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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
Corrections
7 October 2015: A correction was made to the corresponding author designation.
1. Introduction
The discovery of graphene in 2004 [1] has opened up a door to a novel world of 2-dimensional materials. Since then, many researchers have investigated all kinds of optical and electrical characteristics of graphene such as saturable absorption, four wave mixing, etc [2–7]. Many novel photoelectric devices have been invented based on graphene [8, 9]. The prevalence of graphene has also guided researchers in exploring more analogous 2-dimensional materials, among which transition metal dichalcogenides (TMDs) are well-known. Typical TMDs include molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2) and tungsten diselenide (WSe2). 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 [10–12]. 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 [19–22]. Q-switching operation can be classified into 2 categories: active Q-switching and passive Q-switching. The former usually utilizes an acousto-optic or electro-optic 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) [24, 25], carbon nanotubes (CNTs) [26, 27], graphene [28] and others. Some new saturable absorbers such as topological insulator [29, 30] and black phosphorus [31] can also be used in fiber lasers. Recently, lots of researches with TMDs to generate Q-switched laser pulses have been reported. R. I. Woodward et al. reported a tunable ytterbium-doped Q-switched fiber laser based on few layer MoS2, tunable from 1030nm to 1070nm [32]. Besides MoS2, their team has also researched Q-switched MoSe2 fiber lasers with Yb-, Er-, and Tm-doped fiber [33]. Z. Luo et al. used a broadband few-layer MoS2 saturable absorber to Q-switch 1, 1.5 and 2 μs fiber laser [34]. Sahar Hosseinzadeh Kassani et al. reported a WS2 based Q-switched laser with tunable repetition rates from 82 kHz to 134 kHz [35]. Besides Q-switching operation, some researchers have achieved mode-locking operation with TMDs. H. Zhang et al. reported a MoS2 based mode-locked fiber laser, which could generate pulses centered at 1054.3 nm with bandwidth of 2.7 nm and pulse duration of 800ps [36]. H. Liu et al. used MoS2 to generate 710fs mode-locked pulses in an erbium-doped fiber laser [37]. Reza Khazaeinezhad et al. reported a mode-locked fiber laser based on WS2 [38]. Peiguang Yan et al. reported a 675 fs WS2 based mode-locked fiber of which the signal-to-noise ratio is 65dB [39]. Other works related to mode-locked fiber laser based on WS2 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 MoS2, MoSe2, WS2, and WSe2. 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 erbium-doped 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.
2. 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 MoS2 water dispersions are prepared with sodium cholate (SC) as surfactant. The detailed preparation of MoS2 dispersions can be referred to Ref [42]. Meanwhile, 50mg/ml polyvinyl alcohol (PVA) aqueous solution is also prepared. Then we mix 2ml MoS2 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.
Fig. 1 (a) Fabrication procedures of TMDs-PVA films, taking MoS2 for example. (b)~(e) show the photos of fabricated TMDs-PVA polymer films. Insets: SEM images of film edges with x1500 magnification.
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 E12g mode is related to an in-plane motion of TMD molecular and A1g 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. B2g 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 MoS2, A1g mode is observed at 407.2 cm−1 and E12g mode is observed at 381.3 cm−1. For MoSe2, A1g, E12g and B2g modes are observed at 239.6 cm−1, 283.9 cm−1 and 354.0 cm−1, respectively. For WS2, A1g mode is found at 419.5 cm−1 and E12g mode is found at 355.4 cm−1. For WSe2, A1g, E12g and B2g 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.
Fig. 2 (a)-(d)Raman spectroscopy and (e)-(h) Transmission Electron Microscopy photos of four TMDs materials.
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.
Fig. 3 (a) Setup of saturable absorption experiment and (b)~(e) results of 4 different TMDs materials.
