Open Access
Add to BibSonomy
Abstract
In this paper, we first investigated the nonlinear optical properties of Cu1.8S nanocrystals (NCs) by using Z-scan and balanced twin detector measurement technologies, and then demonstrated a passively Q-switched laser at the 2 µm wavelength. The Cu1.8S NCs were synthesized by a solventless thermolysis method and then mixed with sodium carboxymethylcellulose to form the Cu1.8S NCs film. The film exhibited broadband surface plasmon resonance (SPR) absorption from 800 nm to 2000 nm. By incorporating the Cu1.8S NCs film into a thulium doped fiber laser cavity pumped by a 1570 nm fiber laser, stable passive Q-switching at ∼1975.16 nm was obtained for a threshold pump power of ∼931 mW, and ∼3.4 µs pulse duration with a pulse repetition rate of ∼35.14 kHz was also obtained for a pump power of ∼996 mW. Our results showed that Cu1.8S NCs is a promising saturable absorber (SA) for pulse laser generation at the 2 µm wavelength.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, Cu2-xS nanocrystals (NCs) have been successfully used in bioimaging, sensing, catalysis, surface-enhanced Raman scattering, and photonic devices due to the presence of collective oscillations of electrons effects, which was called by surface plasmon resonance (SPR) [1–9]. The SPR frequency defines the operating wavelength range of photonic devices and systems, so several approaches have been developed to tune the SPR frequency in order to realize various photo-electronics applications at different wavelength [10–15]. In comparison with metal NCs, the SPR frequency of Cu2-xS NCs could be tuned in a wide spectral range through doping concentration, temperature, and phase transitions as their size and morphology were unchanged [16–21]. In addition, the Cu2-xS NCs also showed weak photo-thermal effect compared to metal NCs. These results showed that the Cu2-xS NCs are good candidate for nonlinear optical materials. Despite their success in nonlinear optical fields, one of the nonlinear optical properties of Cu2-xS NCs previously considered but now recognized as potentially useful is the saturable absorption. Saturable absorber (SA) is a key device in pulsed laser, which was based on saturable absorption property. SA induces a high optical absorption for low optical intensities and a low optical absorption for high optical intensities. In order to investigate the potential of Cu2-xS NCs as SA for pulse generation, several works have been done recently. In 2016, Qiu et al. first reported wideband (1 µm, 1.56 µm and 2.8 µm) optical modulation for ultrashort pulse generation enabled by Cu2-xS NCs as SAs [20]. Very recently, our group demonstrated Q-switched erbium-doped fiber lasers operating at 1.56 µm by using Cu1.8S NCs as SA [21]. Despite recent progress in this field, the broadband saturable absorption characteristics of Cu2-xS NCs have not yet been fully exploited for constructing pulsed lasers with a broadband operating wavelength. Especially, pulse laser generation at 2 µm wavelength by using a Cu2-xS NCs as SA has not yet been demonstrated, to the best of our knowledge. However, 2 µm wavelength pulse lasers have wide applications including biomedical diagnostics, surgery, molecular spectroscopy, remote sensing and free-space communication, due to the broad gain spectrum of thulium doped fiber from 1.8 to 2.1 µm, possibility to produce ultrashort pulses in the “eye-safe” wavelength range [22–27]. Hence, it is necessary to check whether the Cu2-xS NCs SA can be used to induce 2 µm pulse laser operation or not.
In this paper, we first investigated the nonlinear optical properties of Cu1.8S NCs by using Z-scan and balanced twin detectors measurement technologies, and then demonstrated a passively Q-switched laser at 2 µm wavelength. The Cu1.8S NCs were synthesized by a solventless thermolysis method, and then mixed with sodium carboxymethylcellulose (NaCMC) to form the Cu1.8S NCs film. The Cu1.8S NCs film exhibited broadband SPR absorption from 800 nm to 2000 nm. By inserting the Cu1.8S NCs film into an thulium doped fiber laser (TDFL) cavity pumped by a 1570 nm fiber laser, stable passive Q-switching at ∼1975.16 nm was obtained for a threshold pump power of ∼931 mW, and ∼3.4 µs pulse duration with a pulse repetition rate of ∼35.14 kHz were obtained for a pump power of ∼996 mW. The maximum average output power is about 1.97 mW and the corresponding pulse energy is about 0.06 µJ. Our results showed that Cu1.8S NCs is a promising SA for pulse laser generation at 2 µm wavelength.
