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Abstract
We have successfully achieved the synthesis of heterojunction consisting of WSe2 and BN, by using a liquid phase exfoliation method, and characterization of the prepared materials under the microstructure. The WSe2/BN heterojunction was used as a saturable absorber in the Tm:YAP laser for passively Q-switched operation, and a pulsed laser with an output wavelength around 2 µm range was successfully obtained. After comparing the effects of resonators composed of different cavity mirrors, it is concluded that when the curvature radius of the input mirror is 250 mm and the transmittance of the output coupler is 2.5%, the best output performance was obtained. The maximum average output power of 834 mW was achieved, with a pulsed repetition frequency of 43.51 kHz and a minimum pulse duration of 1.28 µs, corresponding to a peak power of 14.97 W and a maximum single pulse energy of 19.17 µJ.
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1. Introduction
Lasers in the mid-infrared (MIR) 2 µm range with short pulses are promising for scientific research, medical diagnostics and environmental monitoring, among many other areas [1–5]. In order to obtain pulsed lasers with high power and short pulse width in the 2 µm wavelength range, the passively Q-switched (PQS) technique is one of the most effective methods used by researchers, due to its compactness and simplicity of design. The output performance of PQS laser mainly depends on the optical properties of the selected saturable absorber (SA). However, the traditional SA materials have been limited by narrow bandwidths, difficult preparation techniques, and the expensive costs, so it is a key issue for the realization of PQS laser to study new and effective SA materials. Two-dimensional (2D) materials have received a large number of attentions owing to their good nonlinear absorption properties [6]. Graphene [7,8], transition metal dihalides (TMDs) [9–12], black phosphorus (BP) [13], boron nitride (BN) [14,15], topological insulators (TIs) [16,17], transition metal oxides (TMOs) such as α-Fe2O3 [18], MXene [19,20], and other 2D materials have been applied as SAs to passively Q-switched (PQS) laser, and generated pulsed lasers in various wavelengths on account of their high thermally induced damage threshold, ultra-fast saturation recovery time, moderate saturation intensity, controllable modulation depth, broadband saturable absorption, as well as simple fabrication processes.
But the single 2D material also has its limitations employed as SA, for example, stripped BP is oxidized when exposed to air and water, and single atomic layer of graphene has finite absorption of laser, making the optical modulation depth is extremely insignificant. In order to compensate for the shortcomings of a single material and fully utilize the 2D materials, different 2D materials are layered and stacked to form van der Waals (VDW) heterostructures to obtain better performance than a single 2D material [21–27]. For these hetero- structures, each material could maintain its own properties due to the weak VDM forces between the layers. Meanwhile, the heterojunction not only can obtain superior optical properties based on the original properties, but also achieve some new performance because of the optical complementary properties [28,29], such as greater modulation depth, wider absorption bandwidth, and better stability, which leads to high performance pulsed lasers. Consequently, combining the optical merits of two or more 2D materials to form heterojunctions to obtain novel materials has become a promising development trend. Much studies have been reported on the generation of 2 µm pulsed lasers. In 2017, You et al. prepared a heterostructure composed of Bi2Te3 and graphene for the first time by using a fast self-assembly solvothermal-synthesized technique, which was used as a SA in a 2 µm Tm:YAP pulsed laser, and a pulse width of 238 ns was successfully generated [30]. In 2018, Xue et al. reported a 2 µm laser with a pulse width of 488 ns by using MoS2/BP composite as SA, and its output performance was superior to that of a single material, such as MoS2 and BP, for SA respectively [31]. Previously, by using a graphene/BN heterojunction as the SA, our group demonstrated a passively Q-switched Tm:YAP laser, producing the pulsed laser with a pulse duration of 607 ns. Our experimental results demonstrated that a pulsed laser with a heterojunction SA could be obtained better output performance than a single material as the SA [32].
