Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2

In this paper, we report 4 different saturable absorbers based on 4 transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2) and utilize them to Q-switch a ring-cavity fiber laser with identical cavity configuration. It is found that MoSe2 exhibits highest modulation depth with similar preparation process among four saturable absorbers. Q-switching operation performance is compared from the aspects of RF spectrum, optical spectrum, repetition rate and pulse duration. WS2 Q-switched fiber laser generates the most stable pulse trains compared to other 3 fiber lasers. These results demonstrate the feasibility of TMDs to Q-switch fiber laser effectively and provide a meaningful reference for further research in nonlinear fiber optics with these TMDs materials. ©2015 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (140.3540) Lasers, Q-switched; (140.3500) Lasers, erbium. References and links 1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. 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Introduction
The discovery of graphene in 2004 [1] has opened up a door to a novel world of 2dimensional materials. Since then, many researchers have investigated all kinds of optical and electrical characteristics of graphene such as saturable absorption, four wave mixing, etc [2][3][4][5][6][7]. Many novel photoelectric devices have been invented based on graphene [8,9]. The prevalence of graphene has also guided researchers in exploring more analogous 2dimensional materials, among which transition metal dichalcogenides (TMDs) are wellknown. Typical TMDs include molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ). They are actually semiconductors with indirect bandwidth in bulk states. When split into monolayer or few layers, they turn into semiconductors with direct bandgap, which indicates good photoluminescence ability [10][11][12]. Besides, they own some potential optoelectronic properties, especially the nonlinear optical property [13-17]. Q-switched fiber laser is a kind of pulse laser which could generates high energy pulses up to several milli-joules [18]. It has gained significant applications in science researches and medical treatment [19][20][21][22]. Q-switching operation can be classified into 2 categories: active Q-switching and passive Q-switching. The former usually utilizes an acousto-optic or electrooptic modulator to modulate the loss of laser resonator [23], while the latter works with a saturable absorber, which could be semiconductor saturable absorber mirror (SESAM) [ [38]. Peiguang Yan et al. reported a 675 fs WS 2 based mode-locked fiber of which the signal-to-noise ratio is 65dB [39]. Other works related to mode-locked fiber laser based on WS 2 saturable absorber have been reported this year [40,41]. These results encourage us to investigate optoelectronic characteristics of more similar TMDs materials.
In this paper, we demonstrate the feasibility of Q-switching operation with 4 typical TMDs MoS 2 , MoSe 2 , WS 2 , and WSe 2 . The TMDs-PVA saturable absorbers are fabricated and characterized with Raman spectroscopy and Transmission Electron Microscopy. The saturable absorption experiment is carried out to measure the modulation depth of home-made saturable absorbers. Lastly, with these saturable absorbers we construct a ring-cavity erbiumdoped Q-switched fiber laser and the results of different materials are compared. The analysis of different Q-switch performance of 4 TMDs materials provides guidance for selecting suitable TMD saturable absorber to satisfy specific requirements of Q-switched fiber lasers and offer a good reference for future researches on nonlinear optical characteristics of TMDs.

