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We prepared multi-layered MoS2 nanosheets and hierarchical MoS2 nanospheres composed of ultrathin nanosheets via a facile hydrothermal method. The presence of excessive thiourea played a critical role in the formation of nanospheres. Both the passively Q-switched single- and dual-wavelength Yb3+:GdAl3(BO3)4 (Yb:GAB) laser were achieved by using the multi-layered nanosheets and hierarchical nanospheres as saturable absorbers (SAs). As far as we know, the saturable absorption characteristic of hierarchical nanospheres was reported for the first time. Compared with nanosheets, the as-prepared nanospheres SA exhibited improved saturable absorption property with shorter pulse width and higher pulse energy, which should be attributed to the unique hierarchical structure.
© 2015 Optical Society of America
1. Introduction
Since Geim and Novoselov et al firstly report the graphene [1], which is a mono-layer carbon material with Dirac-like gapless electronic band structure, a greatly increased attention has been focused on the 2D layered-structure materials in the field of optoelectronic application [2–5], especially as a saturable absorber to achieve nanosecond to femtosecond laser pulses [6]. Graphene has become the most representative 2D materials in optoelectronic researches in the past few years [7–12]. The great success of graphene encourages researchers to exploit the optical properties of other 2D layered materials and their application potentials. Following that, topological insulators (TIs), a new family of 2D SA materials with a narrow gap in the bulk state (0.1-0.3 eV) and Dirac cone on the surface/edge [13], have caused continuous attentions both in solid-state laser and fiber laser [14–19].
Moreover, atomic layered transition-metal dichalcogenides (TMDs) have exhibited outstanding optical saturation absorption properties. As one of the representatives, MoS2 has a sandwich structure which consists of covalently bound S-Mo-S tri-layers held together by van der Waals interaction [20]. It possesses a thickness dependent band structure that bulk MoS2 is an indirect gap semiconductor with a bandgap of 1.8 eV while mono-layer MoS2 has a direct gap of 1.2 eV due to quantum confinement effect [21,22]. Wang et al. firstly demonstrated the saturable absorption of few layer MoS2 under femtosecond pulse laser excitation at 800 nm [23]. After that, passively mode-locked or Q-switched fiber laser with few layer MoS2 saturable absorber has been realized at 1.0, 1.5, and 2.0 μm [24–28]. As known, solid-state laser is more suitable for high-energy short pulse generation than fiber laser, whereas the application of MoS2 SA on solid-state lasers are less addressed so far [29–32]. By introducing suitable defects, Wang et al. have realized a broadband MoS2 SA and firstly performed Q-switched solid-state lasers operating at 1.06, 1.42, and 2.1 μm with a minimum pulse width of 970, 729, and 410ns, respectively [29]. Xu et al. achieved 227 ns Q-switched Nd:YAP laser at 1079 nm using few-layer MoS2 [30]. Zhan et al. reported a passively Q-switched Yb3+-doped solid-state laser, while only achieved a minimum pulse width of 12 μs [31]. Lou et al. firstly studied the dual-wavelength Q-switched laser [32]. These previous reports are based on mono-layer or few-layer MoS2, whereas the saturable absorption property of MoS2 nanosheets with tens of layers is seldom studied to the best of our knowledge. In additions, there are no reports on saturable absorption property of MoS2 with other morphologies such as the nanospheres. The unique flower like MoS2 nanospheres exhibit greatly enhanced lithium storage properties, which encourages us to pay attention on them.
In this work, we firstly demonstrated the Q-switched single- and dual-wavelength Yb3+:GdAl3(BO3)4 (Yb:GAB) laser with multi-layered MoS2 nanosheets and hierarchical MoS2 nanospheres as saturable absorbers (SAs), which were synthesized via a facile hydrothermal process with different thiourea/Na2MoO4 molar ratios.
2. Fabrication and Characterization of MoS2 SAs
In the previous reports, few-layer MoS2 sheets were mostly prepared by CVD method [26,29] or hydrothermal intercalation/exfoliation approach from bulk crystal [23–25]. In this work, the MoS2 nanosheets and nanospheres were synthesized by a facile hydrothermal process. All chemicals used in our experiments were of analytical purity and used directly without further purification. In a typical synthesis of MoS2 nanosheets, 0.1 g Na2MoO4-2H2O was dissolved in 13 ml deionized water and then 0.072 g NH2CSNH2 was added into the solution under constant stirring. To obtain MoS2 nanospheres, 0.25 g NH2CSNH2 and 0.1 g vinyl pyrrolidone (PVP) were also required. The mixture was transferred into a 22 ml Teflon-lined stainless steel autoclave and heated at 230 °C for 24 hr. After cooling naturally, the black precipitates were collected by centrifugation with DI water and ethanol for several times.
