Charge transfer in graphene/WS2 enhancing the saturable absorption in mixed heterostructure films

Improving the nonlinear optical (NLO) property of two-dimensional (2D) materials is a challenging research problem for the nonlinear optical applications in optoelectronics. Taking an advantage of the van der Waals interactions, graphene/WS2 heterostructure films have been prepared by a facile liquid phase exfoliation method combined with a filtration method. The NLO properties of these graphene/WS2 films have been enhanced compared with that of WS2 and graphene, however with no evident dependency on thickness and pump fluence. The imaginary part of the third-order nonlinear optical susceptibility (Im chi((3))) and figure of merit (FOM) for graphene/WS2 are approximately four times larger than that for WS2 and two times larger than that for graphene. We have explained the mechanism of enhancement based on the rate equation models along with Runge-Kutta algorithm. Our results suggest the enhancement could be attributed to the charge transfer from WS2 to graphene due to the existence of donor-acceptor structure. First-principles calculations confirm that the van der Waals bonding at interface could accelerate charge transfer and electron-hole pair separation efficiently after the heterostructure formation. Our findings provide a way for designing NLO properties of 2D materials with large FOM and Im chi((3)) for nonlinear photonic devices such as mode locking or Q-switching in ultrafast lasers.