The results of saturable absorption experiments of 4 types of materials are shown in Figs. 3(b)-3(e). Fitting with the formula [31]:
where T is transmittance, ΔT is modulation depth, I is intensity of laser, Isat is saturation power intensity and Ans is non-saturable absorbance. The modulation depths of MoS2, MoSe2, WS2, WSe2 are 2.15%, 6.73%, 2.53% and 3.02% respectively. The corresponding saturation intensities are 129.4, 132.5, 148.2 and 270.4MW/cm2, respectively. And the non-saturable absorbances of 4 TMDs saturable absorbers are 63.1%, 39.2%, 58.3% and 61.5%, respectively. It can be observed that MoSe2 exhibits the highest modulation depth and smallest non-saturable absorbance among these materials. By contrast, MoS2-PVA polymer film has a relatively poor modulation depth and a quite high non-saturable absorbance. WSe2-PVA polymer film might suffer from two-photon absorption (TPA) under high incident optical power since its transmittance drops when the intensity of input light exceeds around 550 MW/cm2 [36]. TPA is a process where two photons are absorbed simultaneously by the material. The strength of TPA is proportional to the optical intensity and therefore TPA is usually observed under high input optical intensity. TPA has been reported in TMDs materials by a few works [16, 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 WSe2 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 cm2/MW.3. 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 erbium-doped 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.
Fig. 4 The setup of ring cavity Q-switched fiber laser. LD: laser diode; WDM: wavelength division multiplexer; SMF: single mode fiber; EDF: Erbium-doped fiber; PC: polarization controller; TMDs-PVA SA: transition metal dichalcogenides- polyvinyl alcohol saturable absorber; PII: polarization independent isolator.
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 MoS2 Q-switching operation starts when pump power is higher than 50mW, MoSe2 Q-switching operation starts when pump power is higher than 570mW, WS2 starts Q-switching as the pump power is higher than 400mW, WSe2 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.
Fig. 5 The output power variation with pump power within Q-switching range, (a)~(d) delegates the results of MoS2, MoSe2, WS2 and WSe2, respectively.
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. MoSe2 and WS2 Q-switching operation needs high pump power, WS2 has the highest output power under identical pump condition compared to other three materials. Furthermore, WSe2 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.
Figure 6 gathers output pulses of Q-switched fiber lasers based on 4 TMDs-PVA polymer saturable absorbers. Q-switched pulses have considerable wide pulse duration [18]. In our experiment, MoS2 Q-switched fiber laser generate 12.9 μs pulses when it’s pumped at 100mW. At 650mW pump power, MoSe2 and WS2 Q-switched fiber lasers output pulses with width 4.2 and 4.1 μs, respectively. WSe2 Q-switched fiber laser generates 4.8 μs pulses under 500 mW pump power.
Fig. 6 (a) Pulse train with MoS2-PVA SA under 100 mW pump. (b)Pulse train with MoSe2 under 650 mW pump power. (c) Pulse train with WS2 under 650 mW pump power. (d) Pulse train with WSe2 under 500 mW pump power. Insets are corresponding single pulse.
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 WS2 Q-switched fiber laser works with a highest stability, the extinction ratio of RF signal can reach up to 54.2dB. MoSe2 Q-switched fiber laser is least stable as its extinction ratio is only 31.3dB. MoS2 and WSe2 Q-switched fiber lasers have a moderate performance as the extinction ratios are 48.5 dB and 41.9 dB respectively.
Fig. 7 RF spectra of Q-switched pulses, insets show spectrum at fundamental frequency. (a)~(d) shows the results of MoS2, MoSe2, WS2 and WSe2, 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 MoS2 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 MoSe2 and WSe2 show some superposition of continuous wave while WS2 has a smoother optical spectrum. As a result, the Q-switching operation of WS2 is more stable and effective than other saturable absorbers.
Fig. 8 Optical spectra of Q-switched pulses. (a)~(d) show the measurement results of MoS2, MoSe2, WS2 and WSe2, respectively.
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 WS2 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 MoS2-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.