Figure 1 shows the scanning electron microscopy (SEM) images of the as-produced MoS2 nanosheets and nanospheres at different magnifications. From Fig. 1(a) and 1(b), it can be clearly observed that large-scale isolated nanosheets with lateral dimension of 400-600 nm are widespread. The thicknesses are measured to be about 25-30 nm, indicating that the MoS2 nanosheets have about 40 layers since the thickness of single layer is 0.65 nm [33]. Figures 1(c) and 1(d) show the morphology of the MoS2 nanospheres with sizes of ∼400 nm, which are composed of ultrathin sheet-like subunits.
Fig. 1 SEM images of (a,b) the as-prepared MoS2 nanosheets and (c,d) flower like MoS2 nanospheres at different magnifications.
To better understand the role of excessive thiourea, we further prepared some MoS2 samples at two other amount of NH2CSNH2 (0.14 and 0.25 g) without PVP. It is found that MoS2 sheets reunite easily at the high amounts case, forming nanoflower like struction (0.14 g case) and even 3D flower like MoS2 microspheres (0.25 g case). Due to the laminar growth habit of MoS2, the agglomerated nuclei grow into a flower-like microstructure. The excessive thiourea increases the concentration of S2- in the solution, which accelerates the formation of MoS2 nanosheets and thus promotes the self-assembling process. As shown in Fig. 1(c), large-scaled uniform flower like MoS2 nanospheres are obtained.
To apply the MoS2 samples in solid-state laser, the MoS2 produces were mixed with 1% PMMA (polymethyl methacrylate) -toluene solution by a 60 min high-power ultrasonic treatment. The MoS2/PMMA dispersion was then spin-coated onto a 1mm thick quartz substrate to form film. The optical saturable absorption properties were investigated by the pump-probe measurement. A home-made acousto-optic Q-switching solid-state laser at 1.0 μm was used as the laser source. The corresponding result is shown in Fig. 2.
By fitting the curve with equation
where is the transmittance, is the modulation depth, is the input intensity, is the saturation intensity, and is nonsaturable loss, the saturation intensity and the modulation depth of multi-layered MoS2 nanosheets SA are extracted to be 8.66 kW/cm2 and 12.45%, respectively. There is still a large difference of the saturation intensity compared to the hydrothermal exfoliated MoS2 layers, which can be attributed to the different fabrication techniques that determined the size and quality of the MoS2 nanosheets. However, such a low saturable intensity can significantly reduce the Q-switched laser threshold and thus is beneficial for Q-switched pulse generation. The saturation intensity and the modulation depth of hierarchical MoS2 nanospheres SA are 4.75 kW/cm2 and 6.56%, respectively. The nonsaturable loss is 6.26%. Considering the transmittance of quartz substrate is about 96% at 1.0 μm, the loss of the nanospheres SA is about 2.26%, indicating a thin film on the quartz substrate.The GdAl3(BO3)4 (GAB) crystal has been demonstrated to be a good host material for solid-state lasers because of its good chemical and physical properties. The Yb3+ ions doped GAB crystal is a good host for emitting stable single-, dual- and tri-wavelengths lasers around 1045 nm which has been systemic studied in our group [17,34–36]. We thus choose Yb:GAB crystal as the laser medium to study the wavelength-insensitive saturable absorption of the multi-layered MoS2 nanosheets and hierarchical MoS2 nanospheres at 1.0 μm solid state laser.
As shown in Fig. 3, a 20 mm long plano–concave cavity was designed to value the performance of the MoS2 samples in the Q-switched laser. A 3*3*2 sized Yb:GAB crystal with doping concentration of 10 at.% was used as the laser crystal. The crystal was end-pumped by a fiber-coupled continuous wave diode laser at 976 nm with a numerical aperture of 0.22 and a core diameter of 200 μm. The input mirror (IM) was a plane mirror with antireflection (AR) coated at 976 nm and high-reflection (HR) coated at 1020-1080 nm. A concave mirror with 3% transmittance at 1045 nm and a curvature radius of 75 mm was used as the output coupler (OC). The crystal was wrapped in foil and mounted in a bronze holder with water-cooled at 18 °C.