Transition metal dichalcogenides (TMDs) are a category of inorganic two- dimensional layered materials. The atoms inside the facets are bonded together by strong chemical bonds, and the layers outside the facets interact with each other due to weak VDW forces, allowing them not only to stack on top of each other, but also to exfoliate them into thin nanosheets, which was used in the fabrication of an extensive range of high-performance optoelectronic devices [33–35]. As a typical representative of TMDs, Tungsten diselenide (WSe2) has a low thermal conductivity, a higher nonlinear two-photon absorption coefficient (1.9 × 10−9 cm/W), and a tunable bandgap that varies with the number of layers (1.2 eV indirect bandgap for bulk materials and 1.65 eV direct bandgap for single-layer structures), thus the response wavelengths can be extended to the mid-infrared spectral region [36–38]. The above investigations show that the WSe2 has potential application in Q-switched and mode- locked lasers.
In 2D materials, boron nitride (BN) has similar properties to graphene in some respects, such as favorable thermal stability characteristics, corrosion resistance, superb chemical stability, insulating properties, and tunable band gap [14]. Therefore, BN also plays an important role in the preparation of van der Waals hetero structures. On the one hand, BN has profound thermal conductivity, which is conducive to heat dissipation and can reduce the thermotropic damage of the heterojunction and improve the thermal and mechanical properties of the composites. On the other hand, due to its adjustable wide electronic bandgap and excellent dielectric properties, BN is often used as an optoelectronic device by forming heterojunctions with other 2D materials, which promotes the optical and electrical properties of the composites [39].
In this paper, we selected transition metal dichalcogenides (TMDs) WSe2 to form functional van der Waals heterostructure with BN. Since the optical properties of heterojunctions formed by stacking different 2D materials are also related to the size and morphology, hence, it is also crucial to choose a suitable preparation method. The liquid phase exfoliation (LPE) method [40], which is relatively simple and easily realized under the present conditions, was used to fabricate WSe2/BN heterojunctions. The prepared WSe2/BN heterojunction was applied to a 2 µm a-cut Tm:YAP pulsed laser. The output performance of pulsed lasers was compared with different cavity couplers. When the resonator with a 250 mm radius curvature of input mirror and T = 2.5% output coupler, the best output performance was achieved at 1989nm, with a pulsed output power of 834 mW, a repetition frequency of 43.51 kHz, a pulse width of 1.28 µs, a peak power of 14.97 W, as well as a single-pulse energy of 19.17 µJ. To the best of our knowledge, this is the first time a WSe2/BN heterojunction has been applied to a passive Q-switched laser and successfully generated a pulsed laser. The experiment results indicated that a WSe2/BN heterojunction was a potential candidate SA material in the high-performance mid-infrared pulsed lasers.
2. Preparation and characterization of WSe2/BN hetero- structure films
WSe2/BN heterojunction films were prepared by the low-cost, high-yield, and easy-to-implement liquid phase exfoliation (LPE) method combined with vacuum filtration technique, as shown in Fig. 1. First, 100 mg of WSe2 was taken and added to a solvent which had a mixing ratio of isopropanol (IPA) to deionized water of 3:7, and 100 mg of BN powder was added to 100 ml of pure isopropanol solvent, and the two solutions were placed in an ultrasonic cleaner (KO-400ES) and sonicated for more than 10 hours in a constant-temperature water bath, and after that, the sonicated solutions were mixed and then were sonicated for an additional 30 minutes. In the second step, the mixed solution was centrifuged at 8000 rpm for 10 minutes using a centrifuge (TG20-WS) to remove large particles and obtain the supernatant. Next, making use of a cellulose acetate membrane (pore size of 0.22 µm), 10 ml of the supernatant was vacuum-evacuated and filtered, which can deposit the 2D material onto the filtration membrane, and then the obtained filter paper with a film of saturable absorber was dried for 12 h at room temperature. The final step is transfer, where the WSe2/BN composite films were transferred onto quartz substrates. Before transfer, the substrate was washed in acetone, ethanol and deionized water for 15 min by using an ultrasonic cleaner to remove impurities. After that, the substrate was placed in a Petri dish and a few drops of isopropyl alcohol solution were added on it, so that the filter film could be adhered to the substrate, and at the same time, it was pressed by hand to make it fit to the substrate. Then, acetone solution, which can dissolve cellulose acetate membrane, was added drop by drop along the Petri dish to cover the quartz substrate completely. After waiting for 30 minutes, the waste solution was aspirated and the dissolution was repeated several times until the membrane was completely dissolved, as well as the substrate was free of impurities. Finally, the substrate with WSe2/BN film was dried at 40°C for 30 minutes to obtain the final heterojunction film. The WSe2/BN heterostructure surface morphology was characterized and analyzed by using a scanning electron microscope, as shown in Fig. 2 (a), from which we can observe that the thin films on the substrate are more uniformly distributed and show a clear layer structure. As well as the Raman spectra of the WSe2/BN samples excited by 785 nm laser are shown in Fig. 2 (b).