Fig. 9 Variation of repetition rate and pulse duration with pump power. (a)~(d) show the measurement results of MoS2, MoSe2, WS2 and WSe2, respectively.
4. 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.
Table 1. Bandgap values of four TMDs materials.
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 MoS2 increases with the decrease of MoS2 platelet size [50]. Similar analysis has also been performed in [32]. Defects in the materials can also create defect states and result in sub-bandgap absorption. Wang et al. have demonstrated the reduction of the bandgap of MoS2 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 MoSe2, WS2 and WSe2 have similar structure and material properties as MoS2, 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, MoSe2 has the highest modulation depth and the least non-saturable absorbance, but the extinction ratio in RF spectrum of MoSe2 based laser is low which means suffering much noise during Q-switching operation. WS2 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. MoS2 and WSe2 based lasers have more stable Q-switching operation than MoSe2 based laser, but are not as good as WS2 based laser.
Table 2. Comparison of four TMDs-polymer PVA saturable absorbers
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−1K−1 for MoS2, 0.85 Wm−1K−1 for MoSe2, 2.2 Wm−1K−1 for WS2 and 0.9 Wm−1K−1 for WSe2, respectively [52]. WS2 has the highest thermal conductivity, meaning a best thermal stability among four materials. This is consistent with the experimental results that WS2-based laser has the best noise properties, shown in Table 2. It can also be noted that although MoSe2 has the lowest non-saturable loss, MoSe2 based laser does not has the best performance due to the low thermal conductivity of MoSe2. Besides, since the maximum single pulse energy of femtosecond laser is 0.54 nJ, while in the Q-switched 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/cm2 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 WS2-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 MoS2-PVA, MoSe2-PVA and WSe2-PVA SAs. Especially, MoSe2-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.
Fig. 10 (a) Experimental setup for comparing the stability of the linear optical transmission of 4 TMDs materials under high input power and (b)-(e) measurement results of 4 TMDs materials. CW: continuous wave; EDFA: erbium doped fiber amplifier; SA: saturable absorber; Att.: attenuator.
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 [53–55].
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 Q-switched pulses, WS2 is a good choice. To get wide pump tuning range or wide repetition rate tuning rage of Q-switched pulses, WeS2 seems better. MoSe2 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 MoS2 and WS2 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 MoS2, WS2 and WSe2 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 MoSe2 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].
5. Conclusions
We have fabricated 4 typical TMDs-PVA polymer saturable absorber (MoS2, MoSe2, WS2, 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 MoSe2 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. WS2 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.
Acknowledgments
This work was partially supported by Shanghai Yangfan Program (No. 14YF1401600), the State Key Lab Project of Shanghai Jiao Tong University (No. GKZD030033), NSFC (No. 61178007, No. 51302285), STCSM (Nano Project No. 11nm0502400, the External Cooperation Program of BIC, CAS (No. 181231KYSB20130007). J. W. thanks the National 10000-Talent Program and CAS 100-Talent Program for financial support.
