Efficient Energy Transfer Enabled by Dark States in van der Waals Heterostructures

Dark exciton states show great potential in condensed matter physics and optoelectronics because of their long lifetime and rich distribution in band structures. Therefore, they can theoretically serve as efficient energy reservoirs, providing a platform for future applications. However, their optical-transition-forbidden nature severely limits their experimental exploration and hinders their current application. Here, we demonstrate a universal dark state nonlinear energy transfer (ET) mechanism in monolayer WS₂/CsPbBr₃ van der Waals heterostructures under two-photon excitation, which successfully utilizes the enormous energy reserved in the dark exciton state of CsPbBr₃ to significantly improve the photoelectric performance of monolayer WS₂. We first propose the scenario of resonant ET between the dark state of CsPbBr₃ and WS₂, and then reveal that this is a typical Förster resonant ET and belongs to the 2D-2D category. Interestingly, the dark state ET in CsPbBr₃ is identified as a long-range donor-bridge-acceptor hopping mode, with a potential distance exceeding 200 nm. Finally, we successfully achieve nearly an order of magnitude enhancement in the near-infrared detection performance of monolayer WS₂. Our results enrich the theory of dark exciton states and ET, and they provide a way of using dark exciton states for future practical applications.