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Strong Coulomb interaction in monolayer semiconductors allows them to hostoptically active large many-body states, such as the five-particle state,charged biexciton. Strong nonlinear light absorption by the charged biexcitonunder spectral resonance, coupled with its charged nature, makes it intriguingfor nonlinear photodetection - an area that is hitherto unexplored. Using thehigh built-in vertical electric field in an asymmetrically designed few-layergraphene encapsulated 1L-WS$_2$ heterostructure, here we report a large, highlynonlinear photocurrent arising from the strong absorption by two chargedbiexciton species under zero external bias (self-powered mode). Time-resolvedmeasurement reveals that the generated charged biexcitons transfer to thefew-layer graphene in a timescale of sub-5 ps, indicating an ultrafastintrinsic limit of the photoresponse. By using single- and two-colorphotoluminescence excitation spectroscopy, we show that the two biexcitonicpeaks originate from bright-dark and bright-bright exciton-trion combinations.Such innate nonlinearity in the photocurrent due to its biexcitonic origin,coupled with the ultrafast response due to swift inter-layer charge transfer,exemplifies the promise of manipulating many-body effects in monolayers towardsviable optoelectronic applications.

Highly nonlinear biexcitonic photocurrent from ultrafast inter-layer charge transfer