3. Results and discussion
Firstly, we study the single wavelength Q-switched laser operation. After optimizing the CW laser operation with a threshold absorbed pump power of 1.04 W, we place the MoS2 nanosheets and nanospheres SA inside the laser cavity, respectively, corresponding to a threshold absorbed pump power of 1.48 and 1.38 W. The different nonsaturable loss of the SAs can account for the difference of threshold absorbed pump power. Figure 4(a) shows the relationships between the average output power and the absorbed pump power. When we continuously increase the absorbed pump power up to 2.54 W, the output powers increase linearly. The pulsed laser with nanosheets SA reaches a maximum value of 170 mW with a slope efficiency of 15.4%. Further increasing the pump power, cluttered pulse trains appear due to oversaturation of nanosheets SA. While the pulsed laser oscillation using nanospheres SA remain stable, and no saturation is observed until the absorbed pump power increased over 2.81 W where an average output power of 282 mW is obtained, corresponding to a slope efficiency of 17.5%. The same output power can recur with decreasing the pump power, indicating no damage to the MoS2 SAs during the laser operations.
Fig. 4 Characteristics of the single wavelength Q-switched laser at 1044.6 nm. (a) Average output power and pulse energy as a function of absorbed pump power. (b) Evolution of pulse repetition rate and pulse width with absorbed pump power, (c,d) 281 ns pulse trains and single pulse profile, (e,f) 209 ns pulse trains and single pulse profile.
The pulse width and repetition rate are recorded by a digital oscilloscope (DSO-X 3052A, Agilent), with the datum shown in Fig. 4(b). The trends of pulse width and repetition rate with the pump level obey the typical feature of a passively Q-switched laser. As the absorbed pump power increases to 2.54 W, the pulse width with nanosheets SA drops from 1.17 μs to 281 ns monotonously while the repetition rate increases from 64.8 to 206.2 kHz. For the pulse laser with nanospheres SA, as the absorbed pump power increases from threshold to 2.81 W, a decrease in pulse width from 1 μs to 209 ns and an increase in repetition rate from 68.1 to 217.4 kHz are observed. The calculated pulse energy based on the measured average output power and repetition rate is shown in Fig. 4(a). It is found that the maximum pulse energy of 1.3 μJ with nanospheres SA is much larger than the 0.82μJ with nanosheets SA, also a bit higher than the Q-switched solid state laser with graphene [37] and Tis [16]. Figures 4(c) and 4(d) and Figs. 4(e) and 4(f) show the pulse trains and single pulse profile at pulse energy of 1.3 and 0.82μJ, respectively. They possess good intensity stability since the CAJs of the pulse trains are calculated to be 6.94% and 6.18%, respectively. The emission wavelength of the Q-switched laser is recorded by using a spectrometer (WaveScan, APE, GmBH). As shown in Fig. 5(a), it centers at 1044.6nm with a FWHM of 0.62 nm.
Furthermore, we test the dual-wavelengths Q-switched laser performance of the multi-layer MoS2 nanosheets and hierarchical MoS2 nanospheres. By adjusting the inclination of the output coupler to alter the laser oscillation and introduce additional loss, we achieve an orthogonally polarized dual-wavelength simultaneously Q-switched Yb3+:GAB solid state laser with MoS2 SAs. The dual-wavelength laser spectrum is showed in Fig. 5(b) at the absorbed pump power of 2.6 W. The o-emission is near 1045.7 nm and the e-one centers at near 1043.4 nm. The corresponding frequency difference is 0.63 THz. Figure 6 shows the characteristics of the dual wavelengths Q-switched lasers. By using multi-layer MoS2 nanosheet SA, the shortest pulse duration of 216 ns is obtained with an average output power of 172 mW and a repetition rate of 266 kHz, giving pulse energy of 0.65 μJ. It is excited that we still obtain a better result with hierarchical MoS2 nanospheres SA. The shortest pulse width, largest output power and highest pulse energy are 198 ns, 352 mW and 1.36μJ, respectively. The clock amplitude jitters (CAJs) of the pulse trains in Fig. 6(c) and Fig. 6(e) are 4.82% and 3.75%, revealing a good stable intensity. Our results indicate that both the multi-layer MoS2 nanosheet and MoS2 nanospheres have broadband absorption which is suitable for dual-wavelength simultaneously Q-switched solid-state laser.
Fig. 6 Characteristics of the dual-wavelength Q-switched laser operations. (a) Average output power and pulse energy as a function of absorbed pump power. (b) Evolution of pulse repetition rate and pulse width with absorbed pump power, (c,d) 216 ns pulse trains and single pulse profile, (e,f) 198 ns pulse trains and single pulse profile.
