Synchronous dual-wavelength pulsed laser is well suitable to generate coherent terahertz (THz) radiation^[1] and applied in pump–probe processes,^[2] nonlinear frequency conversion,^[3] etc. It has been reported that difference frequency generation of two Q-switched near infrared laser pulses could generate high peak power THz radiation.^[4,5] Passively Q-switched solid-state laser favored the advantages of compactness, flexibility for generation of dual-wavelength laser. In previous works, many saturable absorbers (SAs) such as carbon materials^[6–8] (carbon nanotubes, graphene, and graphene oxide), two-dimensional materials including topological insulators^[9] and molybdenum disulphide (MoS₂)^[10] have been applied in solid-state lasers for Q-switching or mode locking. Recently, passively Q-switched dual-wavelength solid-state lasers have been realized based on graphene,^[11] graphene oxide,^[12] single-wall carbon nanotubes,^[13] topological insulator,^[14] and MoS₂ saturable absorber,^[15] respectively.

In two-dimensional materials, graphene research has reached a mature stage. However, as saturable absorber, the optics damage threshold of single-layer graphene is as low as 14 mJ/cm².^[16] Graphene-like material such as tungsten disulphide (WS₂) has attracted much attention for its physical properties.^[17] Monolayers of tungsten disulphide are proved to be stable under ambient conditions (room temperature in air)^[18] and have emerged as candidate materials for future saturable absorbers. Recently, WS₂ has been applied in fiber lasers. Mao et al. demonstrated an Er-doped mode-locked fiber laser with WS₂ nanoplatelets (NPs).^[19] Yan et al. realized 1 GHz harmonic mode-locking with pulse duration of 452 fs at 1.5 μm.^[20] Wu et al. obtained a mode-locked Er-doped fiber laser at 1550 nm.^[21] Zhao et al. obtained passively Q-switched laser operation with 60 ns pulse width and ultrafast mode locking with 8.6-ps pulse width.^[22] These research results proved WS₂ to be a potential high-power flexible saturable absorber for lasers.

In this paper, we demonstrate a Q-switched solid-state Yb:LSO laser based on WS₂ nanoplatelet film. Q-switched pulses as short as 1.6 μs were obtained at the pump power of 9 W. The Q-switched laser delivered dual-wavelength output at wavelength of 1034.3 nm and 1056.8 nm simultaneously. At the maximum pump power of 9 W, the average output power is 250 mW, corresponding to pulse energy of 3.25 μJ. To the best of our knowledge, this is the first time to realize a passively Q-switched, dual-wavelength solid-state laser using WS₂ as saturable absorber.

WS₂ nanoplatelets could be fabricated by liquid-phase exfoliation due to the weak bonding between the WS₂ layers, which made it possible to separate WS₂ bulk into single molecular layers.^[23] In our experiment, solution of WS₂ nanoplatelets was fabricated by the liquid exfoliation.^[24] The WS₂ powder was dispersed with ethanol and water at the volume ratio of 50:50, and then the mixture was ultrasonically agitated for about 200 min. After ultrasonication, the solution was centrifuged at 3000 r/min for several times to move out the larger WS₂ NPs. Hence, the WS₂ solution was completed as shown in Fig. 1(a). Figure 1(b) shows the atomic force microscope (AFM) image of WS₂ sample.

The thickness of WS₂ NPs ranges from 20 to 70 nm. The width and length of WS₂ NPs fall within the range of about 200 nm. We test the Raman spectra of WS₂ sample. The two characteristic peaks and A_1g appear at 352.2 cm⁻¹ and 420.5 cm⁻¹, which are in agreement with the earlier reports.^[17,19] The nonlinear transmission of the WS₂ film was measured by the near-field Z-scan technique shown in Fig. 2(a). The result is shown in Fig. 2(b) and the modulation depth of WS₂ film sample could be derived to be about 12.7%. Meanwhile, the saturable optical intensity of the WS₂ film is 0.106 mJ/cm², which is higher than reported typical values of graphene oxide,^[25] carbon nanotubes,^[26] or topological insulators,^[27] which implies that such high saturable intensity entails large optical intensity for excitation and the threshold of Q-switching operation with the WS₂ film would be higher. The damage threshold of the WS₂/PVA film was measured to be 1.14 mJ/cm² by a picosecond Yb-doped fiber laser amplifer system with an output power up to 6.5 W.

To prepare the WS₂-SA, we used a BK7 glass substrate with high refractive index thin polymer of 96% at 1030 nm with a bandwidth of about 60 nm. After mixing WS₂ solution with polyvinyl alcohol (PVA), WS₂/PVA was spin coated on the substrate with low speed. Then the WS₂ saturable absorber mirror (SAM) was finished.

The WS₂ saturable absorber was inserted to a solid-state Yb:LSO laser and the schematic of the laser is shown in Fig. 3. The laser gain medium was a 5 at.%-doped Yb:LSO crystal size of 5 mm×6 mm×3 mm. For stable operation, the Yb:LSO crystal was wrapped with indium foil, mounted on a copper heat sink, and water cooled down to 15.3 °C. The pump source was a fiber-coupled diode laser at 976 nm. The core diameter of the tail fiber was 105 μm and the numerical aperture was 0.22. The pump laser beam was focused by two lenses with an imaging ratio of 1:2. The cavity was composed of three mirrors. M1 was a flat dichroic mirror with antireflection at 980 nm and high reflection at 1020–1100 nm. OC was an output mirror (T = 2.5%) with a radius of curvature of 100 mm. WS₂-SAM was a high reflective mirror coated with WS₂, as shown in Fig. 3.

