Pulse lasers have a wide range of applications in the medical treatment, communication, material processing, and other fields. Saturable absorber (SA) is a key element in triggering pulse operation, which can switch between different absorption states and produce ultrafast lasers on ultrafast timelines. In the past few years, various two-dimensional materials have been applied to ultrafast laser research, such as graphene, transition metal oxides, topological insulators, black phosphorus, and MXenes. The transition metal dichalcogenides (TMDs) put transition metals (Mo, W, Ti, Re, Hf) sandwiched between two chalcogenides (S, Se, Te) planes. This structure has reliable optical, mechanical, and electronic properties. It has become a new two-dimensional material. As SA, TMDs are widely used in pulse lasers, such as MoS₂, ReS₂, and WS₂. However, TMDs typically have a high saturation light intensity (tens of GW/cm²). Ternary metal sulfides (TMSs) nanomaterials have been explored by many researchers due to their ultra-wideband nonlinear optical response, high carrier mobility, and excellent air stability. The TMSs ZnIn₂S₄ (ZIS) has a layered structure composed of two-dimensional [S-Zn-S-In-S-In-S] layers with tunable electronic and optical properties. Compared with traditional binary metal sulfides such as CdS and Sb₂S₃, ZIS has the advantages of low toxicity, good chemical stability, simple preparation method, and abundant sources. In terms of optical properties, ZIS has a high optical absorption coefficient and strong optical stability. In addition, the charge transport characteristics of ZIS are changed due to the presence of abundant sulfur vacancies. It is necessary to study the layered ZIS with sulfur vacancies as efficient SA.

In this paper, we synthesize ZIS nanoflowers with sulfur vacancies by solution method. The morphology, phase structure, and ultraviolet-visible (UV-Vis) diffuse reflection spectra of ZIS nanoflowers are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet-visible-near infrared (UV-Vis-NIR) spectra. The nonlinear SA characteristics of ZIS are studied by the Z-scan method, and the optical performance and nonlinear optical response of ZIS are verified by constructing a pulsed laser experimental setup. Finally, the passive Q-switched laser output based on ZIS is realized.

We represent the characteristics of ZIS nanoflowers and show SEM images of ZIS-SA in Figs. 1(a) and (b), XRD spectrum of ZIS-SA in Fig. 1(c), absorptance of the ZIS-SA in Fig. 1(d), and EPR spectroscopy of the ZIS-SA in Fig. 1(e). As shown in Fig. 2, we verify the optical properties of the SA. The fitting results show that the saturation intensity of the ZIS-SA is 675 MW/cm² and the modulation depth is 7.8%. Second, we verify the modulation performance of ZIS as SA in pulsed lasers. Fig. 3 shows the schematic of the 1 μm pulsed laser experimental setup. Fig. 4 shows the output performance of the passive Q-switched laser. As shown in Fig. 4(a), when the pump power is 5.31 W, the continuous output power reaches 2.67 W. Passive Q-switched laser is achieved by inserting SA and adjusting its position in the cavity so that the SA is in the best position. When the pump power is 4.10 W, a stable Q-switched pulse can be obtained. When the pump power is increased to 5.31 W, the maximum output power of the laser is 240 mW. As shown in Fig. 4(b), with an increase in pump power, the pulse repetition rate gradually increases and the pulse width decreases. When the pump power increases from 4.10 W to 5.31 W, and the pulse repetition rate frequency increases from 594.6 kHz to 629.08 kHz, and the pulse width decreases from 560 ns to 388 ns. As shown in Fig. 4(c), single pulse energy and peak power of the Q-switched laser. It can be seen that the peak power and single pulse energy increase with the growing pump power. When the pump power is 5.31 W, the single pulse energy is 0.38 μJ, and the corresponding peak power is 0.98 W. As shown in Fig. 5, we record the shortest pulse at different time scales. Fig. 5(a) shows the pulse sequence at a 20 μs time scale, and Fig. 5(b) shows the pulse sequence at a 2 μs time scale. The minimum pulse width is 388 ns. As shown in Fig 6, we measure the beam quality M2 factors. The transverse beam quality factor (Mx2) is 1.83 and the longitudinal beam quality factor (My2) is 1.65 which indicate that the Q-switched pulse laser has a high beam quality.

Two-dimensional zinc sulfide indium nanoflowers are synthesized by solution method. The Q-switched Nd∶YVO₄ laser was realized by ZIS as SA. When the maximum pump power is 5.31 W, the maximum pulse repetition rate frequency of the pulsed laser is 629.08 kHz, the pulse width is 388 ns, and the maximum average output power is 240 mW. The corresponding single pulse energy and peak power are 0.38 μJ and 0.98 W. The results show that although the band gap of indium zinc sulfide is 2.32 eV and its absorption edge is located at 534 nm, the presence of sulfur vacancies introduces intermediate energy levels in the near-infrared band and enhances its light absorption. Therefore, the nanomaterials of indium zinc sulfide can still exhibit good saturation absorption characteristics in the near-infrared region. The laser with a high repetition rate and short pulse width can be obtained by ZIS as SA in the laser resonator. Therefore, zinc indium sulfide nanomaterials have a good application prospect in Q-switched pulse lasers.