  Strong Coulomb interaction in monolayer semiconductors allows them to host
optically active large many-body states, such as the five-particle state,
charged biexciton. Strong nonlinear light absorption by the charged biexciton
under spectral resonance, coupled with its charged nature, makes it intriguing
for nonlinear photodetection - an area that is hitherto unexplored. Using the
high built-in vertical electric field in an asymmetrically designed few-layer
graphene encapsulated 1L-WS$_2$ heterostructure, here we report a large, highly
nonlinear photocurrent arising from the strong absorption by two charged
biexciton species under zero external bias (self-powered mode). Time-resolved
measurement reveals that the generated charged biexcitons transfer to the
few-layer graphene in a timescale of sub-5 ps, indicating an ultrafast
intrinsic limit of the photoresponse. By using single- and two-color
photoluminescence excitation spectroscopy, we show that the two biexcitonic
peaks 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 towards
viable optoelectronic applications.
