Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions

Two-dimensional (2D) topological insulators (TIs) have attracted much attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as mode-locking solid-state lasers. In this work, through fabricating graphene/Bi2Te3 heterojunction which combines the monolayer graphene and Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated via varying the thickness of the Bi2Te3 as synthesizing the heterojunctions. Furthermore, by the aid of the estimated junction electron escaping time, the model about photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can be accordingly realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work exhibit the functionality of the optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and their potential for applications.