Interface Charge Transfer in MXenes/Graphene Heterostructures for Ultrafast Photonics

Abstract Two‐dimensional (2D) transition‐metal carbides, nitrides, and carbocyanides, known as MXenes, have attracted tremendous attention in photonics and electronics because of their promising optical, electrical, and mechanical properties. However, the potential of MXenes‐based heterostructures for ultrafast photonics has not yet been explored. In this study, a Ti 3 C 2 T x /graphene vertical heterostructure (TG‐VHS), an excellent saturable absorber (SA), is successfully used to generate femtosecond laser pulses via passive mode‐locking. The modulation depth of the TG‐VHS is measured to be 28%, which is ≈12.7 and 1.5 times higher than that of Ti 3 C 2 T x MXenes and graphene, respectively. Density functional theory calculations show that various heterostructure systems are formed owing to the presence of various functional groups in Ti 3 C 2 T x , and multiple charge transfers significantly enhance the nonlinear optical characteristics of TG‐VHS. Moreover, utilizing TG‐VHS as the SA, a stable mode‐locked operation with a pulse duration as short as 356 fs at a wavelength of 1559.16 nm is achieved; harmonic mode‐locking with the highest repetition rate of 603.1 MHz is also obtained. This study provides an important strategy for developing VHS SA for ultrafast pulse fiber lasers and opens new perspectives for designing advanced MXenes‐based photonic devices.