Generation of Bright–Dark Pulses in Erbium-Doped Fiber Lasers Based on a Germanene-Saturable Absorber

As a group IV graphene-like material, germanene has fast carrier relaxation time and excellent nonlinear saturable absorption properties, which make it have a broad application prospect in the field of optics. However, in the field of laser applications, it has not been sufficiently studied. In this study, the energy band and optical property calculations of germanene with two structural defects were performed based on the density functional theory. The germanene nanosheet prepared by a liquid-phase exfoliation method was systematically characterized and analyzed. High-quality germanene-saturable absorbers were selected for application in erbium-doped fiber lasers. By adjusting the device, a bright–dark soliton with a fundamental frequency of 6.28 MHz and a single-pulse energy of 2.58 nJ was obtained. Meanwhile, the dark–dark–bright, dark–bright–dark, and dark–bright–bright solitons were also experimentally observed and characterized. The study shows that germanene nanosheets have excellent nonlinear saturable absorption properties for bright–dark soliton’s modulation. It also provides theoretical and experimental bases for the application of germanene in the field of ultrafast lasers.