An all-photonic isolator using atomically thin (2D) bismuth telluride (Bi₂Te₃)

Here, we demonstrate two-dimensional (2D) Bi₂Te₃ with broadband Kerr nonlinear optical response, which can be used for nonreciprocal light propagation in passive photonic isolators. The self-induced diffraction patterns generated at various wavelengths (650 nm, 532 nm, and 405 nm) in the far field are investigated to calculate the nonlinear refractive index (n₂) and third-order nonlinear susceptibility χ(3)total of the synthesized 2D Bi₂Te₃ using the SSPM (spatial self-phase modulation) spectroscopy method. 2D Bi₂Te₃ exhibits a significant nonlinear refractive index on the order of ≈10^-4 cm² W^-1, which is higher than that of graphene. The laser-induced hole-coherence effect accounts for the significant magnitude of the third-order nonlinear susceptibility χ(3)monolayer (on the order of 10^-7 e.s.u.). Surface engineering is applied to realize a fast-response photonic system. Bader charge analysis (ab initio simulations) was performed to probe the interaction between 2D Bi₂Te₃ and different solvent molecules. 2D hBN exhibits reversible saturation, reducing the intensity of the propagating beam. Leveraging the enhanced Kerr nonlinearity of 2D Bi₂Te₃, a nonlinear photonic isolator (2D-Bi₂Te₃-2D-hBN heterostructure) that disrupts time-reversal symmetry has been successfully demonstrated, enabling unidirectional light propagation. This demonstration of the photonic isolator shows Bi₂Te₃ as a novel 2D material, expanding its potential applications across multiple photonic devices, including detectors, modulators, and switches.