2. Synthesis and characterization of Cu1.8S NCs
Cu1.8S NCs used in our experiment were prepared by using the hot-injection method as reported previously [21,28,29]. Firstly, Copper precursor was prepared by mixing 0.01 mol CuCl with a mixture of 4 mL oleic acid (OA) and 5 mL oleylamine (OAm) at 130°C under continuous stirring. Then, the as-prepared copper precursor was cooled to 25 °C. Secondly, sulfur precursor was prepared in a three necked flask by dissolving 0.01 mol sulfur powder into 40 mL Octadecene (ODE) at 200 °C under continuous stirring. Subsequently, the temperature of sulfur solution was set to 180 °C, followed by a swift injection of copper precursor. Cu1.8S NCs were obtained under the above conditions. The obtained products were washed with cyclohexane and ethanol after cooling down to room temperature naturally. Then centrifuged under the 5000 rpm for 10 min. Finally, the obtained samples were dried at 70 °C for 24 h in a drying oven. Figure 1(a) shows the transmission electron microscopy (TEM) images of Cu1.8S NCs, the NCs were sphere-like with an average size of ∼10 nm. The NCs were all monodispersed and homogeneous. The inset in Fig. 1(a) shows the photograph of Cu1.8S NCs solution, the color of the solution is gray-green. X-ray diffraction (XRD) analysis was used to identify the crystal structure of the as-synthesized Cu1.8S NCs. Four main peaks at 2θ of 27.78°, 32.16°, 46.18°and 54.7° were observed in the XRD pattern (black solid line) as shown in Fig. 1(b). All the diffraction peaks given in Fig. 1(b) are in accordance with the rhombohedral CuS (JCPDS No. 47-1748, red curve).
Fig. 1. (a)TEM image and (b) XRD diffraction pattern of Cu1.8S NCs. The inset in Fig. 1(a): Photograph of the aqueous solution of Cu1.8S NCs.
In order to identify the elements compositions of the NCs, we use an energy-dispersive X-ray spectrograph (EDX) to test the elements ratio of Cu and S in our materials as shown in Fig. 2. Figure 2(a) and (b) confirms the uniform distribution of S an Cu elements over the NCs, and the atomic ratios of Cu to S were determined to be 1.82 as shown in Fig. 2(c). Hence, the Cu2-xS NCs used in our experiment was Cu1.8S NCs.
We also measured the absorption characteristics of the Cu1.8S NCs powder (black solid curve) and Cu1.8S NCs film (red solid curve) by using an ultraviolet (UV)-visible-near infrared (NIR) spectrophotometer (UV-3600, Shimadzu) as shown in Fig. 3. The Cu1.8S NCs powder were obtained by dispersing the Cu1.8S NCs into cyclohexane and depositing them on a silica glass, then drying at room temperature for 24 hours. The Cu1.8S NCs powder only have a weak absorption peak at 1218 nm, as shown in Fig. 3 (black line). In order to investigate the absorption characteristic at 2 µm, the absorption spectrum of Cu1.8S NCs was measured with a filmy sample to avoid the strong absorption between 1.4 µm and 2.1 µm of liquid water. The Cu1.8S NCs film was formed by mixed Cu1.8S NCs with 3.2 wt.% aqueous solution of NaCMC through ultrasonication process firstly and casting the solution onto a flat substrate, then followed by a slow drying at room temperature. In comparison with the Cu1.8S NCs powder, the Cu1.8S-NaCMC film (red line) has a broad absorption band from 800 nm to more than 2000 nm due to the aggregation of the Cu1.8S NCs in the film [21].
High optical nonlinearity is indispensable to enable efficient optical modulation. In order to investigate the nonlinear optical properties of Cu1.8S NCs, the open Z-scan measurement technology was performed firstly, the experiment setup was shown in Fig. 4(a) [30]. The Cu1.8S NCs sample was irradiated by a 1930nm mode-locked pulse laser (AdValue Photonics AP-ML1, pulse duration: 2.8 ps, repetition rate: 32.3 MHz). The incident beam was divided into two laser beams. One laser beam as the reference beam was monitored with photo-detector 1, while the other laser beam was focused by a CaF2 lens (focal length: 150 mm), which perpendicularly to the sample and further monitored with photo-detector 2. During the experiment, the sample was put on a motorized translation stage and was irradiated by the focused beam. The optical powers of reference and transmission beam were collected using the detectors at the same time. Figure 4(b) shows the open-aperture Z-scan measurement trace of Cu1.8S NCs at 1930nm. The characteristic of the upward peak at the focus manifesting saturable absorption of Cu1.8S NCs. The Z-scan trace can be fit by the following equation:
In Eq. (1), q0=βI0Leff, where β is the nonlinear absorption coefficient, I0 means the optical intensity at the focus. Leff=(1-e-αL) is the sample’s effective length, L is the sample length and α is the linear absorption coefficientis. z0 =kω20/λ is the diffraction length of the beam, and the waist radius ω0 of the focused beam is 35 µm measured by the knife edge method. By fitting the curve, we can obtain the value of the nonlinear absorption coefficient is about -1.99*10−8 m/W. In addition, saturable absorption is also a very important property for realized pulse laser generation. To investigate the saturable absorption property of the Cu1.8S NCs film, we used a balanced twin detectors measurement technology as shown in Fig. 4(c). We measured the dependence of the transmission ratio on the pump peak intensity of the ∼400 fs pulse laser at ∼1980nm. By fitting the data shown in Fig. 4(d) with the equation α (I) = αs /(1 + I/Is) + αns (where α (I) is the absorption coefficient, αs and αns are the saturable and nonsaturable absorption components, and I and Is are input and saturation intensities, respectively), the modulation depth, non-bleachable loss and saturation intensity were determined to be ∼11.9%, ∼15.9%, ∼98 MW/cm2, respectively, which clearly characterizes the saturable absorption feature. The above results showed that the Cu1.8S NCs film could be used as SAs for constructing pulsed lasers.Fig. 4. The nonlinear optical properties of the prepared Cu1.8S NCs film were investigated by an open-aperture Z-scan technique and balanced twin detector measurement technology, respectively. (a) Experiment setup of open aperture Z-scan measurements of Cu1.8S NCs sample at 1930nm. (b) The open-aperture Z-scan measurement of Cu1.8S NCs. (c) Setup of open saturable absorption characteristics measurements of Cu1.8S NCs sample. (d) Saturable absorption of Cu1.8S NCs.