Fig. 1. Schematic diagram of the procedure for fabricating WSe2/BN heterostructures by liquid phase exfoliation (LPE).
3. Experimental setup
The experimental setup is shown in Fig. 3. A fiber-coupled laser diode (core: 105 µm, NA: 0.22) was used as the pump source. A laser water cooler was utilized for dispersing the heat from the diode, and the temperature of the diode was controlled to be at 20 °C throughout the whole experimental process. Near the threshold output power, the output wavelength of the LD was measured to be stable at 789 nm. The pump laser of the LD was focused into the gain mediumby a collimated focusing system, which consisting of two plano-convex mirrors with focal lengths of 25 and 75 mm, respectively.
An experimental setup with an L-cavity structure was used in our experiment, where the pump light was filter out by placing a 45° dichroic mirror M3, which was coated with highly transmissive to 780-820 nm and highly reflective to 1.9-2.1 µm. The above scheme can minimize the thermotropic damage of the pump power to the saturable absorber. In addition, the laser resonator also consists of an input mirror M1, a Tm:YAP crystal and an output coupler M2. When replacing different cavity mirrors, the physical cavity length is adjusted in time to ensure optimal output power. Here we provide three different input mirrors, i.e., one flat-flat mirror, two flat-concave mirrors with the curvature radius of 150 mm and 250 mm, respectively, to comparing the output performances under plane-concave resonator and plane-plane resonator. The three input mirrors were coated with a high transmittance material of 780-800 nm on the front face, and coated with a high reflectance material of 1.9-2.1 µm on another surface. Two output mirrors for 1900-2100 nm transmittance of T = 2.5% and T = 1.5% was employed in the cavity. An a-cut 3 at. % Tm:YAP with dimensions of 3 × 3 mm2 in cross-section and 8 mm in length was used as the gain crystal, it was wrapped with indium foil and put in a copper block cooled with temperature keep at 20 °C by the circulating water to reduce the thermal loads, as well as the dual end faces of the laser medium were coated cover anti-reflective films at 780-800 nm and 1.9-2.1 µm, respectively. About 70% of the pump laser is absorbed through Tm:YAP crystals with no lasing. SA was placed between M1 and M3 and as close as to M1 to achieve Q-switched operation. The output performance of the Q-switched pulse was recorded by a digital oscilloscope (DSOX4104A, 1 GHz bandwidth, 5 GS/s sampling rate) and a power meter (PM00- 19C).
4. Experimental results and analysis
Continuous wave (CW) and passively Q-switched (PQS) mode operations were performed using the above experimental setup. Six resonator combinations were obtained by using different input and output mirrors. Under CW mode operation, the output powers of the Tm:YAP laser were shown in Fig. 4. The highest CW output power is achieved when the radius of curvature of the input mirror is 250 mm and the transmittance of the output coupler is 2.5%. The average output power is 2.55 W when the absorbed pump power is 21.4 W, corresponding with a slope efficiency of 14.4%. When the input mirror is a plane mirror, it constitutes a plane-plane resonator with the worst CW output performance. As the pump power increases, the output power of plane-plane resonators increases at a small rate, while that of concave-plane resonators increases at a big rate, which can be attributed to a good thermal stability of the latter.
