Fig. 1. Fabrication methods of two-dimensional heterostructure saturable absorbers.

Fig. 2. (a) Schematic of α-MoTe₂/MoS₂ heterostructure prepared by mechanical exfoliation^[40]; (b) the optical microscopy image^[40].

Fig. 3. (a) Illustration of preparation procedures of WS₂/Graphene heterostructure films by liquid phase exfoliation, a series of of WS₂/Graphene heterostructure films with different thickness obtained from different filtration volume; (b) atomic force microscopy image of WS₂/Graphene heterostructure films; (c) X-ray diffraction patterns; (d) absorption as a function of filtration volume at 800 nm; (e) open-aperture Z-scan results of WS₂, Graphene, and WS₂/Graphene heterostructure films with the thickness of ～135 nm; (f) histogram of the imaginary part of the third-order nonlinear coefficient Imχ⁽³⁾ and figure of merit (FOM) of WS₂, Graphene, and WS₂/Graphene heterostructure^[41].

Fig. 4. (a) Illustration of preparation procedures of MoTe₂/MoS₂ heterostructure films by liquid phase exfoliation; (b) Z-scan results of MoTe₂, MoS₂ and MoTe₂/MoS₂ heterostructure films under the pump intensity of 606 GW·cm^–2 with the thickness of ～80 nm; (c) Z-scan results of MoTe₂/MoS₂ heterostructure films with thickness of 30, 60, 80, 100, 120 nm at 606 GW·cm^–2, respectively^[47].

Fig. 5. (a) Schematic and (b) Raman spectrum of MoS₂/WS₂ heterostructure^[53]; (c) optical microscope photograph of monolayer triangular WS₂ grown on monolayer MoS₂ nanosheet; (d) photoluminescence (PL) spectrum of WS₂ monolayer, MoS₂ monolayer and MoS₂/WS₂ heterostructure^[54]; (e) schematic diagram of as-grown Bi₂Te₃/Graphene heterostructure on SiO₂/Si substrate; (f) absorption spectrum of Graphene and Bi₂Te₃/Graphene heterostructure^[55].

Fig. 6. (a) AFM image of Bi₂Te₃/Graphene heterostructure fabricated by two-step CVD method on the SiO₂ substrate; (b) thickness profiles along line 1 in (a); (c) absorption spectrum of Bi₂Te₃/Graphene heterostructure from 900−2000 nm^[56]; (d) schematic of WS₂/ Graphene heterostructure after transferring successfully; (e) Z-scan graph of WS₂, Graphene and WS₂/Graphene heterostructure^[32]

Fig. 7. Scanning electron microscope images of MoS₂/WS₂ heterostructure from the top view (a) and side view (b); (c) Raman spectrum of MoS₂, WS₂ and MoS₂/WS₂ heterostructure; (d) transmission of MoS₂/WS₂ heterostructure with respect to the power intensity of incident light^[57].

Fig. 8. (a) Illustration of band alignment and carrier mobility of the type-II MoS₂/WS₂ heterostructure^[63]; (b) band alignment of semiconductor type-II MoTe₂/MoS₂ heterostructure^[47]; (c) diagram of the charge-transfer process in a MoS₂/graphene heterostructure^[64]; (d) energy band diagram of Bi₂Te₃/graphene heterojunction, the blue dots stand for the photogenerated electrons, while red hollow dots stand for holes^[55].

Fig. 9. Recorded results of Te/BP heterojunction SAM-based mode-locked laser: (a), (b) measured autocorrelation trace of 404 fs and the corresponding spectrum; (c), (d) recorded frequency spectrum with a wide and a narrow span respectively^[65]; (e) schematic setup of the Q-switched mode-locking (QML) laser and (f) the output power versus pump power of the continuous wave (CW) and QML operation for MoS₂/Graphene heterostructure^[34]

Fig. 10. (a) Schematic of graphene/MoS₂ heterojunction mode-locked laser device; (b) pulse trains; (c) spectrum; (d) autocorrelation race for 92 fs duration; (e) frequency spectrum^[66].

Fig. 11. (a)−(c) Mode-locking performance of Graphene/WS₂ heterostructure: (a) Optical spectrum; (b) pulse trains; (c) autocorrelation trace^[69]. (d)−(f) Mode-locking performance of Graphene/Mo₂C heterostructure: (d) Optical spectrum; (e) pulse trains; (f) autocorrelation trace^[70]. (g)−(i) Mode-locking performance of Graphene/phosphorene heterostructure: (g) Optical spectrum; (h) pulse trains; (i) autocorrelation trace^[71].

Fig. 12. (a) Schematic of Graphene/Bi₂Te₃ heterostructure on the end-facet of fiber connector; (b)−(d) Mode-locking characteristics of Graphene/Bi₂Te₃ heterostructure: (b) Optical spectrum; (c) pulse trains; (d) autocorrelation trace^[72]. (e) Schematic of Er-doped fiber laser. (f)−(h) Mode-locking characteristics of Bi₂Te₃/FeTe₂ heterostructure: (f) Optical spectrum; (g) pulse trains; (h) autocorrelation trace^[73]. (i) Schematic of Er-doped fiber laser. (j)−(l) Mode-locking characteristics of Graphene/MoS₂ heterostructure: (j) Optical spectrum; (k) pulse trains; (l) autocorrelation trace^[74].

Fig. 13. (a) Experimental setup of mode-locked fiber laser with 1550 nm QD-SESAM; Inset: cross-sectional transmission electron microscope image of the QD-SESAM and 1 μm × 1 μm AFM image of the 1550 nm QDs. (b)−(e) Characteristics of mode-locked the developed fiber laser of InAs/GaAs QD: (b) Output power versus pump power; (c) output optical spectra; (d) RF spectrum of the mode-locked fiber laser; (e) autocorrelation trace^[75].

Fig. 14. (a) Schematic of deposition of the Te/Se sample on the microfiber. (b)−(d) Mode locking performance of the Te-based fiber laser: (b) Optical spectrum; (c) pulse trains; (d) autocorrelation trace. (e) Te/Se samples under microscope with 50 μm scale. (f)−(h) Self-starting mode locking performance of the Yb-doped fiber laser: (f) Optical spectrum; (g) pulse trains; (h) autocorrelation trace^[76].

Fig. 15. (a)−(c) Mode-locking performance of MoS₂/WS₂ heterostructure: (a) Optical spectrum; (b) pulse trains; (c) autocorrelation trace^[57]. (d)−(f) Mode-locking performance of MoS₂/Sb₂Te₃/MoS₂ heterostructure: (d) Optical spectrum; (e) pulse trains; (f) autocorrelation trace^[77]. (g)−(i) Mode-locking performance of WS₂/MoS₂/WS₂ heterostructure: (g) Optical spectrum; (h) RF spectrum; (i) autocorrelation trace^[63].