Black phosphorus-enabled harmonic mode locking of dark pulses in a Yb-doped fiber laser

The harmonic mode locking of dark pulses based on black phosphorus is proposed and observed in a Yb-doped fiber laser for the first time, to the best of our knowledge. The order of harmonic mode locking could be readily switched by adjusting the intra-cavity polarization states only. Up to an 11th order harmonic dark pulse was observed. A numerical investigation based on the extended nonlinear Schrödinger equation is employed to confirm the generation of dark pulses in the laser, which proves that harmonic dark pulses are an intrinsic feature of fiber lasers. A black phosphorus saturable absorber is fabricated by the modified mineralizer-assisted chemical vapor transport method. This work illustrates that black phosphorus is a promising material in laser optics. We anticipate that our work will stimulate the applications of black phosphorus in many other photonic systems.