Violet phosphorene as a saturable absorber for controllable soliton molecule generation in a mode-locked fiber laser

The recently discovered phosphorus allotrope, violet phosphorus, has predictable ultrafast nonlinear optical (NLO) properties and potential for photonics applications. Here, we examined the ultrafast carrier dynamics and NLO characteristics of two-dimensional (2D) few-layered violet phosphorene (VP) in the near-infrared band. Excited carriers in the 2D VP featured fast relaxation lifetimes on 100 fs timescales and exceptional polarization-dependent saturable absorption properties. The modulation depth varied from 1.18% to 7.35% under different intracavity polarization states. We further demonstrated a 1.5 mu m erbium-doped fiber laser using VP nanoflakes as the saturable absorber (SA), generating stable conventional soliton mode-locking pulses with a 765-fs pulse width. The VP SA-based bound soliton molecules with tunable pulse numbers and time separations were also observed for the first time to our knowledge. Our work reveals the immense potential of 2D VP for near-infrared ultrashort pulse modulation and contributes to the understanding of complex soliton dynamics in nonlinear optical systems. Controllable soliton molecules were generated in a Er-doped mode-locked fiber laser with a few-layered violet phosphorene saturable absorber.