Charged-exciton complexes induced by interlayer carrier transfer in 2D Van der Waals heterostructure

High-order correlated excitonic states in semiconductors, such as trion, biexciton and polaron, etc. hold a great promising platform in contemporary quantum and nonlinear optics. The recently found 2D TMDs further gives such excitonic states additional valley properties, with bound states of excitons in opposite valleys in momentum spaces. Despite great efforts have been made on the emission properties of the excitonic states, their absorption feature, especially the ultrafast absorption dynamics, are rarely reported. Here, we reported the enhanced optical absorption of the charged-excitonic states in monolayer WS2, including singlet and triplet trions, charged biexcitons and semidark trions, by utilizing the interlayer charge transfer-induced photo-doping effect in graphene-WS2 heterostructure. Depending on the recombination rates of the doping electrons, the absorption intensity of the charged complexes exhibits ultrafast decay dynamics, with lifetime of several picoseconds. Due to many-body interaction, both increasing pump intensity and lattice temperature can broaden these fine excitonic absorption peaks and even reverse the shape of the transient absorption spectrum completely.