Unidirectional ultrabroadband and wide-angle absorption in graphene-embedded photonic crystals with the cascading structure comprising the Octonacci sequence

Using the transfer matrix method, a unidirectional absorber with an ultrabroadband absorption bandwidth and angular stability is realized in the graphene-embedded photonic crystals (GPCs) arranged by the cascading structure formed with the periodic sequence and the quasi-periodic Octonacci sequence in the terahertz regime. As a result, the surface conductivity of the graphene sheet can be modulated via the chemical potential, and the characteristics of the proposed absorber are tunable. Compared to the structure spliced by the diverse periodic sequences, the relative absorption bandwidth of the proposed composite construction is up to 94.53%, which far outweighs that of the periodic one. We compare the Octonacci sequence, the Fibonacci sequence, and the Thue–Morse sequence, and the calculated results reveal the advantage of the Octonacci sequence in the expansion of the absorption bandwidth. Under the optimization of the related parameters, the incident wave is primarily reflected in the forward propagation and absorbed in a wide range of θ under the TM mode in backward propagation, which shows the splendid unidirectionality and angular stability. The impacts of the chemical potential, structural thicknesses, and stack numbers on the absorption properties are also investigated in detail. Additionally, the impedance match theory and the interference field theory are introduced to explain the intrinsic absorption mechanism of the presented GPCs. In short, the unidirectional broadband and angle-insensitive absorber has extensive application prospects in optical sensing, optical filtering, photodetection, and solar energy collection.