Black phosphorus-based anisotropic absorber at terahertz frequency

As an emerging anisotropic two-dimensional (2D) material, the monolayer black phosphorus (BP) shows promising applications in optoelectronics. However, the atomic-scale thickness of BP results in low absorption, limiting its development in the field of photonic devices. Therefore, the enhancement of light-matter interactions in the monolayer BP is crucial for the development of advanced photonic devices. In this paper, we propose a BP-based tunable anisotropic absorber for enhancing light absorption in the terahertz frequency range. The structure excites the surface plasmon polaritons of BP by exciting the guided mode resonance in the MgF2 grating, which enhances the interaction of light with the monolayer BP. The unique anisotropy of BP leads to the structure exhibiting different absorptions in different directions, i.e. 99.9% at 8.564 THz in the armchair (AC) direction and 85.5% at 8.516 THz in the zigzag (ZZ) direction of BP. In addition to the high absorption, the structure exhibits actively tunable absorption by varying the electronic doping, geometrical parameters and incidence angle of BP. Furthermore, the refractive index sensitivities of the proposed absorber are 2.14 THz/RIU and 1.94 THz/RIU in two different directions of BP, respectively. The proposed BP-based anisotropic absorber has great potential for various optoelectronic applications and opens up a new avenue for the development of tunable terahertz devices based on the monolayer BP.