Material preparation and characterization
Saturable absorber with thin-film form has advantages in the mass preparation, uniform quality and flexibility of usage. In our experiment, all the four TMDs are embedded in polyvinyl alcohol (PVA) to fabricate TMDs-PVA saturable absorber films. The whole fabrication process has been shown in Fig. 1 (a). For a meaningful comparison among these TMDs materials, all four TMDs are processed with the same procedures and parameters to keep the sample concentration constant. Therefore, the conclusions of comparison of four samples in this work should indicate the difference of the samples. Here MoS2 is used as an example in Fig. 1 (a). First of all, 5mg/ml MoS 2 water dispersions are prepared with sodium cholate (SC) as surfactant. The detailed preparation of MoS 2 dispersions can be referred to Ref [42]. Meanwhile, 50mg/ml polyvinyl alcohol (PVA) aqueous solution is also prepared. Then we mix 2ml MoS 2 dispersions with 10ml PVA aqueous solution for 24 hours with a magnetic stirrer. After that, the mixture is processed for another 4 hours by ultrasonic water bath device. The uniform mixture is then dropped onto the surface of a clean plastic dish and dried under 50°C air condition for 3~4 days. Finally, the high quality transparent films are obtained. Figures 1(b)-1(e) show the well-fabricated 4 types of TMDs-PVA polymer films. Although there is corrugation on the films, the films are cut into very small pieces (1x1 mm) for experimental usage. Moreover the light beam diameter is 10 μm and the films are very flat on this scale. Scanning electron microscope (SEM) is used to confirm this, shown in the insets of Figs. 1(b)-1(e). Very flat edges can be observed of four TMDs-PVA films.  Raman spectroscopy is utilized to characterize the atomic structures of the fabricated films. The Raman spectra of 4 types of TMDs-PVA films are displayed in Fig. 2. According to [43], the E 1 2g mode is related to an in-plane motion of TMD molecular and A 1g mode is corresponded to out-of-plane motion of TMD molecular. And the separation between the two modes is positively correlated to material thickness and is a good indicator of TMDs material layers. B 2g mode is due to the breakdown of translation symmetry in few-layer TMD material. The details for determining layers of different TMDs materials can be referred to Ref [44,45].For MoS 2 , A 1g mode is observed at 407.2 cm −1 and E 1 2g mode is observed at 381.3 cm −1 .
For MoSe 2 , A 1g , E 1 2g and B 2g modes are observed at 239.6 cm −1 , 283.9 cm −1 and 354.0 cm −1 , respectively. For WS 2 , A 1g mode is found at 419.5 cm −1 and E 1 2g mode is found at 355.4 cm −1 . For WSe 2 , A 1g , E 1 2g and B 2g modes are observed at 250.7 cm −1 , 245.1 cm −1 and 315.6 cm −1 , respectively. The measuring results are analogous to the results of few-layer TMDs materials reported at Ref [43], which mean a good thickness of these TMDs films. The Transmission Electron Microscopy (TEM) gives an auxiliary demonstration of thickness of TMDs films. TEM photos are gathered in Figs. 2(e)-2(h). The darker a region is in TEM photo, the thicker it is and the more layers exist in that region. There are 1~5 layers in these TMDs films, as indicated by TEM photos. The power dependent nonlinear transmission is the key parameter to evaluate a saturable absorber. The saturable absorption of our TMDs-PVA polymer films are investigated by standard two-arm experiment. The experiment setup is given in Fig. 3(a). The mode-locked laser generates femtosecond laser pulses with a repetition rate of 37MHz and pulse width of 560 fs. The output power of mode-locked laser can be tuned with a maximum power of 20mW and a single pulse energy of 0.54nJ. The output pulses propagate through a 90:10 coupler so that 10% optical power is measured by power meter 1 as a reference and 90% optical power passes through the TMDs-PVA polymer films, which are cut into 1mm × 1mm squares and sandwiched by a pair of FC/PC connectors. The power of light transmitted through the sample is measured by power meter 2. A 10-dB attenuator is applied before the power meter 2 to adapt to the measurement range of the power meter. The transmission at different optical intensity is obtained by adjusting the output power of mode-locked laser.  46] as well as in other saturable absorber materials [47]. To demonstrate that the reverse saturable absorption observed in our work is caused by TPA, a modified formula with a TPA term is used to fit the data of WSe 2 in Fig. 3 (e): where β is the TPA coefficient. We find a better fitting result (the green line) with this modified formula than the original one (the red line) and β is approximately 7 × 10 −5 cm 2 /MW.

