Fig. 1. Q-switched laser schematic design. Pr:YLF crystal, 444 nm; LD, LSR444SD, 3.5 W; M1, plane coating, R_444 nm < 0.1%, concave coating, T_444 nm > 99.9%, and R_{522 nm (640 nm or 720 nm)} > 99.9%; M2, concave coating, T_{522 nm (640 nm or 720 nm)} = 4%.

Fig. 2. (a) Crystal structure of HfGeTe; (b) bulk electronic structure of HfGeTe without spin-orbit coupling (SOC)^[32,36,37]. Two kinds of Dirac points: red circle, a diamond-shaped Dirac nodal line; blue square, a quartic degenerated Dirac point. (c) Atomic force microscopy morphology of the HfGeTe/silicon slice; (d) Raman spectrum of the HfGeTe/silicon slice and HfGeTe bulk crystal.

Fig. 3. (a) Schematic diagram of nonlinear saturable absorption at the Dirac point; (b) absorption spectra of the HfGeTe/quartz glass and quartz glass; relationship between transmittance and optical intensity at (c) 515 nm, (d) 640 nm, and (e) 720 nm, respectively.

Fig. 4. Average output power versus pump power at (a) 522 nm, (c) 640 nm, and (e) 720 nm. The inserted figure is the related lasing spectra. Change of pulse width and pulse repetition frequency of the (b) 522-nm, (d) 640-nm, and (f) 720-nm Q-switched lasers.

Fig. 5. (a) Pulse profile at 522 nm under a pump average power of 3.74 W; (b) pulse profile at 640 nm under a pump average power of 2.87 W; (c) pulse profile at 720 nm under a pump average power of 2.43 W; (d) pulse train of 640-nm pulsed laser under a pump average power of 2.87 W.

Fig. 6. (a) Saturation energy density of several low-dimensional saturable absorbers; (b) pulse widths of Pr-based red pulsed lasers based on different saturable absorbers; the numbers correspond to Refs. [42–46" target="_self" style="display: inline;">–46,48–56" target="_self" style="display: inline;">–56] (42, MoS₂; 43, WS₂; 44, Bi₂Se₃; 45, graphene; 46, CdTe/CdS quantum dots; 48, graphene oxide colloids; 49, Au nanorods; 50, few-layer MXene Ti₃C₂T_x; 51, 1T-titanium selenide; 52, Bi₂Se₃; 53, black phosphorus; 54, black phosphorus; 55, graphene oxide; 56, graphene).