Fig. 1.

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	Fig. 1. (a) WS2 dispersion after ultrasonication and centrifuging. (b) The AFM scan image of WS2 NPs and thickness of the WS2 film.

Fig. 2.

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	Fig. 2. (a) The schematic of z-scan system. (b) The nonlinear transmission curve of WS2 film.

Fig. 3.

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	Fig. 3. Schematic of Yb:LSO laser with a WS2-SAM.

The average output power of Yb:LSO laser with and without WS₂-SA was plotted in Fig. 4(a). Initially, we investigate the performance of continuous wave (CW) Yb:LSO laser by replacing the WS₂-SAM with a high reflective mirror. The pump threshold was measured to be 2.8 W. With the pump power increasing to 9 W, the average output power increases to 2.77 W, corresponding to an optical-to-optical efficiency of 30.8%. When the WS₂-SAM was used as the end mirror, the laser threshold was up to 6 W. Q-switching began at the pump power of 6.8 W, and the maximum output power was measured to be 250 mW. Note that the output power of Q-switched laser is much less than that of CW laser, which is primarily attributed to the large loss introduced by both of WS₂/PVA and BK7 substrate.

Fig. 4.

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	Fig. 4. (a) Average output power versus incident pump power for continuous wave and Q-switched operation. (b) Pulse width and repetition rate versus incident pump power for Q-switched operation.

The laser performance was observed using a 1 GHz digital oscilloscope (Agilent, 54833A). Figure 4(b) shows the pulse width and repetition rate versus incident pump powers. The pulse width decreases from 8 μs to 1.6 μs, while the pump power increases from 6.8 W to 9 W, and the pulse repetition rate increases from 34.5 kHz to 76.9 kHz. The low repetition rate of Q-switched solid-state laser should be attributed to the higher saturable optical intensity of the WS₂-SA. Higher saturable energy led to shorter buildup time for the pulse generation,^[28] so the Q-switched solid-state laser could sustain with a low repetition rate. Note that the repetition rate and the pulse width are not completely linearly dependent on pump power. This is attributed to the gain competition between two wavelengths of 1034 nm and 1056 nm. The gain competition would affect the inversion population. As a result, the non-saturable inversion population changes the output characteristics of Q-switched pulses. Figure 5(a) shows the shorted pulse width of 1.6 μs obtained in our Q-switched Yb:LSO laser based on WS₂ saturable absorber, which is similar to the reported passively Q-switched fiber lasers with WS₂-SAs.^[23] The pulse width could be reduced by reducing cavity length and optimizing the WS₂-SA. The energy of Q-switched pulses as a function of incident pump power is shown in Fig. 5(b). When increasing the pump power to 9 W, the maximum pulse energy of 3.25 μJ could be obtained. However, from the evolution of the pulse energy, larger pulse energy could be expected since the pulse energy of Q-switched pulses is not saturated.

Fig. 5.

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	Fig. 5. (a) Temporal profile of the shortest pulse under pump power of 9 W. (b) Pulse energy as a function of incident pump power.

Finally, we observed the spectra for the CW and Q-switching operation. The results are shown in Figs. 6(a) and 6(b). It could be seen in continuous wave state, the center wavelength is 1056.6 nm. However in Q-switching state, it turns out to be dual-wavelength operation, and the center wavelengths are about 1034.3 nm and 1056.8 nm. The output power at 1056.8 nm is larger than at 1034.3 nm for a larger gain cross section. In experiments, the surface of gain medium was deliberately tuned with a slight deviation angle to M1, thus the Fabry–Perot effect between mirror M1 and Yb:LSO crystal could be excluded. One reason for dual-wavelength Q-switching is that the laser crystal Yb:LSO has a large emission area. The emission bandwidth of Yb:LSO is as broad as 73 nm^[29] and it could be tuned from 1016 nm to 1086 nm,^[30] which underlay the dual-wavelength operation. Another reason is the shift of spectrum peak caused by non-stability. It is noteworthy that the output spectra of the Q-switched laser (1034.3 nm and 1056.8 nm) correspond to two of five peaks of the emission spectrum for Yb:LSO crystal (1032 nm and 1055 nm).^[31] At last, the non-saturable loss introduced by WS₂ film contributes to a significant decrease of output power, and the tungsten disulphide saturable absorber film also has a great influence on the emission spectrum of the laser.

Fig. 6.

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	Fig. 6. (a) The spectrum of CW operation. (b) The spectrum of Q-switching operation.

Fig. 7.

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	Fig. 7. Q-switched pulse trains under incident pump powers of (a) 6.8 W, (b) 8 W, and (c) 8.5 W.

Figure 7 shows pulse trains of Q-switched Yb:LSO laser under different pump powers, in which no satellite pulse as reported in Refs. [11], [32], and [33] is observed, which indicates that the dual-wavelength is time-overlapped under Q-switching regime.

A passively Q-switched dual-wavelength solid-state laser based on WS₂ saturable absorber was demonstrated. Q-switching operated simultaneously at the center wavelengths of 1034.3 nm and 1056.8 nm. The pulse width and repetition rate were measured from 8 μs to 1.6 μs, and 34.5 kHz to 76.9 kHz, respectively. The maximum output power was 250 mW, corresponding to a single pulse energy of 3.25 μJ. To the best of our knowledge, it is the first time to obtain dual-wavelength Q-switching operation based on WS₂-SA in solid-laser lasers.