Q-switching operations
A ring-cavity fiber laser is constructed to verify the functionality of our home-made TMDs-PVA films as saturable absorbers. The setup is shown in Fig. 4. A 980nm diode laser with tunable output power is used as a pump source. A 980/1550nm wavelength division multiplexer (WDM) couples the pump light into the ring cavity. We use a section of erbiumdoped fiber (EDF) as the gain medium. 2 polarization controllers (PC 1 & PC2) are used to adjust polarization of optical light and birefringence of fiber cavity. A 90/10 coupler extracts 10% optical power as output. A polarization independent isolator (PII) assures the unidirectional running of fiber laser. As for the output, an optical spectrum analyzer (OSA, YOKOGAWA AQ6370C) is used to monitor the optical spectrum of output laser. We also use a photodetector (PD) to transform the optical signals into electrical signals. An oscilloscope (Agilent Technologies, DSO9254A) and an electric spectrum analyzer (ESA, ROHDE & SCHWARZ) are used to monitor these electrical signals in time and frequency domain, respectively. TMDs-PVA saturable absorber is constructed by sandwiching pieces of square TMDs-PVA polymer films by a pair of FC/PC connectors. The total cavity length is approximately 17.4 m. The architecture of fiber laser is kept completely same for all 4 different TMDs-PVA polymer saturable absorbers. Q-switching operation is obtained as following: When the pump power is increased gradually, the free running of continuous wave (CW) laser is first observed which is indicated by a single narrow peak in OSA. Many longitudinal modes compete in cavity simultaneously when the pump power is further increased. Then the optical polarization is adjusted by tuning 2 PCs. Q-switched pulses will be generated at a certain pump power for different TMDs materials. Specifically, The MoS 2 Q-switching operation starts when pump power is higher than 50mW, MoSe 2 Q-switching operation starts when pump power is higher than 570mW, WS 2 starts Q-switching as the pump power is higher than 400mW, WSe 2 starts Q-switching as the pump power is higher than 280mW. Moreover, it is found that Q-switching operation can only happen when fiber laser is pumped with appropriate power and maintain stable pulses in a certain pump power range. Figure 5 shows the relationship of pump power with the output power of Q-switched lasers for four TMDs materials. The nonlinear relation between the output power and pump power is due to the generation of unstable CW lasing when Q-switching operation is still dominated.
As shown in Fig. 5, the power of Q-switched lasers rises following the increase of pump power. MoS2 Q-switching operation happens in low pump power range, its output power is lowest and the maximum output power doesn't exceed 0dBm. MoSe 2 and WS 2 Q-switching operation needs high pump power, WS 2 has the highest output power under identical pump condition compared to other three materials. Furthermore, WSe 2 has the broadest Q-switching range as it Q-switches laser starting from 280mW to 720mW and limited to the maximum output of pump LD.    As for the stability of Q-switching operation in the above pump condition, we measure the signal to noise ratio of electrical signal transformed by PD and analyze electrical spectrum with the help of ESA. With a resolution bandwidth of 100Hz, the results are gathered in Fig.  7. It shows WS 2 Q-switched fiber laser works with a highest stability, the extinction ratio of RF signal can reach up to 54.2dB. MoSe 2 Q-switched fiber laser is least stable as its extinction ratio is only 31.3dB. MoS 2 and WSe 2 Q-switched fiber lasers have a moderate performance as the extinction ratios are 48.5 dB and 41.9 dB respectively.
Optical spectrum measurement results are given in Fig. 8. The pump condition is identical with Fig. 6. Results show all four Q-switched fiber laser output spectra centered on the vicinity of 1560 nm. These Q-switched operations have different optical spectrum profile. The optical spectrum of MoS 2 has a strong continuous wave which superposes in the center of spectrum. This continuous wave occupies much power and leads to a relatively low output power of Q-switched pulses as well as degraded stability. Similarly, the optical spectrum of MoSe 2 and WSe 2 show some superposition of continuous wave while WS 2 has a smoother optical spectrum. As a result, the Q-switching operation of WS 2 is more stable and effective than other saturable absorbers.