Like Black phosphorous (BP) [38], MoS2 presents a thickness dependent gap-band structure. With the increase of the layer number, the gap-band and optical properties of MoS2 change greatly. Mono-layer MoS2 has enhanced non-linear optical properties and has been proved to be a promising saturable absorber. The results in this work suggest that the multi-layered MoS2 sheets synthesized by hydrothermal route with 40 layers are good enough for building a Q-switched laser, since our shortest pulse width 281 ns and 216 ns for single- and dual-wavelength lasers are comparable to the record minima 227 ns and 182 ns with few-layer MoS2 exfoliated from bulk crystal in [30] and [32]. The improved performance of multi-layer MoS2 SA in this work can be attributed to the defects introduced during the hydrothermal synthesis process, which can broaden and enhance the saturable absorption [29]. On the other hand, compared to the other techniques, hydrothermal synthesis route has the advantages of cost efficiency, simplicity and large scale. Our works indicate it a potential and more economical method to prepare the MoS2 sheet for the further investigations and applications in optoelectronic devices.
For hierarchical MoS2 nanospheres composed of nanosheets, it is studied as a nonlinear optical switcher for the first time. Unexpectedly, although with a thinner film, the as-prepared nanospheres SA shows improved saturable absorption with higher pulse energy and shorter pulse width in comparison to nanosheets SA. The enhanced Q-switching property might be attributed to the unique hierarchical structure of MoS2 nanospheres. For few layer MoS2, the saturable absorption is attributed to the interesting badgap, which is the blending of the 1T (metallic-like) and 2H (semiconducting) phases. Similar to TIs, the saturable absorption thus can be simply regarded as being proportional to the surface area. The self-assembly of MoS2 ultrathin nanosheets into nanospheres account for the presence of saturable absorption, and lead a larger area which thus leads an improved saturable absorption. Moreover, there are some interactions that affect the saturable absorption. A detailed discuss still need further investigations.
When a thicker nanospheres SA of a transmittance of about 85% is inserted into the laser cavity, stable laser pulses with a pulse width of 400 ns and a single pulse energy of 1.94μJ are achieved, which is the largest pulse energy of MoS2 SA obtained in reported bulk laser to the best of our knowledges. However, before saturation, the SA is damaged when we further increase the pump power. In general, a larger nonsaturable loss leads a larger impurity absorption in the SA, which will cause a larger heat accumulation. Moreover, the loose nanospheres structure might go against the heat diffusion, especially in a thicker one. In this case, using a cooling device or a substrate with higher thermal conductivity, larger pulse energy and shorter pulse width can be expected.
4. Conclusions
In this work, multi-layered MoS2 nanosheets and hierarchical MoS2 nanospheres composed of ultrathin nanosheets are fabricated via a facile hydrothermal route. As the thiourea/Na2MoO4 molar ratio increase, the nanosheets develop into nanospheres self-assembly. The two samples are used as saturable absorbers (SAs) in the passively Q-switched single- and dual-wavelength Yb3+:GdAl3(BO3)4 (Yb:GAB) laser for the first time. At the single wavelength case, the largest pulse energy and shortest pulse width with nanosheets and nanospheres SA are 0.82 μJ, 281 ns and 1.3 μJ, 209 ns, respectively. At the dual wavelength case, the results of nanosheets SA are tested to be 0.65 μJ and 216 ns as compared to 1.36 μJ and 198 ns for nanospheres SA. It is thus evident that both multi-layered MoS2 nanosheets and hierarchical MoS2 nanospheres are promising saturable absorber for pulsed bulk laser. Moreover, the higher pulse energy and shorter pulse width indicate an improved potential of hierarchical MoS2 nanospheres as nonlinear optical switches, which might be attributed to the unique hierarchical structure. Our works also provide a potential and more economical method to prepare the MoS2 samples with different morphologies for the further investigations and applications in optoelectronic devices.
Acknowledgments
This work is supported by National Natural Science Foundation of China (NSFC) (51472240, 61078076, 91122033 and 11304313), Knowledge Innovation Program of Chinese Academy of Sciences (CAS) (KJCX2-EW-H03), Key Laboratory of Functional Crystal Materials and Device (No. JG1403, Shandong University, Ministry of Education), State Key Laboratory of Rare Earth Resource Utilization (No. RERU2015018, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences), Science and Technology Plan Cooperation Project of Fujian Province (2015I0007) and Nature Science Foundation of Fujian Province (2015J05134).
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