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Abstract
We fabricated a molybdenum disulfide (MoS2) saturable absorber (SA) by coating MoS2 thin film on a quartz plate with the Langmuir-Blodgett (LB) method. To the best of our knowledge, it is the first time that saturable absorbers have been fabricated with the LB method. The film thickness can be adjusted freely in the LB method process. In addition, the surface of the absorber film made by the LB method was very smooth, which can decrease the optical scattering on the absorber, thus reducing the non-saturable absorption losses. With such an absorber, higher average output power two-dimensional material absorber mode locked lasers are possible. In this paper, a stable passively Q-switched (QS) Nd:GdVO4 laser was realized. The shortest pulse duration was 269.2 ns with a repetition rate of 1.03 MHz. The corresponding highest average output power, single pulse energy, and peak power were 1.39 W (35.85% conversion efficiency), 1.35 µJ, and 5.01 W, respectively. Therefore, the LB method may be a good choice to make practical two-dimensional material absorbers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In order to satisfy the requirement of pulsed lasers, some new style SAs with the characteristics of simple structure, easy manufacture, and low cost, like carbon nanotubes [1–3], graphene [4–7], and transition-metal dichalcogenide [8–12], are always pursued by researchers related to this field. Meanwhile, the saturable absorbers also impel passive mode locked and Q-switched lasers to produce better parameters of laser pulses. In our paper, as a kind of graphene-like two-dimensional (2D) material, MoS2 was applied to a passively Q-switched Nd:GdVO4 laser. The MoS2 nanomaterial possesses an indirect band gap (1.0~2.0 eV) and its band gap increases with the decreasing number of the layers [13]. Besides, the indirect band gap turns into a direct band gap when the bulk material is exfoliated into individual layers [14]. Determined by the unique band gap, MoS2 displays excellent optical nonlinear saturable absorption and broad wavelength operation range. However, the inhomogeneous sizes and distributions of some reported MoS2 nanomaterial SAs will lead to the poor repeatability of laser experimental.
The LB technique is always used to prepare highly ordered monomolecular film [15,16]. Based on the technique, the thickness of thin film can be controlled in the nanometer-level. In addition, the prepared nanomaterial flakes are distributed uniformly on the film [17–19]. Moreover, unlike magnetron sputtering and ion beam aided evaporation techniques, the membrane forming conditions in LB technique only requires normal temperature and pressure conditions, which avoid the damage of the nanomaterial and membrane structures. Therefore, the technique is usually applied to the studies on physical properties of nanomaterial films, including electrical properties [20], biological properties [21,22], and optical properties [23–25].
As far as we know, it was the first time to apply the LB film based on MoS2 nanomaterial in Nd: GdVO4 laser and the experimental results verified the superiority of MoS2 LB film SA in all-solid-state lasers.
2. Fabrication and characterization of MoS2 LB film SA
2.1. Suspension configuration
The bulk MoS2 material was exfoliated into few-layer nanoplates by the liquid phase exfoliation (LPE) method [26,27]. The 10 mg bulk MoS2 powder were dissolved in the 10 ml deionized water, then the solution was ultrasonicated for 12 hours to get few-layer 2D nanomaterial. After that, we mixed the supernate of MoS2 solution, methanol, and chloroform with the volume ratio of 2:8:1.5. The dispersants of methanol and chloroform enhanced the spread of nanoplates solution over the water subphase in the next step. Figure 1(a) shows the image of the prepared mixed solution.
Fig. 1 (a) The image of the prepared MoS2 solution. (b) The Raman spectrum of MoS2 nanosheets. The SEM images from the (c) surface and (d) cross section of the MoS2 film SA. (e) AFM image of MoS2 film. (f) The thickness of MoS2 film.
A typical Raman spectrum of as-prepared few-layer MoS2 were measured by a Raman spectrometer excited by a 532 nm laser source, as shown in Fig. 1(b). Two Raman peaks were located at 379 cm−1 and 404 cm−1, respectively, which were in agreement with the Raman characteristic of reported few-layer MoS2 [28].
2.2 Preparation of LB film
The MoS2 film was made by using a system (JML04C1) with a double barrier Langmuir trough, a cool-water machine, and a force transducer. Firstly, we dipped a 1 mm thickness hydrophilic quartz plate vertically in the Langmuir trough half filled with the water subphase. Then, the prepared MoS2-methanol-chloroform solution was dropped slowly on the surface of the water subphase (pH = 7.0, at 20°C) in the Langmuir trough at a rate of 0.1 ml/min for 30 minutes. After 0.5 h volatilization of the dispersants, a pair of mobile barriers was used to compress the floating monolayers at the air-water interface. Under a surface pressure of 22 mN/m, the quartz plate was lifted at a rate of 0.8 mm/min so that the floating layer was transferred on the surfaces of quartz plate. Finally, we put the quartz plate in an drying oven for evaporation. The temperature of the oven was kept at ~60°C. After two hours, the MoS2 absorber coated on the surfaces of quartz plate was achieved. The Fig. 1(c) and the Fig. 1(d) display the scanning electron microscope (SEM) images from the surface and the cross section of the MoS2 film SA. It could be seen that the MoS2 have the layer structure and there was a thin uniform-thickness MoS2 film on the surface of quartz plate.
