Absorption enhancement effect of black phosphorus induced by quasi-bound states in the continuum and its application

Black phosphorus (BP), as a new two-dimensional (2D) semiconductor material, has attracted much attention in the field of optoelectronic devices due to its direct band gap and unique lattice structure. However, the atomically thin thickness of monolayer BP leads to its inherently low optical absorption capacity, which limits its practical applications. In this study, a composite structure based on the monolayer BP Fabry-Pérot (F-P) cavity is proposed, which breaks through the limitations of the conventional absorption enhancement methods by exciting quasi-bound states in the continuum (q-BIC). By systematically modulating four parameters, such as the radius of the air cylinders, the thickness of the dielectric layers, and the incident angle of the light, etc, the polarization-sensitive Fabry-Pérot BIC (F-P BIC), Friedrich-Wintgen BIC (F-W BIC), and symmetry-protected BIC (S-P BIC) supported by the proposed structure are excited. Compared with the traditional BIC modulation, it is more flexible and diverse, which improves the freedom and practicality of the structural design. The synergistic modulation of multiple parameters provides a design idea for the realization of multi-channel stable absorbers and multi-frequency optical switches. Due to the in-plane optical anisotropy of BP, the proposed structure can achieve a multi-frequency absorption optical switch, and its modulation depth (MD) is high up to 97.1%, while the insertion loss (IL) is low to 0.013 dB. Using the dynamic tunability of BP, it is found that the proposed structure exhibits excellent dual-narrowband absorption effect in a wide tuning range of the relative Fermi energy. Particularly, when the relative Fermi energy of the BP EF−EC=0.388 eV, two narrowband near-perfect absorption peaks with extremely high absorption rates of 99.916% and 99.853% appear in the absorption spectrum, which can be attributed to critical coupling. The proposed structure has broad application prospects in the fields of perfect absorbers and optical switches.