The WSe2/BN heterojunction prepared by using LPE method was employed as the SA to perform the PQS operation, and the average output power is shown in Fig. 5. Like CW mode, in all of these demonstrations, the highest average output power operated at the PQS mode, which is 834 mW when the absorbed pump power is 21.4 W with a corresponding slope efficiency of 5.5%, was also achieved at the input mirror of R = 250 mm and the output mirror of T = 2.5%. Switched to T = 1.5% output coupler, when the absorbed pump power is greater than 20 W, the gain saturation phenomenon occurred, and the CW output power stabilized at about 2.38 W. At this time, when the absorbed pump power is increased to 20.2 W, thepulsed output power is 721 mW, corresponding with aslope efficiency of 4.6%. The lower slope efficiency indicates that the mode matching between the pump laser and the oscillating laser in this cavity is less than expectation. In addition, owing to the insertion loss of the SA, the output power in the PQS mode is much less than CW mode, and the threshold is higher than CW mode.
Comparing the output performance of the Tm:YAP lasers when three different input mirrors (R = inf, R = 150 mm, R = 250 mm) and two output couplers (T = 2.5%, T = 1.5%) are used, the curves of pulse duration and pulse repetition rate of the oscillating laser with the change of absorption pump power is obtained, as shown in Fig. 6 (a, b, c). In all measurements, the repetition frequency was proportional to the absorbed pump power and the pulse width was inversely proportional to the absorbed pump power. The corresponding single pulse energy and peak power versus absorbed pump power are shown in Fig. 6 (d, e, f). According to Figs. 4, 5, 6 (a), and 6 (d), it can be seen that the lowest threshold and the best output performance are achieved when M1 with a radius of curvature of R = 250 mm was used as the input mirror. Moreover, the output performance with transmittance of output mirror T = 2.5% is better than the output performance with an output mirror transmittance of T = 1.5%. Using the output coupler with T = 1.5%, when the absorbed pump power is increased to 20.2 W, the pulse duration of 1.51 µs was obtained, corresponding with a pulsed repetition frequency of 42.08 kHz, a single pulse energy of 17.13 µJ and a peak power of 11.34 W. In contrast, the absorbed pump power increases to 21.42 W with T = 2.5% output coupler, the shortest pulse duration of 1.28 µs was achieved, which corresponds to a pulsed repetition frequency of 43.51 kHz, the largest single-pulse energy of 19.17 µJ, and the highest peak power of 14.97 W. However, it is worth mentioning that the single pulse energy was gradually approaching saturation with the increasing pump power reaches a certain point. With the increase of the pump power, the pulse frequency increases linearly, and the saturable absorber gradually reaches the effective saturated absorption, which leads to the gradual approximation of the single pulse energy reaches a stable value. Based on the above experiment results, we can see that the prepared Heterojunction thin film is not good because this preparation method leads to uneven distribution of the thin film material on the quartz substrate and the thickness of the heterojunction is not easy to control. By improving the preparation method, we expect to obtain WSe2/ BN heterojunctions with the right number of layers in the future, i.e., with the right modulation depth, which will lead to shorter pulse width and higher peak power. Comparisons of PQS solid-state laser with different Heterojunction SAs were shown in Table 1.
Fig. 6. Output performance of the PQS Tm:YAP laser with different output couplers (T = 1.5%, T = 2.5%). (a, d) R = 250 mm, (b, e) R = 150 mm, (c, f) R = inf.
Table 1. Comparisons of PQS solid-state laser with different Heterojunction SAs.
Single pulse and pulse sequence images were recorded at the maximum output power with R = 250 mm and T = 2.5%. Figure 7 shows that the pulse repetition rate of 43.51 kHz and the minimum pulse duration of 1.28 µs was achieved when the average output power is 834 mW. A slight fluctuation in the output pulse trains was observed, which was attributed to the heat accumulation on the WSe2/BN heterojunction at high pumping levels, which could not be eliminated in time within a short period of time, leading to the deterioration of the pulse output performance of the Q-switched laser. At this moment, the peak laser pulse fluctuation is 10.7% at the maximum output power. The output performance of the pulsed lasers with different cavity mirrors is shown in Table 2.
Fig. 7. Shortest#single pulse profile (a) and typical pulse train (b) of the PQS Tm:YAP laser at the maximum average output power.
Table 2. Comparisons of PQS laser with different cavity mirrors.
