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Abstract
Recently, two-dimensional vanadium disulfide (VS2) materials, as typical TMDs, have been successfully prepared and applied to lasers. Here, multilayer VS2 films were used as a saturable absorber (SA) in an all-solid-state visible laser. The VS2 films have a modulation depth of 34.1% and a saturation intensity of 27.5 µJ/cm2. Three wavelength passively Q-switched lasers were located at 522.7 nm, 639.4 nm, and 720.9 nm, respectively. The shortest pulse width for the three Q-switched lasers were 120 ns, 93 ns, and 108 ns. The experimental results indicate that VS2 is a promising SA material in an all-solid-state visible laser.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
All-solid-state passively Q-switched lasers have attracted extensive research interest for their application value and potential in industrial processing, manufacturing, medical treatment, etc. In recent decades, owing to the development of two-dimensional (2D) saturable absorber (SA) materials, passively Q-switched lasers have made significant progress in narrow pulse width, high repetition rate and multi-wavelength lasers. To date, graphene, transition metal dichalcogenides (TMDs), topological insulators (TIs), black phosphorus (BP) and many other 2D materials have been studied [1–8]. Among them, TMDs materials have attracted much attention due to their large number of members, such as MoS2, SnS2, ReSe2, and MoTe2 [9–12]. Each material has unique outstanding characteristics. Moreover, due to the large modulation depth, TMDs are more suitable for passively Q-switched lasers. In this work, we demonstrate vanadium disulfide (VS2), a type of TMDs material, as a SA in passively Q-switched visible lasers.
VS2 materials have “sandwich-like” structures. The transition metal V atoms are sandwiched between two layers of S atoms to form an S-V-S structure through covalent bonding [13–15]. Layers interact with each other by van der Waals forces [13–17]. VS2 materials have a larger interlayer spacing of 0.57-0.58 nm so that VS2 has better energy storage, which could enable VS2 to be used in alkaline metal ion batteries [13,18,19]. Research has shown that the band structures of the monolayer and bulk VS2 are almost the same, indicating that the interlayer coupling effect is feeble and no need to strictly control the number of layers different from MoS2 [18]. Moreover, the weaker interlayer coupling makes it easier to fabricate layered VS2 materials. According to the different arrangements of atoms, the structures of VS2 have 1T-VS2 and 2H-VS2 [20]. These two structures both exhibit metallic character, which makes VS2 own excellent electrical conductivity [14,21]. Because of its excellent properties, VS2 materials are ideal for lithium-ion batteries, catalysts and electronic devices [21–23].
Since MoS2 was discovered to have saturable absorption properties in 2013, various TMDs materials have been identified as SA for lasers [3]. In 2020, the nonlinear optical properties of VS2 materials were reported, and VS2 materials were utilized as SA in fiber lasers [24,25]. Li et al. studied the saturable absorption characteristics of 2D nanomaterial VS2. The Er-doped Q-switched fiber laser obtained a pulse duration of 854 ns with an output power of 43 mW [24]. Using VS2 as SA, Pang et al. obtained the mode-locking Er-doped fiber laser with a pulse duration of 169 fs [25]. These two reports demonstrate that VS2 materials have the ability to act as a SA. To our knowledge, VS2 materials have not yet been realized for creating passively Q-switched visible lasers based on bulk laser crystals.
In this paper, incorporating VS2 SA, we demonstrate the passively Q-switched Pr:YLF lasers. For VS2 SA, the modulation depth and the saturation intensity were measured to be 34.1% and 27.5 µJ/cm2. The Q-switched Pr:YLF lasers at 523 nm, 639 nm, and 721 nm wavelengths were achieved by adjusting the coating parameters of the resonator cavity mirror, and the corresponding maximum average output power was 41 mW, 67 mW, and 54 mW, respectively. The shortest pulse width and the highest repetition rate of 523 nm laser were 120 ns and 100 kHz. At 639 nm laser, the shortest pulse width was 93 ns and the highest repetition rate was 139 kHz. The shortest pulse width was 108 ns with a repetition rate of 120 kHz at 721 nm laser. The experiment results show that the multilayer VS2 SA is suitable for passively Q-switched lasers.
2. Fabrications and characteristics of VS2 SA
The 2D VS2 films were fabricated by the liquid-phase exfoliation (LPE) method. The VS2 powder with a purity of 99.9% was dispersed in an alcohol suspension. Then, it was ultrasonicated for 4 hours to get suspension with VS2 films. After that, the suspension was centrifuged at 8000 r/min for 15 minutes to remove aggregated powders. Finally, the extracted VS2 films supernatant was transferred onto a sapphire substrate by spin coating at 300 r/min for 10 seconds and dried under an infrared oven lamp.
