Synergistic Interfacial Charge Transfer and Plasmon-Accelerated Carrier Dynamics in SnSe for Enhanced Nonlinear Optical Absorption

Two-dimensional (2D) tin selenide (SnSe) has emerged as a promising saturable absorber (SA) for ultrafast photonics, although its practical implementation is constrained by intrinsically slow carrier relaxation dynamics. Herein, we demonstrate an interfacial engineering strategy that significantly accelerates carrier dynamics by constructing a copper nanoparticle-decorated tin selenide nanosheet heterostructure (SnSe/Cu). The carrier relaxation times in the SnSe/Cu are reduced by an order of magnitude compared to those of pristine SnSe. In detail, photo-generated carriers undergo sub-picosecond hot-carrier cooling and tens-of-picosecond ground-state recovery. This dual-acceleration phenomenon originates from a synergistic mechanism involving rapid hot-carrier extraction via interfacial charge transfer (ICT) and efficient dissipation of excitation energy through a non-radiative channel enabled by plasmon-mediated resonant energy transfer. This accelerated carrier dynamics finally translates into superior nonlinear optical (NLO) saturable absorption, enhancing modulation depth by more than fivefold while significantly reducing non-saturable loss. When implemented in erbium-doped fiber lasers, the SnSe/Cu SA enables stable mode-locking with a 791-fs pulse duration and diverse bound-state soliton operations. The narrower pulse width and diverse mode-locking states achieved by the SnSe/Cu are entirely attributable to its faster and stronger saturable absorption. This work establishes plasmon modulation based on metallic nanoparticles as an effective approach for engineering carrier dynamics in 2D materials and developing high-performance SAs for ultrafast photonics applications.