π–π Stacked Conjugated MOF/Graphene Nanosheet Heterostructures as Saturable Absorbers for Ultrafast Photonics Applications

Metal–organic frameworks (MOFs) exhibit high specific surface areas, tunable porosity, well-defined primary ligands, and highly ordered crystalline structures, which have aroused extensive research interest across various scientific disciplines. As a two-dimensional (2D) conductive metal–organic framework, Cu₃(HITP)₂ (HITP: 2,3,6,7,10,11-hexaaminotriphenylene) is composed of extended π-conjugated organic linkers and copper ions coordinated in a planar geometry. It possesses a π-conjugated system, significant in-plane charge transfer capability, and excellent carrier mobility. However, research on its application in the field of ultrafast photonics has yet to be explored. Herein, two types of Cu₃(HITP)₂/graphene nanosheet van der Waals π–π stacked heterostructures (CG-VHS-F and CG-VHS-T) are used as excellent saturable absorbers (SAs) and integrated into an erbium-doped fiber (EDF) laser. CG-VHS-F SA achieves fundamental mode-locking (FML) with a pulse duration of 449 fs and ninth-order harmonic mode-locking (HML). CG-VHS-T SA achieves an FML and 19th-order HML, demonstrating superior nonlinear optical performance. Under the influence of the π–π stacking interaction between Cu₃(HITP)₂ and graphene, CG-VHS exhibits enhanced interfacial charge transfer performance, electron cloud expansion effects, and superior nonlinear optical (NLO) properties. The experimental results indicate that CG-VHS SAs hold significant potential for generating ultrafast and highly stable pulses in ultrafast photonics applications. Meanwhile, this research also provides a promising strategy for the design and development of conjugated MOFs.