A Raman spectrometer was used to measure the atomic structure of VS2 films. Figure 1(a) shows the two peaks E1g and A1g. The in-plane vibration mode (E1g) locates at 212 cm−1, and the A1g peak locates at 466 cm−1, corresponding to the out-plane vibration mode. The transmittance of the VS2 films is investigated and is shown in Fig. 1(b). The transmittance spectrum displays good light transmittance in the 400 nm to 1000 nm wavelength range, with a transmittance of more than 50%. For the research objectives 523 nm, 639 nm and 721 nm, the transmittance is 79.1%, 70.5% and 71.1%, respectively.
The atomic force microscopy (AFM) image and height profiles are given in Fig. 2(a) and (b). The measured height planer map exhibits that the thickness of the material is approximately 3.8 nm, and the corresponding number of layers of the VS2 sample is about 7 (the thickness of monolayer VS2 is about 0.576 nm [18,24]).
An actively Q-switched laser system at a central wavelength of 756 nm with a pulse width of 800 ns and a repetition frequency of 3 kHz was used to confirm the saturable absorption characteristic of VS2 films. According to the intensity-dependent nonlinear optical absorption theory, the experimental results were fitted by the formula (1):
where T(I) is the transmission, Tns is the non-saturable absorbance, ΔT is the modulation depth, I is the input intensity of the laser, and Isat is the saturation intensity. The experimental data and fitting curve are shown in Fig. 3. The results indicate that the VS2 SA sample had saturable absorption characteristics with a modulation depth of 34.1% and a saturation intensity of 27.5 µJ/cm2.3. Passively Q-switched experiments
The experimental setup is shown in Fig. 4. A InGaN LD was employed as the pumping source, whose maximum output power was 3 W. The wavelength of the LD was 444 nm with a core diameter of 200 µm. A compact two-mirror cavity was adopted as a resonant cavity, which consisted of a plane mirror (M1) and a concave mirror (M2). The M1 acted as the input mirror, and the M2 worked as the output couplers. The length of the two-mirror cavity was approximately 46 mm. For the three Q-switched lasers, we individually used three sets of resonator cavity mirrors. M1s are all coated with antireflection for the pump wavelength of 444 nm, while painted on high reflection for 523, 639 and 721 nm lasers, respectively. Three M2s all had a radius of 50 mm, and individually had an output transmittance of 2% at the three wavelengths. The focal length of the focusing lens (M3) was 75 mm. A Pr:YLF crystal with a dimension of 3 mm × 3 mm × 6 mm was used as the laser gain medium whose rare-earth ion doping concentration was 0.5%. The Pr:YLF crystal was wrapped in indium foil and cooled by the circulating water system with a temperature of 17 °C. The experiment measured that the absorption efficiency of the Pr:YLF crystal was 81.5%. After obtaining the continuous laser (CW) laser, the passively Q-switched operation was investigated by inserting the VS2 SA in the cavity.
Figure 5(a) demonstrates the average output power characteristics of the Pr:YLF CW and Q-switched lasers. The CW laser had a slope efficiency of 32.8% and a maximum average output power of 506 mW. For the generated Q-switched lasers, the threshold absorbed pump power was 1.16 W. Under an absorbed pump power of 2.23 W, the maximum output power was 41 mW, with a slope efficiency of 3.7%. Compared to CW lasers, the slope efficiency and the average output power of Q-switched lasers are greatly reduced. Many factors contribute to this phenomenon. In our experiments, the loss in the VS2 SA and the experimental manipulations to improve the pulse quality play significant role.
Fig. 5. (a) Average output power and (b) repetition rate and pulse width versus the absorbed pump power at 523 nm.
When the absorbed pump power increased from 1.47 W to 2.23 W, the Q-switched laser was measured the pulse width and repetition rate. As depicted in Fig. 5(b), for the 523 nm laser, the minimum pulse width of 120 ns at a repetition rate of 100 kHz was generated. The single pulse energy and the peak power were 0.41 µJ and 3.42 W.
Figure 6(a) shows that the average output power of CW and Q-switched lasers increase almost linearly with the increase of the absorption pump power. The threshold absorbed pump power of CW and Q-switched lasers was 215 mW and 843 mW. The efficiency of the linear function was 42.8% and 4.8%, respectively. The maximum average output power was 862 mW and 67 mW.