Repetition rate and pulse duration are another two parameters of Q-switched fiber laser. Here, pulse duration is measured with oscilloscope by measuring the full width at half maximum (FWHM) of a pulse. These two parameters vary with pump power, as shown in Fig. 9. It concludes that WS 2 shows an obviously stable variation trend than other three materials which is consistent with the analysis that an optical spectrum without parasitic CW lasing has the best stability. The fluctuation of pulse duration with increasing pump power of MoS 2 -PVA saturable absorber is related to parasitic CW operation. Parasitic CW operation competes with normal Q-switching operation. The existence of parasitic CW can partially bleach the saturable absorber, change its saturable absorption and affects the pulse duration of Q-switched pulses [48,49]. Therefore, under different pump power, the power of parasitic CW operation fluctuates and thus causes pulse duration to oscillate with increasing pump power.

Discussion
The photon energy of generated 1550nm Q-switched pulses is approximately 0.8 eV. However, the bandgap values of four TMDs materials are larger than 0.8 eV. The bandgap values of these TMDs materials are summarized in Table 1. This sub-bandgap absorption is attributed to the edge states and the defect states of the TMDs nanosheets. The bandgap for TMDs materials is obtained in the assumption of an infinite lattice without any defect. However, TMDs nanosheets prepared in our work have limited size and thus high edge-to-surface ratio, which results in the existence of edge states with sub-bandgap absorption. In the early experiment, it has been shown that the sub-bandgap absorption in MoS 2 increases with the decrease of MoS 2 platelet size [50]. Similar analysis has also been performed in [32]. Defects in the materials can also create defect states and result in subbandgap absorption. Wang et al. have demonstrated the reduction of the bandgap of MoS 2 from 1.08 eV to 0.08 eV and transformed this material into a broadband saturable absorber by adding defects to the material [51]. Because MoSe 2 , WS 2 and WSe 2 have similar structure and material properties as MoS 2 , it is reasonable to deduce that the sub-bandgap absorption in these three TMDs materials are also due to the absorption induced by the edge states and defect states.
Based on the above experiments, material properties and Q-switching operations of four TMDs-PVA saturable absorbers are summarized in Table 2, which is meaningful to compare material characteristics of different TMDs materials. In this table, MoSe 2 has the highest modulation depth and the least non-saturable absorbance, but the extinction ratio in RF spectrum of MoSe 2 based laser is low which means suffering much noise during Q-switching operation. WS 2 based laser exhibits the best performance: the extinction ratio of RF signal reaches to 54.2 dB, optical spectrum has little glitch compared with other materials indicating a weaker influence by continuous wave. Besides, repetition rate and pulse duration vary with pump power smoothly and give a clear variation trend. MoS 2 and WSe 2 based lasers have more stable Q-switching operation than MoSe 2 based laser, but are not as good as WS 2 based laser.
This difference in the laser behaviors indicates that four lasers suffer different noise levels. The different noise levels come from different thermal stability of TMDs materials. Thermal stability is dominated by the thermal conductivity of a material. Material with higher thermal conductivity can dissipate heat faster and thus can endure higher absorption loss, allow higher input optical intensity and have a higher damage threshold. The high-thermal-conductivity material can sustain a stable optical property in the laser cavity and results in a low-noise laser operation. On the contrary, material with lower thermal conductivity becomes less stable and results in a noisy laser operation. It is also easy to be damaged at high input optical intensity. The thermal conductivity is 1.05 Wm −1 K −1 for MoS 2 , 0.85 Wm −1 K −1 for MoSe 2 , 2.2 Wm −1 K −1 for WS 2 and 0.9 Wm −1 K −1 for WSe 2 , respectively [52]. WS 2 has the highest thermal conductivity, meaning a best thermal stability among four materials. This is consistent with the experimental results that WS 2 -based laser has the best noise properties, shown in Table 2. It can also be noted that although MoSe 2 has the lowest non-saturable loss, MoSe 2 based laser does not has the best performance due to the low thermal conductivity of MoSe 2 . Besides, since the maximum single pulse energy of femtosecond laser is 0.54 nJ, while in the Qswitched fiber laser, the intra-cavity pulse energy can exceed several hundreds of nJ or even a few μJ. Therefore the thermal stability of four TMDs-PVA SAs cannot be observed in the nonlinear transmission experiments, but can be observed in the Q-switched fiber laser cavity. To further confirm the thermal stability of four TMDs-PVA SAs, an auxiliary experiment has been carried out to compare the stability of linear optical transmission of four SAs under different input power, shown in Fig. 10. Here an EDFA is used to amplify the output of a 1550nm CW laser. The amplified 1550 nm optical light passes through a 99:1 coupler, 1% optical light is measured by a power meter (Power Meter 1) as reference. 