Anisotropy-induced Fano resonance and strong coupling in a two-dimensional black phosphorus-based infrared metasurface

Inherent losses in noble metals have been a significant concern for designing efficient metasurface devices. One of the methods to excite low-loss resonances in otherwise lossy systems is to utilize Fano resonances, which arise from the interference between two modes possessing different quality factors. In this study, a two-dimensional material, namely, black phosphorus (BP), has been used to design the metasurface structure for the observation of highly tunable Fano resonances. The design demonstrates improved excitation of interfering modes intrinsically, due to the inherent anisotropic optical properties of BP layers, thereby overcoming the requirement of asymmetry introduced by the structural design. The two interfering modes are identified as the dipole and sextupole modes, which are excited in black phosphorus nanopatches, analogous to the Hall effect. As black phosphorus possesses thickness-dependent optical properties, a comprehensive analysis of thickness-dependent coupling between longitudinal and transverse dipole modes is carried out using bi-layer, quad-layer, and hexa-layer BP. A coupled harmonic oscillator model is employed to determine the coupling strength across both weak and strong coupling regimes. Structural parameters and doping levels are identified as key factors in tuning the interaction between dipole and sextupole modes, enabling the transition from weak to strong coupling and resulting in the emergence of Fano resonances and hybrid states. This work presents a cost-effective and compact design for highly tunable two-dimensional metasurfaces, offering excellent potential for on-chip integration with optoelectronic devices and a wide range of applications in nanophotonics and optoelectronics.