At the maximum output power, the laser spectrometer (APE WAVE, 800-2600 nm) with a resolution bandwidth of 0.5 nm was used to measure the output spectra which were shown in Fig. 8. Under CW mode, the central output wavelength of 1994.7 nm was obtained from the Tm:YAP laser. Under PQS mode, the center output wavelength of the Tm:YAP laser is around 1989.4 nm. Compared with CW mode, the output wavelength of PQS mode has a blue shift and a smaller number of emission peaks near the central lasing. Because of the additional cavity loss brought by the WSe2/BN, the gain-loss relation in the cavity made the short wavelengths with large photon energies easier to generate.
In addition, the net gain difference between the various longitudinal modes is increased by inserting the WSe2/BN SA, which plays the role of a Fabry-Perot interferometer, making the larger gain emission output near the center frequency.
5. Conclusion
In summary, we have fabricated WSe2/BN heterojunctions by using liquid phase exfoliation method, which was used as SA in the 2 µm passively Q-switched lasers with good output performance. When the absorbed pump power reaches 21.42 W, the average output power, pulse frequency, and pulse duration are 834 mW, 43.51 kHz, and 1.28 µs, respectively, and the peak power and single pulse energy are 14.97 W and 19.17 µJ, respectively. We believe that better pulsed laser performance can be expected if the structure of the resonant cavity is further optimized. For example, by increasing the size of the laser crystal, improving the parameters of the cavity mirrors, varying the thickness of the SA film, or improving the preparation method. Still, the present experimental results can demonstrate that heterostructure consisting of two 2D materials, the transition metal dichalcogenides WSe2 and BN, have promising research potential as novel nonlinear optical materials for generating mid-infrared pulsed lasers.
Funding
National Natural Science Foundation of China ((NSFC, 61805209)); Key Laboratories for National Defense Science and Technology ((JCKY202210C006)); Natural Science Foundation of Shandong Province ((ZR2023MA046)).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. L. Xu, S.Y. Zhang, and W.B. Chen, “Tm:YLF laser-pumped periodically poled MgO-doped congruent LiNbO3 crystal optical parametric oscillators,” Opt. Lett. 37(4), 743–745 (2012). [CrossRef]
2. S. Cha, K.P. Chan, and D.K. Killinger, “Tunable 2.1-microm Ho lidar for simultaneous range-resolved measurements of atmospheric water vapor and aerosol backscatter profiles,” Appl. Opt. 30(27), 3938–3943 (1991). [CrossRef]
3. A Godard, “Infrared (2-12 µm) solid-state laser sources: a review,” C. R. Phys. 8(10), 1100–1128 (2007). [CrossRef]
4. J. Lin, “Progress of medical lasers: Fundamentals and Applications,” Med. Devices Diagn. Eng. 1(2), 36–41 (2016). [CrossRef]
5. C Brian, G Lew, M. Stephen, et al., “Compact and efficient mid-IR OPO source pumped by a passively Q-switched Tm:YAP laser,” Opt. Lett. 43(5), 1099–1102 (2018). [CrossRef]
6. C.Y. Ma, C. Wang, B. Gao, et al., “Recent progress in ultrafast lasers based on 2D materials as a saturable absorber,” Appl. Phys. Rev. 6(4), 041304 (2019). [CrossRef]
7. J. Hou, B.T. Zhang, J.L. He, et al., “Passively Q- switched 2 µm Tm:YAP laser based on graphene saturable absorber mirror,” Appl. Opt. 53(22), 4968 (2014). [CrossRef]