99% optical light passes through the TMDs-PVA saturable absorbers and is measured by another power meter (Power Meter 2). A 20-dB attenuator is added before the power meter to meet its measurement range. The maximum output power of EDFA is 500 mW, corresponding to an optical intensity of ~6.3 MW/cm 2 in optical fiber, which is much smaller than the saturation intensity of the SAs. Therefore this experiment investigates the linear optical transmission of the SAs. If the SAs can maintain a stable material properties under different incident power, a linear relationship between input power and output power is expected. The experimental results are shown in Figs. 10(b)-10(e). It can be observed that WS 2 -PVA SA has the best linearity, meaning a stable optical transmission in the whole range of input power. However, change of the linearity can be clearly observed for MoS 2 -PVA, MoSe 2 -PVA and WSe 2 -PVA SAs. Especially, MoSe 2 -PVA SA significantly changes its transmission near the input power of 337.8 mW, meaning the worst stability among the samples. These results are consistent with the discussion about thermal stability of 4 TMDs materials. Actually, TMDs-PVA polymer films were sandwiched between fiber connectors and illuminated by lasers perpendicularly in our scheme. This required high thermal stability for our home-made films and became a challenge for the saturable absorbers under high pump power. Some other schemes may help to overcome this problem. For example, TMDs can be deposited on the surface of a tapered fiber or side-polished fiber to construct saturable absorber which interacts with the evanescent field of light beam [53][54][55].
Moreover, Table 2 may provide a reference for us to fabricate TMDs-based Q-switched fiber laser to meet special requirements. For example, to obtain high-power and stable Qswitched pulses, WS2 is a good choice. To get wide pump tuning range or wide repetition rate tuning rage of Q-switched pulses, WeS 2 seems better. MoSe 2 may be a good candidate to mode-lock a fiber laser as long as solving the problem of thermal stability and would become a next research hotspot.
Our work demonstrates the feasibility of four typical TMDs to Q-switch a ring-cavity fiber laser. The mode locking operations based on MoS 2 and WS 2 have been reported [37,38,40,42]. The non-saturable absorbance of our saturable absorbers is slightly high. The values of non-saturable absorbance for MoS 2 , WS 2 and WSe 2 are around 60%. High non-saturable absorbance can cause laser to become less efficient and increase the tendency for Q-switched instabilities [56]. Therefore, Q-switching operation is more favorable than mode-locking operation in the laser cavity based on our TMDs saturable absorbers. Although MoSe 2 has a smaller non-saturable absorbance of 39.2%, the weak thermal stability of this material leads to the noisy Q-switched operation. An improvement on the material preparation may help to reduce the non-saturable absorbance and increase the possibility of obtaining mode-locking operation. Other meaningful nonlinear optical phenomenon such as four wave mixing [57], which has been generated with graphene, is also expected to be observed with these TMDs materials [58,59].

Conclusions
We have fabricated 4 typical TMDs-PVA polymer saturable absorber (MoS 2 , MoSe 2 , WS 2 , and WSe2) and utilize them to Q-switch the same ring-cavity erbium-doped fiber laser. The saturable absorption of these SAs is characterized and MoSe 2 exhibits the highest modulation depth. Q-switching operations of a same erbium-doped fiber laser based on these SAs are achieved and compared in the aspects of RF spectrum, optical spectrum, repetition rate and pulse duration. WS 2 Q-switched fiber laser outputs the most stable pulse trains with the cleanest optical spectrum and highest extinction ratio in the RF spectrum. These results demonstrate the feasibility of TMDs to Q-switch fiber laser effectively and provide a meaningful reference for further research in nonlinear fiber optics with these TMDs materials.