References and links
1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef] [PubMed]
2. A. Martinez and S. Yamashita, “10 GHz fundamental mode fiber laser using a graphene saturable absorber,” Appl. Phys. Lett. 101(4), 041118 (2012). [CrossRef]
3. Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
4. Y. Wu, B. Yao, Y. Cheng, Y. Rao, Y. Gong, X. Zhou, B. Wu, and K. S. Chiang, “Four-wave mixing in a microfiber attached onto a graphene film,” IEEE Photonics Technol. Lett. 26(3), 249–252 (2014). [CrossRef]
5. J. L. Xu, X. L. Li, Y. Z. Wu, X. P. Hao, J. L. He, and K. J. Yang, “Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser,” Opt. Lett. 36(10), 1948–1950 (2011). [CrossRef] [PubMed]
6. B. V. Cunning, C. L. Brown, and D. Kielpinski, “Low-loss flake-graphene saturable absorber mirror for laser mode-locking at sub-200-fs pulse duration,” Appl. Phys. Lett. 99(26), 261109 (2011). [CrossRef]
7. Y. Feng, N. Dong, Y. Li, X. Zhang, C. Chang, S. Zhang, and J. Wang, “Host matrix effect on the near infrared saturation performance of graphene absorbers,” Opt. Mater. Express 5(4), 802–808 (2015). [CrossRef]
8. P. Blake, P. D. Brimicombe, R. R. Nair, T. J. Booth, D. Jiang, F. Schedin, L. A. Ponomarenko, S. V. Morozov, H. F. Gleeson, E. W. Hill, A. K. Geim, and K. S. Novoselov, “Graphene-based liquid crystal device,” Nano Lett. 8(6), 1704–1708 (2008). [CrossRef] [PubMed]
9. F. Xia, T. Mueller, Y. M. Lin, A. Valdes-Garcia, and P. Avouris, “Ultrafast graphene photodetector,” Nat. Nanotechnol. 4(12), 839–843 (2009). [CrossRef] [PubMed]
10. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist, and V. Nicolosi, “Two-dimensional nanosheets produced by liquid exfoliation of layered materials,” Science 331(6017), 568–571 (2011). [CrossRef] [PubMed]
11. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). [CrossRef] [PubMed]
12. E. S. Reifler, N. T. Nuhfer, and E. Towe, “Structure and optical properties of some layered two-dimensional transition-metal dichalcogenides: molybdenum disulfide, molybdenum diselenide, and tungsten diselenide,” Microsc. Microanal. 20(S3), 1752–1753 (2014). [CrossRef]
13. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]
14. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nat. Nanotechnol. 7(11), 699–712 (2012). [CrossRef] [PubMed]
15. D. Liu, S. Fu, M. Tang, P. Shum, and D. Liu, “Comb filter-based fiber-optic methane sensor system with mitigation of cross gas sensitivity,” J. Lightwave Technol. 30(19), 3103–3109 (2012). [CrossRef]
16. K. Wang, J. Wang, J. Fan, M. Lotya, A. O’Neill, D. Fox, Y. Feng, X. Zhang, B. Jiang, Q. Zhao, H. Zhang, J. N. Coleman, L. Zhang, and W. J. Blau, “Ultrafast saturable absorption of two-dimensional MoS2 nanosheets,” ACS Nano 7(10), 9260–9267 (2013). [CrossRef] [PubMed]
17. X. Zhang, S. Zhang, C. Chang, Y. Feng, Y. Li, N. Dong, K. Wang, L. Zhang, W. J. Blau, and J. Wang, “Facile fabrication of wafer-scale MoS2 neat films with enhanced third-order nonlinear optical performance,” Nanoscale 7(7), 2978–2986 (2015). [CrossRef] [PubMed]
18. B. Hitz, “Photonics research: short Q-switched fiber laser generates millijoules in nanoseconds,” Photon. Spectra 41, 94–95 (2007).
19. U. Sharma, C. Kim, J. U. Kang, and N. M. Fried, “Highly stable tunable dual-wavelength Q-switched fiber laser for DIAL applications,” in Laser Applications to Chemical and Environmental Analysis(Optical Society of America, 2004), p. B3.
