2D Transition Metal Carbo‐Chalcogenide Nb₂CSe₂: A Combination of Transition Metal Dichalcogenide and MXene for Ultrafast Photonics Application

Abstract Transition metal carbo‐chalcogenides (TMCCs), a novel class of 2D materials, combining the distinctive properties of transition metal dichalcogenides (TMDs) and MXenes, eliciting considerable interest for their excellent properties across various fields. However, TMCCs' optical properties and potential in ultrafast photonics remain unexplored. This study investigates Nb 2 CSe 2 , a novel developed TMCC, which exhibits superior optical properties and robust environmental stability. Density functional theory calculations reveal solid near‐infrared absorption and a high density of states near the Fermi level in Nb 2 CSe 2 , indicative of high carrier mobility and robust optical responses. The nonlinear absorption coefficient of Nb 2 CSe 2 is measured as 0.431 cm GW −1 , significantly outperforming most TMDs and MXenes. Its fastest relaxation lifetime of excited carriers is on the order of hundreds of femtoseconds. The wavelength‐dependent transition of carrier dynamics and excitation intensity‐dependent transition of absorption mechanism is observed. For the first time, a TMCC‐based saturable absorber is integrated into an erbium‐doped fiber laser cavity, the evolution from Q‐switched operation to conventional soliton operation with sub‐picoseconds pulse width and dual‐wavelength complex soliton dynamics are realized, underscoring the material's superior capabilities in enhancing ultrafast laser performance. This research provides a promising platform for fundamental studies and practical applications of TMCCs in ultrafast photonics.