Nonlinear, Ultra-fast photocurrent generation using charged biexciton

Monolayer transition metal dichalcogenides host several many-body systems under suitable excitation and doping condition. We probe the formation of charged biexciton, a five-particle state in monolayer WS₂, through the photoluminescence excitation spectroscopy, and demonstrate that the formation is favoured when the excitation laser is in resonance with the neutral exciton line. We further probe the generation by using a dual-wavelength excitation, demonstrating a favoured generation when the two excitations match with the exciton and trion lines. Using a few-layer graphene capped WS₂ sample, we demonstrate a photocurrent generation mechanism that is highly nonlinear in nature. Further, we show that the lifetime of the charged biexciton reduces to sub-5 ps due to graphene capping, suggesting an ultra-fast photocurrent generation rate.