20. M. Skorczakowski, J. Swiderski, W. Pichola, P. Nyga, A. Zajac, M. Maciejewska, L. Galecki, J. Kasprzak, S. Gross, A. Heinrich, and T. Bragagna, “Mid-infrared Q-switched Er:YAG laser for medical applications,” Laser Phys. Lett. 7(7), 498–504 (2010). [CrossRef]
21. X. T. Xu, J. P. Zhai, J. S. Wang, Y. P. Chen, Y. Q. Yu, M. Zhang, I. L. Li, S. C. Ruan, and Z. K. Tang, “Passively Q-switching induced by the smallest single-walled carbon nanotubes,” Appl. Phys. Lett. 104(17), 171107 (2014). [CrossRef]
22. M. Xiang, S. Fu, M. Tang, H. Tang, P. Shum, and D. Liu, “Nyquist WDM superchannel using offset-16QAM and receiver-side digital spectral shaping,” Opt. Express 22(14), 17448–17457 (2014). [CrossRef] [PubMed]
23. J. J. Zayhowski and C. Dill III, “Coupled-cavity electro-optically Q-switched Nd:YVO4 microchip lasers,” Opt. Lett. 20(7), 716–718 (1995). [CrossRef] [PubMed]
24. R. Paschotta, R. Häring, E. Gini, H. Melchior, U. Keller, H. L. Offerhaus, and D. J. Richardson, “Passively Q-switched 0.1-mJ fiber laser system at 1.53 µm,” Opt. Lett. 24(6), 388–390 (1999). [CrossRef] [PubMed]
25. C. Gao, W. Zhao, Y. Wang, S. Zhu, G. Chen, and Y. Wang, “Passive Q-switched fiber laser with SESAM in ytterbium-doped double-clad fiber,” in 27th International congress on High-Speed Photography and Photonics(International Society for Optics and Photonics, 2007), p. 62794G. [CrossRef]
26. K. Jiang, S. Fu, P. Shum, and C. Lin, “A wavelength-switchable passively harmonically mode-locked fiber laser with low pumping threshold using single-walled carbon nanotubes,” IEEE Photonics Technol. Lett. 22(11), 754–756 (2010). [CrossRef]
27. J. Li, Y. Song, Z. Yu, C. Tian, Y. Li, and Y. Wang, “Passively Q-switched Nd: YCOB laser with a single-walled carbon nanotube saturable absorber,” in Photonics Asia(International Society for Optics and Photonics, 2012), p. 855119.
28. J. L. Xu, X. L. Li, J. L. He, X. P. Hao, Y. Yang, Y. Z. Wu, S. D. Liu, and B. T. Zhang, “Efficient graphene Q switching and mode locking of 1.34 μm neodymium lasers,” Opt. Lett. 37(13), 2652–2654 (2012). [CrossRef] [PubMed]
29. Z. Luo, C. Liu, Y. Huang, D. Wu, J. Wu, H. Xu, Z. Cai, Z. Lin, L. Sun, and J. Weng, “Topological-insulator passively Q-switched double-clad fiber laser at 2 m wavelength,” IEEE J Sel. Top. Quantum Electron. 20, 1–8 (2014). [CrossRef]
30. P. Yan, R. Lin, S. Ruan, A. Liu, H. Chen, Y. Zheng, S. Chen, C. Guo, and J. Hu, “A practical topological insulator saturable absorber for mode-locked fiber laser,” Sci. Rep. 5, 8690 (2015).
31. Y. Chen, G. Jiang, S. Chen, Z. Guo, X. Yu, C. Zhao, H. Zhang, Q. Bao, S. Wen, D. Tang, and D. Fan, “Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation,” Opt. Express 23(10), 12823–12833 (2015). [CrossRef] [PubMed]
32. R. I. Woodward, E. J. R. Kelleher, R. C. T. Howe, G. Hu, F. Torrisi, T. Hasan, S. V. Popov, and J. R. Taylor, “Tunable Q-switched fiber laser based on saturable edge-state absorption in few-layer molybdenum disulfide (MoS₂),” Opt. Express 22(25), 31113–31122 (2014). [CrossRef] [PubMed]
33. R. I. Woodward, R. Howe, T. H. Runcorn, G. Hu, F. Torrisi, E. Kelleher, and T. Hasan, “Wideband saturable absorption in few-layer molybdenum diselenide (MoSe2) for Q-switching Yb-, Er-and Tm-doped fiber lasers,” arXiv:1503.08003 (2015).
