Tunable Optical Bistability and High-Sensitivity Terahertz Refractive-Index Sensing Based on a Fabry-Pérot Cavity Incorporating Weyl Semimetals

Optical bistability (OB) is a fundamental nonlinear effect enabling all-optical switching, logic, memory, and sensing, yet its practical implementation is often hindered by high power thresholds and limited tunability. Here, we theoretically demonstrate low-threshold and widely tunable OB in a Fabry-Pérot (FP) cavity integrated with a Weyl semimetal (WSM) layer. By combining the strong local-field enhancement of the FP cavity with the large third-order nonlinear conductivity of the WSM, we achieve pronounced bistable hysteresis loops in both the transmitted field amplitude and transmittance at terahertz frequencies, with incident field switching thresholds on the order of 106 V/m. The bistability thresholds and hysteresis width can be flexibly controlled by tuning the WSM Fermi energy, the position of the WSM layer inside the cavity, and the mirror transmittance. Moreover, exploiting the strong dependence of the down-switching threshold on small changes in the refractive index of the cavity medium, we propose a terahertz refractive-index sensor with a maximum sensitivity of 296.8 GW·m−2·RIU−1. These results provide a theoretical basis for tunable nonlinear photonic devices and threshold-based terahertz refractive-index sensors using topological semimetals.