2.3 Characterizations of LB film
The thickness of MoS2 film was characterized by using an Atomic Force Microscope (AFM), as displayed in the Fig. 1(e) and Fig. 1(f). The surface morphology showed clearly that MoS2 nanosheets were distributed uniformly on a large area of 0.8*1.2 µm2. From the Fig. 1(f), we found that the height of LB film was calculated to be about 6~7 nm, demonstrating that the thickness of MoS2 nanoplates on the film was relatively uniform.
As shown in Fig. 2(b), the linear optical transmission curve of the MoS2 SA was measured by a spectrophotometer (TU-1810). The transparency of the quartz plate coated with MoS2 film SA was 82.5% at 1064 nm. Excluding the quartz material absorption of 9.9%, the linear optical absorption of pure MoS2 film was 7.7%.
Fig. 2 (a) The schematic of the nonlinear optical measurement system. (b) The linear optical transmission curve of the MoS2 SA. (c) The nonlinear optical transmission curve.
In order to confirm the saturable absorption capability of the MoS2 SA at 1 µm, the nonlinear optical characteristic was measured by using a home-made 1064 nm picosecond solid state laser shown in Fig. 2(a). The pulse duration was 20 ps and the repetition rate was 100 MHz. The output laser beam was split into two beams. One was used for the measurement of SA’s nonlinear optical transmission, the other act as the reference light. We recorded the nonlinear optical transmission data while altered the laser power continuously, as revealed in Fig. 2(c). Here, the measured experimental data were precisely fitted with the following equation [28,29]:
where I was the input intensity, Isat was the saturable intensity, Tns was the non-saturable loss, T(I) was the transmission, and ΔT was the modulation depth. From the curve of the nonlinear transmission, the modulation depth (ΔT), saturable intensity (Isat), and non-saturable loss (Tns) were 4.9%, 104 KW/cm2, and 13.5%, respectively.
3. Experimental setup
Figure 3 showed the schematic of passively Q-switched Nd:GdVO4 laser. A 3*3*10 mm3 Nd:GdVO4 crystal was pumped by a 808 nm diode laser (LD) with the maximum output power of 30 W. The 5 cm long laser cavity was designed. The dichroic mirrors M1 (R = ∞) and M2 (R = 50 mm) acted as two end mirrors. The M1 was coated with the film of 808 nm high transmissivity (HT) and 1064 nm high reflection (HR). The M2 was an output coupler (OC) with the transmission of 9%. The crystal mounted by copper was cooled by a 16°C cool-water machine. The MoS2 SA was inserted in the laser cavity close to M2.
4. Experimental results and discussions
Figure 4 displayed the SEM image in large scale (50 μm). Generally, the diameter of the laser spot upon the absorber in Q switching or mode locking operation is from 30 to 100 micrometer. From the SEM image, we can see that the MoS2 distribution in large scale is not the constant value. Therefore, we can adjust the absorption of the absorber by changing the position that the laser spot illuminated.
Figure 5 displayed the Q switched laser results based on as-prepared MoS2 LB film SA. When the pump power rose from 3.9 W to 6 W, a stable passive Q-switching operation was obtained. From the Fig. 5(a), one can see that the repetition rate of Q Switched (QS) laser increased from 654.7 kHz to 1030 kHz with the increase of pump power, and the single pulse duration decreased from 623 ns to 269.2 ns.
Fig. 5 Experimental results: (a) Pulse duration and repetition rate curves. (b) Average output power. (c) Single pulse energy and pulse peak power. (d) The profile of the shortest single laser pulse. (e) Pulse train. (f) The optical spectrum of the Q switched laser output.
The average output power of continuous wave (CW) laser and the QS laser based on the same laser cavity were shown in Fig. 5(b). The average output power of Q Switched laser reached to a maximum value of 1.39 W under 6 W pump power. When the pump power exceeded 6 W, the MoS2 film was damaged by laser-induced heat accumulation [30,31], which resulted in the CW operation. On the whole, the output power of Q Switched laser increased approximately linearly versus pump power with a slope efficiency of 35.85%. The slope efficiency in CW operation is 59.39%. From the Fig. 5(c), it was seen that the maximum single pulse energy was 1.35 µJ and the maximum peak power was 5.01 W. The shortest single pulse of 269.2 ns was shown in Fig. 5(d). The pulse train was displayed in Fig. 5(e). Figure 5(f) displayed the optical spectrum of the Q switched laser output. The central wavelength is 1064.3 nm.
Finally, we got high average output power and slope efficiency of Q-switch, as shown in Table 1. This was attributed to the LB film was smooth, evenly distributed, and low insertion loss. Then we repeated the Q-switched laser measurement three times based on same MoS2 film SA. The similar results were produced in the Nd:GdVO4 laser.
5. Conclusion
In summary, we fabricated a kind of MoS2 saturable absorber by a simple and low-cost LB method. The film made by LB shows some virtues, such as nanoscale thickness, smooth surface, and high transmission and less impurity. These virtues will help to get absorbers with low non saturable absorption losses, which is important in high power Q-switched lasers. By employing the LB film to a diode end-pumped Nd: GdVO4 laser, we achieved a passively Q-switching operation with the slop efficiency of 35% and the high average output power of 1.39 W. The 269.2 ns pulses with the high peak power of 5.01 W were obtained. The corresponding repetition rate and the single pulse energy were measured to be 1.03 MHz and 1.35 µJ.
Funding
Central University (No. GK201702005); Natural Science Foundation of Shaanxi Province, China (No. 2017JM6091); Fundamental Research Funds for the Central Universities (No. 2017TS011).
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