34. Z. Luo, Y. Huang, M. Zhong, Y. Li, J. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, and J. Weng, “1-, 1.5-, and 2-μm fiber lasers Q-switched by a broadband few-layer MoS2 saturable absorber,” J. Lightwave Technol. 32(24), 4679–4686 (2014). [CrossRef]
35. S. H. Kassani, R. Khazaeinezhad, H. Jeong, T. Nazari, D.-I. Yeom, and K. Oh, “All-fiber Er-doped Q-switched laser based on tungsten disulfide saturable absorber,” Opt. Mater. Express 5(2), 373 (2015). [CrossRef]
36. H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P. Loh, “Molybdenum disulfide (MoS₂) as a broadband saturable absorber for ultra-fast photonics,” Opt. Express 22(6), 7249–7260 (2014). [CrossRef] [PubMed]
37. H. Liu, A. P. Luo, F. Z. Wang, R. Tang, M. Liu, Z. C. Luo, W. C. Xu, C. J. Zhao, and H. Zhang, “Femtosecond pulse erbium-doped fiber laser by a few-layer MoS2 saturable absorber,” Opt. Lett. 39(15), 4591–4594 (2014). [CrossRef] [PubMed]
38. R. Khazaeinezhad, S. Hosseinzadeh Kassani, H. Jeong, D. Yeom, and K. Oh, “Passively Mode-locked fiber laser based on CVD WS2,” in CLEO:2015(Optical Society of America, San Jose, California, 2015), pp. W2A–W74A.
39. P. Yan, A. Liu, Y. Chen, H. Chen, S. Ruan, C. Guo, S. Chen, I. L. Li, H. Yang, J. Hu, and G. Cao, “Microfiber-based WS_2-film saturable absorber for ultra-fast photonics,” Opt. Mater. Express 5(3), 479 (2015). [CrossRef]
40. D. Mao, Y. Wang, C. Ma, L. Han, B. Jiang, X. Gan, S. Hua, W. Zhang, T. Mei, and J. Zhao, “WS2 mode-locked ultrafast fiber laser,” SCI REP-UK 5 (2015). [CrossRef]
41. P. Yan, A. Liu, Y. Chen, H. Chen, S. Ruan, C. Guo, S. Chen, I. L. Li, H. Yang, J. Hu, and G. Cao, “Microfiber-based WS2-film saturable absorber for ultra-fast photonics,” Opt. Mater. Express 5(3), 479–489 (2015). [CrossRef]
42. K. Wu, X. Zhang, J. Wang, X. Li, and J. Chen, “WS₂ as a saturable absorber for ultrafast photonic applications of mode-locked and Q-switched lasers,” Opt. Express 23(9), 11453–11461 (2015). [CrossRef] [PubMed]
43. P. Tonndorf, R. Schmidt, P. Böttger, X. Zhang, J. Börner, A. Liebig, M. Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn, S. Michaelis de Vasconcellos, and R. Bratschitsch, “Photoluminescence emission and Raman response of monolayer MoS₂, MoSe₂, and WSe₂,” Opt. Express 21(4), 4908–4916 (2013). [CrossRef] [PubMed]
44. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, and D. Baillargeat, “From bulk to monolayer MoS2: evolution of Raman scattering,” Adv. Funct. Mater. 22(7), 1385–1390 (2012). [CrossRef]
45. H. R. Gutiérrez, N. Perea-López, A. L. Elías, A. Berkdemir, B. Wang, R. Lv, F. López-Urías, V. H. Crespi, H. Terrones, and M. Terrones, “Extraordinary room-temperature photoluminescence in triangular WS2 monolayers,” Nano Lett. 13(8), 3447–3454 (2013). [CrossRef] [PubMed]
46. S. Zhang, N. Dong, N. McEvoy, M. O’Brien, S. Winters, N. C. Berner, C. Yim, Y. Li, X. Zhang, Z. Chen, L. Zhang, G. S. Duesberg, and J. Wang, “Direct observation of degenerate two-photon absorption and its saturation in WS2 and MoS2 monolayer and few-layer films,” ACS Nano 9(7), 7142–7150 (2015). [CrossRef] [PubMed]
47. H. Byun, M. Y. Sander, A. Motamedi, H. Shen, G. S. Petrich, L. A. Kolodziejski, E. P. Ippen, and F. X. Kärtner, “Compact, stable 1 GHz femtosecond Er-doped fiber lasers,” Appl. Opt. 49(29), 5577–5582 (2010). [CrossRef] [PubMed]
48. O. R. Wood and S. E. Schwarz, “Passive Q-switching of a CO2 laser,” Appl. Phys. Lett. 11(3), 88–89 (1967). [CrossRef]
49. G. J. Spühler, R. Paschotta, R. Fluck, B. Braun, M. Moser, G. Zhang, E. Gini, and U. Keller, “Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers,” J. Opt. Soc. Am. B. 16, 376–388 (1999).
