Enhanced Nonlinear Optical Properties of Bi Quantum Dot/Te Nanotube Heterojunctions for Ultrafast Photonics Applications

van der Waals heterojunctions (vdWHs) have emerged as a promising strategy to improve the optoelectronic properties of materials for use in optoelectronic devices. In this study, independent tellurium nanotube (Te NT) structures with diameters of approximately 80 ± 10 nm were first synthesized. Subsequently, zero-dimensional (0D) bismuth quantum dots (Bi QDs) were uniformly deposited on the surface of one-dimensional (1D) Te NTs via a hydrothermal bottom-up assembly process, successfully constructing a mixed-dimensional Bi/Te van der Waals heterojunction (vdWH) with Bi QDs densely distributed around the Te NTs. The nonlinear optical properties of the heterojunction were systematically investigated. Due to the built-in electric field at the tellurium/bismuth interface, these vdWHs show significantly enhanced nonlinear absorption, lower saturation intensity, and improved carrier transport. These properties make Bi/Te vdWHs promising candidates for advanced optical applications. Furthermore, stable ultrashort pulses and bound-state solitons at a 1.5 μm wavelength were generated by incorporating Bi/Te vdWHs as saturable absorbers within an erbium-doped fiber laser cavity. Such findings highlight the potential of Bi/Te vdWHs as a versatile platform for developing high-performance optical devices with enhanced optoelectronic and nonlinear optical properties.