Fig. 1. Development of materials as real saturable absorber (SA) in lasers^[16].

Fig. 2. Atomic structures of two-dimensional (2D) materials: (a) Graphene^[19]; (b) TIs^[23]; (c) TMDs^[28,29]; (d) BP^[36]; (e) MOFs^[41].

Fig. 3. Schematic diagram fabrication methods of 2D materials: Top-down, bottom-up methods and Topological transformation.

Fig. 4. (a) SEM image of the Ni-MOF showing a 2D layer structure; (b) AFM image of the Ni-MOF dissolved in an IPA solution; (c) raman spectrum of the Ni-MOF^[41].

Fig. 5. (a) The setup of balanced twin-detector measurement; (b) the measured saturable absorption data and their corresponding fitting curve under 1934 nm laser irradiation^[41].

Fig. 6. (a) A typical data set from Z-scan experiment of the SA device^[64]; (b) the typical shapes of Z-scan measurements^[55].

Fig. 7. Fiber integration with two-dimensional materials: Transmission integration method ((a) sandwiching structure transferring SA on fiber end^[66]); evanescent-wave integration method ((b) D-typed fiber^[65], (c) tapered fiber^[41]).

Fig. 8. (a) Setup of graphene based mode-locked fiber laser^[56]; (b) autocorrection trace; (c) optical spectrum; (d) setup of the BP mode-locked fiber laser^[40]; (e) autocorrelation trace; (f) optical spectrum.

Fig. 9. The shortest-pulse Tm-doped fiber laser based on BP at 2 μm spectral region: (a) Setup of Tm:fiber mode-locked laser; (b) autocorrelation trace^[83].