Low-Threshold Optical Bistability via Surface Plasmon Polaritons in 3D Dirac Semimetal Multilayer Structures

Three-dimensional Dirac semimetal (3D DSM), characterized by linear band dispersion and strong terahertz nonlinear optical responses, has attracted increasing interest as promising materials for compact nonlinear photonic devices. Optical bistability (OB), which enables two stable output states under the same input condition, is of particular importance to all-optical switching, optical logic gates, and optical memory. However, achieving OB with a sufficiently low switching threshold remains a key challenge. Here, we propose a prism-coupled multilayer structure incorporating 3D DSMs to realize low-threshold, tunable OB by exciting the surface plasmon polaritons (SPPs). The prism-coupling configuration enables efficient excitation of SPPs, producing strong local-field enhancement around the nonlinear 3D DSM layer. This enhanced light–matter interaction, together with the large nonlinear refractive index of the 3D DSM, substantially reduces the electric-field threshold required for bistable switching. Numerical results show that OB can be achieved with an incident electric-field threshold on the order of 105 V/m through optimizing the material and structural parameters. Moreover, the switching threshold and hysteresis loop width can be flexibly controlled by varying the Fermi energy, relaxation time, and geometric parameters of the 3D DSM multilayer structure. These results suggest that SPP-assisted 3D DSM structures provide an effective platform for low-threshold, actively tunable optical bistable devices in integrated terahertz photonic systems.