8. J.G. Wang, X.G. Mu, M.T. Sun, et al., “Optoelectronic properties and applications of graphene-based hybrid nanomaterials and van der Waals heterostructures,” Appl. Mater. Tod. 16, 1–20 (2019). [CrossRef]
9. H.Y. Guoyu, Y.R. Song, K.X. Li, et al., “Mode-locked ytterbium- doped fiber laser based on tungsten disulphide,” Laser Phys. Lett. 12(12), 125102 (2015). [CrossRef]
10. C. Cheng, H.L. Liu, Y. Tan, et al., “Passively Q-switched waveguide lasers based on two-dimensional transition metal diselenide,” Opt. Express 24(10), 10385–10390 (2016). [CrossRef]
11. G. Zhang, Y.G. Wang, J. Wang, et al., “Passively Q-switched and mode-locked YVO4/ Nd:YVO4/Nd:YVO4laser based on a MoS2 saturable absorber at 1342.5 nm,” Opt. Laser Technol. 109, 293–296 (2019). [CrossRef]
12. Z.Q. Niu, T.L. Feng, T. Li, et al., “Layered Metallic Vanadium Disulfide for Doubly Q-Switched Tm:YAP Laser with EOM: Experimental and Theoretical Investigations,” Nanomaterials 11(10), 2605 (2021). [CrossRef]
13. H.K. Zhang, J.L. He, Z.W. Wang, et al., “Dual-wavelength, passively Q-switched Tm:YAP laser with black phosphorus saturable absorber,” Opt. Mater. Express 6(7), 2328–2335 (2016). [CrossRef]
14. X.I. Yang, L.J. Li, L. Zhou, et al., “Passively Q-switched operation of a Tm:YAlO3 laser with a BN saturable absorber mirror,” Laser Phys. Lett. 16(2), 025801 (2019). [CrossRef]
15. G Dmitri, B Yoshio, Yang, et al., “Boron nitride nanotubes and nanosheets,” ACS Nano 4(6), 2979–2993 (2010). [CrossRef]
16. Y.Y. Lin, P. Lee, J.L. Xu, et al., “High-pulse-energy topological insulator Bi2Te3-based passive Q-Switched solid-state laser,” IEEE Photonics J. 8(4), 1502710 (2016). [CrossRef]
17. S.Y. Luo, X.G. Yan, B. Xu, et al., “Few-layer Bi2Se3-based passively Q-switched Pr:YLF visible lasers,” Opt. Commun. 406, 61–65 (2018). [CrossRef]
18. L. Dong, H.W. Chu, Y. Li, et al., “Nonlinear optical responses of α-Fe2O3 nanosheets and application as a saturable absorber in the wide near-infrared region,” Opt. Laser Technol. 136, 106812 (2021). [CrossRef]
19. C.Y. Su, Y.Z. Liu, T.L. Feng, et al., “Optical modulation of the MXene Ti3C2Tx saturable absorber for Er:Lu2O3 laser,” Opt. Mater. 115, 110949 (2021). [CrossRef]
20. Y.Y Hu, X. Yang, L.J. Li, et al., “A passively Q-switched operation of Tm:YAP laser with a Nb2AlC-based saturable absorber,” Optik 256, 168743 (2022). [CrossRef]
21. M.M. He, C.J. Quan, C. He, et al., “Enhanced Nonlinear Saturable Absorption of MoS2/Graphene Nanocomposite Films,” J. Phys. Chem. C 121(48), 27147–27153 (2017). [CrossRef]
22. S.X. Liu, Z.J. Li, Y.Q. Ge, et al., “Graphene/phosphorene nano-heterojunction: facile synthesis, nonlinear optics, and ultrafast photonics applications with enhanced performance,” Photonics Res. 5(6), 662 (2017). [CrossRef]
23. Q. Hong, Y. Jian, Z.Q. Xu, et al., “Broadband photodetectors based on graphene-Bi2Te3 heterostructure,” ACS Nano 9(2), 1886–1894 (2015). [CrossRef]
24. Y. P. Liu, S.Y. Zhang, Jun He, et al., “Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials,” Nano-Micro Lett. 11(1), 13 (2019). [CrossRef]
25. W.J. Liu, Y.N. Zhu, M.L. Liu, et al., “Optical properties and applications for MoS2-Sb2Te3-MoS2 heterostructure materials,” Photonics Res. 6(3), 220–227 (2018). [CrossRef]
26. C.H. Lu, C.J. Quan, K.Y. Si, et al., “Charge transfer in graphene/WS2 enhancing the saturable absorption in mixed heterostructure films,” Appl. Surface Sci. 479, 1161–1168 (2019). [CrossRef]
