Investigation into saturable absorption mechanism of bulk GeS particles at short-wavelength infrared band

Nonlinear optical materials are key platforms in various electronic and photonic devices. This study investigated the feasibility of using bulk germanium monosulfide (GeS) particles to fabricate a saturable absorber (SA) for mode locking of fiber lasers. Bulk GeS particles were prepared via mechanical exfoliation without chemical etching. A theoretical investigation based on density functional theory (DFT) calculations was conducted to determine the origin of the saturable absorption of bulk GeS in the short-wavelength infrared (SWIR) wavelength region despite its large energy bandgap. Theoretically, the experimentally observed SWIR absorption of bulk GeS could be attributed to an energy bandgap reduction caused by strains on the bulk GeS crystal lattice structure. Using the bulk particles, an SA with a saturation power and modulation depth of approximately 38 W and 18 %, respectively, was realized at 1550 nm. The SA was then used in an erbium fiber ring cavity to produce stable optical pulses with a temporal width of -796 fs at -1556 nm. These results indicate that bulk GeS particles can be a low-cost, efficient, and nonlinear optical material for the realization of SAs in the SWIR region.