50. C. B. Roxlo, R. R. Chianelli, H. W. Deckman, A. F. Ruppert, and P. P. Wong, “Bulk and surface optical absorption in molybdenum disulfide,” J. Vac. Sci. Technol. A 5(4), 555–557 (1987). [CrossRef]
51. S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, and J. Wang, “Broadband few-layer MoS2 saturable absorbers,” Adv. Mater. 26(21), 3538–3544 (2014). [CrossRef] [PubMed]
52. J. Kim, S. Choi, W. Seo, and W. Cho, “Thermal and electronic properties of exfoliated metal chalcogenides,” Bull. Korean Chem. Soc. 31(11), 3225–3227 (2010). [CrossRef]
53. M. Jung, J. Koo, J. Park, Y. W. Song, Y. M. Jhon, K. Lee, S. Lee, and J. H. Lee, “Mode-locked pulse generation from an all-fiberized, Tm-Ho-codoped fiber laser incorporating a graphene oxide-deposited side-polished fiber,” Opt. Express 21(17), 20062–20072 (2013). [CrossRef] [PubMed]
54. T. Chen, C. Liao, D. N. Wang, and Y. Wang, “Passively mode-locked fiber laser by using monolayer chemical vapor deposition of graphene on D-shaped fiber,” Appl. Opt. 53(13), 2828–2832 (2014). [CrossRef] [PubMed]
55. L. Gao, W. Huang, J. D. Zhang, T. Zhu, H. Zhang, C. J. Zhao, W. Zhang, and H. Zhang, “Q-switched mode-locked erbium-doped fiber laser based on topological insulator Bi2Se3 deposited fiber taper,” Appl. Opt. 53(23), 5117–5122 (2014). [CrossRef] [PubMed]
56. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek, and J. A. D. Au, “Semiconductor saturable absorber mirrors (SESAM’s) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE J Sel. Top. Quantum Electron. 2(3), 435–453 (1996). [CrossRef]
57. T. Gu, N. Petrone, J. F. McMillan, A. van der Zande, M. Yu, G. Lo, D. Kwong, J. Hone, and C. W. Wong, “Regenerative oscillation and four-wave mixing in graphene optoelectronics,” Nat. Photonics 6(8), 554–559 (2012). [CrossRef]
58. K. Wang, J. Wang, J. Fan, M. Lotya, A. O’Neill, D. Fox, Y. Feng, X. Zhang, B. Jiang, Q. Zhao, H. Zhang, J. N. Coleman, L. Zhang, and W. J. Blau, “Ultrafast saturable absorption of two-dimensional MoS2 nanosheets,” ACS Nano 7(10), 9260–9267 (2013). [CrossRef] [PubMed]
59. K. Wang, Y. Feng, C. Chang, J. Zhan, C. Wang, Q. Zhao, J. N. Coleman, L. Zhang, W. J. Blau, and J. Wang, “Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors,” Nanoscale 6(18), 10530–10535 (2014). [CrossRef] [PubMed]
