Bismuth oxysulfide nanosheets as a novel saturable absorber for Er-doped and Tm-doped ultrafast fiber lasers

Novel layered bismuth oxysulfide (Bi2O2S) has shown great potential in photoelectric detection, photocatalysis, and photochemical cells due to its excellent properties such as excellent environmental stability, high carrier mobility, and favorite band gap (about 1∼1.5 eV). These excellent physical properties also make it advantageous for applications in the field of ultrafast photonics. However, to date, there has been no reported research on Bi2O2S in ultrafast fiber lasers. In this work, we report a facile method to synthesize Bi2O2S nanosheets by a combination of the hydrothermal and liquid-phase exfoliation (LPE) methods with good crystallinity and thickness less than 10 nm. Further, stable mode-locked pulses are achieved by applying the Bi2O2S SA in Er-doped and Tm-doped fiber lasers, respectively. The Er-doped mode-locked fiber laser has a pulse width of 453.6 fs and the optical spectrum is stable, with a maximum output power of 5.1 mW in the mode-locked state. The obtained Tm-doped mode-locked fiber laser based on Bi2O2S has an estimated pulse width of 2.69 ps and a maximum output power of 7.13 mW. Our results provide new opportunities for the development of Bi2O2S together with its application in ultrafast photonics.