27. Q.Y. Zhang, Q. Bai, E.L. Cai, et al., “Nonlinear optical properties of graphdiyne/graphene van der Waals heterostructure for laser modulations,” Results Phys. 38, 105654 (2022). [CrossRef]
28. X.P. Hong, J. J. Kim, S.F. Shi, et al., “Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures,” Nat. Nanotechnol. 9(9), 682–686 (2014). [CrossRef]
29. W.J. Liu, M.L. Liu, B. Liu, et al., “Nonlinear optical properties MoS2-WS2 heterostructure in fiber laser,” Opt. Express 27(5), 6689–6699 (2019). [CrossRef]
30. Z.Y. You, Y.J. Sun, D.L. Sun, et al., “High performance of a passively Q-switched mid-infrared laser with Bi2Te3/graphene composite SA,” Opt. Lett. 42(4), 871 (2017). [CrossRef]
31. Y. Xue, Z.D. Xie, Z.L. Ye, et al., “Enhanced saturable absorption of MoS2 black phosphorus composite in 2 µm passively Q-switched Tm: YAP laser,” Chin. Opt. Lett. 16(02), 020018 (2018). [CrossRef]
32. L.L. Gao, Y. Ding, X.J. Zhai, et al., “Passively Q-switched 2 µm laser based on graphene/BN heterostructure as saturable absorber,” Opt. Laser Technol. 168(4), 109852 (2024). [CrossRef]
33. C.L. Tan and H. Zhang, “Two-dimensional transition metal dichalcogenide nanosheet-based composites,” Chem. Soc. Rev. 44(9), 2713 (2015). [CrossRef]
34. Y.F. Ma, H.Y. Sun, B.F. Ran, et al., “Passively Q- switched Tm:YAlO3 laser based on WS2/MoS2 two-dimensional nanosheets at 2 µm,” Opt. Laser Technol. 126, 106084 (2020). [CrossRef]
35. Y.X. Liu, G. Zhang, F.T. Gao, et al., “Passively Q-switched mode-locked laser based on a MoS2/MoSe2 heterostructure saturable absorber,” Opt. Mater. 133, 112864 (2022). [CrossRef]
36. L.J. Li, W.C. Cui, X.I. Yang, et al., “A high-beam-quality passively Q-switched 2 µm solid-state laser with a WSe2 saturable absorber,” Opt. Laser Technol. 125, 105960 (2020). [CrossRef]
37. H.Z. Huang, Y. Ge, J.H. Li, et al., “Layered WSe2 Q-Switched Tm/Ho composite laser in a resonance-enhanced hybrid cavity,” OSA Continuum 3(3), 542 (2020). [CrossRef]
38. H.W. Hu, H.Z. Huang, J.H. Huang, et al., “Watt-level passively Q-switched Tm:YVO4 laser with few-layer WSe2 saturable absorber,” Infrared Phys. Technol. 113, 103554 (2020). [CrossRef]
39. S.J. Magorrian, A.J. Graham, N. Yeung, et al., “Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures,” 2D Mater. 9(4), 045036 (2022). [CrossRef]
40. N. J. Coleman, M. Lotya, A O’Neill, et al., “Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials,” Science 331(6017), 568–571 (2011). [CrossRef]
41. X.L. Sun, B.T. Zhang, Y.L. Li, et al., “Tunable Ultrafast Nonlinear Optical Properties of Graphene/MoS2 van der Waals Heterostructures and Their Application in Solid-State Bulk Lasers,” ACS Nano 12(11), 11376–11385 (2018). [CrossRef]
42. L. Dong, H.W. Chu, X. Wang, et al., “Enhanced broadband nonlinear optical response of TiO2/CuO nanosheets via oxygen vacancy engineering,” Nanophotonics 10(5), 1541–1551 (2021). [CrossRef]
43. R. Dai, J.H. Chang, Y.Y. Li, et al., “Performance enhancement of passively Q-switched Nd: YVO4 laser using graphene-molybdenum disulphide heterojunction as a saturable absorber,” Opt. Laser Technol. 117, 265–271 (2019). [CrossRef]
44. B.Z. Yan, G.R. Li, B.N. Shi, et al., “2D tellurene/black phosphorus heterojunctions based broadband nonlinear saturable absorber,” Nanophotonics 9(8), 2593–2602 (2020). [CrossRef]