Fig. 6. (a) The output power variation to the CW and Q-switched lasers at 639 nm; (b) Pulse repetition rate and pulse width versus the absorbed pump power at 639 nm.
Based on the saturable absorption characteristics of VS2 SA, a Q-switched laser was obtained at around 639 nm. The performance of the Q-switched repetition rate and pulse width is shown in Fig. 6(b). The pulse width declined from 181 ns to 93 ns. The experiment got single pulse energy of 0.48 µJ and a peak power of 5.16 W with a maximum repetition rate of 139 kHz.
The maximum average output power of the 721 nm CW laser was measured to be 669 mW with a slope efficiency of 33.5%. After inserting the VS2 SA and the absorbed pump power was up to 843 mW, the 721 nm Q-switched laser obtained. The Q-switched lasers acquired the maximum average output power of 54 mW with a slope efficiency of 3.8%, which is illustrated in Fig. 7(a).
Fig. 7. (a) Average output power and (b) pulse repetition rate and duration versus absorbed pump power at 721 nm.
For the Q-switched laser, the repetition rate increased from 46 kHz to 120 kHz, and the pulse width changed from 242 ns to 108 ns (Fig. 7(b)). At the absorbed pump power of 2.23 W, the single pulse energy was 0.45 µJ, and the peak power was 4.17 W.
The pulse profiles are shown in Fig. 8(a)-(c) under the absorbed pump power of 2.23 W for 523 nm, 639 nm and 721 nm lasers. It can be seen that the single pulse and the pulse train all have good waveform. With the different resonant cavity parameters, the center wavelength comparison of Q-switched pulses is shown in Fig. 8(d), when the absorbed pump power was 2.23 W. The output spectrum had a central wavelength of 522.7 nm, 639.4 nm and 720.9 nm, respectively, and the corresponding full width at half maximum (FWHM) of 0.3 nm, 0.2 nm and 0.3 nm. The corresponding three 2D images of the output beam spatial profile show the beam profiles were close to the TEM00 mode. Under the absorbed pump power of 2.23 W, no obvious damages were observed in VS2 SA.
Fig. 8. (a)-(c) The pulse profiles at 523 nm, 639 nm and 721 nm, respectively; (d) Spectrum and 2D image of the output beam spatial profile for Q-switching modes.
SA materials such as quantum dots, crystals, and 2D materials are widely used in visible pulsed lasers [26–34]. The discovery of each new 2D materials sparks a research boom because of their advantages of simple preparation method, low cost, repeated use and wide absorption band. Table 1 lists the data of some experiments using 2D SA materials for visible pulsed lasers in recent years. From the aspects of peak power and pulse energy, it can be seen that our investigations have achieved relatively higher results. Moreover, the pulse width characteristic is also prominent. Using VS2 SA, Q-switched lasers have obtained the shorter pulse widths at three wavelengths. The 34.1% large modulation depth of VS2 SA contributes to the great Q-switched results. The comparative data shows that VS2 materials are an excellent all-solid-state SA material.
Table 1. Comparison of Pr3+-doped Bulk Visible Q-switched Lasers by Different 2D SAs.
4. Conclusion
In summary, three wavelengths of passively Q-switched lasers were investigated using VS2 films as SA. For the nonlinear characteristics of VS2 SA, the modulation depth was 34.1%, and the saturation intensity was 27.5 µJ/cm2. By LD end-pumping, the center wavelengths of the three passively Q-switched lasers were 522.7 nm, 639.4 nm, and 720.9 nm, respectively. The 523 nm Q-switched laser acquired the maximum average output power of 41 mW. The shortest pulse width was 120 ns with a repetition rate of 100 kHz. For the 639 nm laser, the maximum average output power and the slope efficiency were 67 mW and 4.8%, respectively. The shortest pulse width was 93 ns and the highest repetition rate was 139 kHz. The shortest pulse width was 108 ns with the repetition rate of 120 kHz and the average output power of 54 mW when the Q-switched wavelength was 721 nm. Our investigations indicate that VS2 is a great SA for all-solid-state visible lasers.
Funding
Natural Science Foundation of Shandong Province (ZR2021QF128); National Natural Science Foundation of China (12004208); Qilu University of Technology (Shandong Academy of Sciences), Education and Industry Integration and Innovation Pilot (2022PY022,2022JBZ01-04) .
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.
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