, which is spliced of a periodic sequence and a quasi-periodic sequence on a normalized scale. Compared to the periodic sequence, the proposed cascading construction has more preponderance in terms of bandwidth and angular stability. The surface conductivity of the graphene layer, which manipulates the absorption effect, can be modulated by the Fermi level, causing the characteristics of the proposed absorber tuned. The Fermi level of graphene, dielectric constants, and thicknesses of ordinary media, are optimized by the chaotic gray wolf algorithm, contributing to the decent improvement of absorption with a relative bandwidth of up to 127 %. Most importantly, in the 1.5-4.5 THz, the angular stability reaches 86 degrees under the TM mode and 70 degrees in TE one. The impacts of sequence number N of periodic sequence and M of quasi-periodic sequence on absorption in both angle and frequency domains are investigated. Additionally, the impedance match theory is introduced to express excellent absorption performance.