Mode-locked fiber laser based on the broadband germanene saturable absorber

Passively mode-locked fiber lasers have been extensively studied as one of the most effective tools for obtaining ultrashort pulses. As an important part of passively mode-locked fiber lasers, the exploration of saturable absorbers (SAs) based on two-dimensional materials has become one of the research hotspots in the field of ultrafast photonics in recent years. Germanene, as a novel two-dimensional Dirac material with ultrafast optical response and broadband optical absorption properties, has a great potential for SA in mode-locked fiber lasers. In our work, germanium nanosheets are prepared by the liquid-phase exfoliated method, and then the germanene broadband SA is successfully assembled by the laser-induced deposition method. Furthermore, the saturable absorption properties of germanene-SA at three major bands in the range of $1.0\sim 2.0$ μm are measured by balanced synchronous twin-detector measurement system. The modulation depth of the obtained germanene broadband SA is 4.19%, 6.75% and 8.45% at the 1.0, 1.5 and 2.0 $\mu \mathrm{m}$ bands, respectively, and its saturable absorption fluence is 2181.8 $\mu \mathrm{J}/\text{cm}^{2}$ , 44.6 $\mu \mathrm{J}/$ cm and 59.8 $\mu \mathrm{J}/\text{cm}^{2}$ , respectively. $\text{Yb}^{3+}, \text{Er}^{3+}$ and $\text{Tm}^{3+}$ -doped mode-locked fiber lasers based on the germanene-SA are constructed. In addition to the dissipative and conventional soliton pulses with pulse duration of 79.6 ps and 1.61 ps at 1061.1 nm and 1883.5 nm, two operating states are realized in $\text{Er}^{3+}$ . doped mode-locked fiber laser including noise-like mode-locking and conventional soliton mode-locking. The existence of rogue waves is further observed in the operation of noise-like pulses, and the proportion of rogue waves is 0.15% by statistical calculation. To the best of our knowledge, it is the first demonstration of the broadband (up to $\sim$ 1000 nm) operation property of the germanene-SA, laying an important foundation for the application in the broadband ultrafast lasers.