3. Experimental details and results
In order to see whether the Cu1.8S NCs film as SA can be used to induce pulse laser operation or not, we integrated the fiber-compatible Cu1.8S NCs SA into a TDFL cavity. The fiber-compatible Cu1.8S NCs SA was prepared by FC/PC connection with the film deposited at the tip of the fiber. The experimental setup of the Cu1.8S NCs film based TDFL was shown in Fig. 5. The pump light at 1570 nm was inject into the laser cavity via a 1570/2000nm wavelength division multiplexer (WDM). A 20 cm long TDF (Nurfern, SM-TSF-5/125, dispersion: -12 ps2/km at 1.9 µm) was used as the gain medium and the other fibers were ∼7.6 m standard single mode fiber (Coring, SMF-28, dispersion: -67 ps2/km at 1.9 µm). The total net cavity dispersion was estimated to be -0.51 ps2. A polarization controller was used to adjust the cavity birefringence. An isolator was incorporated to guarantee the single-direction operation and a 10 dB output coupler was used to extracted 10% intra-cavity power for output. The optical spectrum was measured by an optical spectrum analyzer (Yokogawa, AQ 6375). The pulse train and single pulse duration were measured by a 12.5 GHz photodetector (Newport, 818-BB-51F) and an oscilloscope (Tektronix, DPO70604C).
Under a pump power of 931 mW, which corresponds to the Q-switched threshold of the current laser cavity, the operation of stable Q-switched pulse was obtained. The relatively high Q-switched threshold was caused by the high insertion loss of the SA and the fusion loss of the SMF-28 and gain fiber. The aggregated of the Cu1.8S nanoparticles in the film may cause the high insertion loss of the SA. In the future, we will decrease the insertion loss of the SA through varying the concentration of the nanoparticles. The laser output properties at a pump power of 996 mW were summarized in Fig. 6. Figure 6(a) shows the emission spectrum of the Q-switched laser, the 3 dB spectral bandwidth was measured to be 6.3 nm (with central wavelength located at 1975.16 nm). Its long range stability had been characterized in Fig. 6(b). The interval between two adjacent pulses was 28.45 µs and the corresponding repetition rate was 35.14 kHz. The single pulse duration of Q-switched laser is 3.4 µs, as shown in Fig. 6(c). Figure 6(d) shows the dependence of pulse width and the repetition rate on the pump power. By increase the pump power from 931 to 996 mW, the pulse width decrease from 4.32 to 3.4 µs and the repetition rate increase from 31.73 to 35.14 kHz. The above results presents a typical feature of passively Q-switched lasers [31–34]. The Q-switched fiber laser exhibited an excellent stability at room temperature. In addition, in order to verify the effects of Cu1.8S NCs on Q-switching, the Cu1.8S NCs film inside the laser cavity was replaced by a bare NaCMC film. By increasing the pump power gradually, the output laser always works in a continuous wave regime. These results confirmed that the above Q-switched laser was induced by the Cu1.8S NCs SA.
Fig. 6. (a) Emission spectrum, (b) pulse train, (c) single pulse duration, and (d) the dependence of pulse width and repetition rate on pump power of Q-switched fiber laser at 1975.16 nm.
The stable Q-switched pulses were first investigated as the pump power increased to 931 mW. Figure 7 depicts the average output power versus pump power of the Q-switched fiber laser. It is obvious that the output power and the pump power have a linear correlation. With an increase in the pump power from 931 to 996 mW, the average output power increased from 0.95 to 1.97 mW, and the corresponding slope efficiency was about 1.04%. The maximum pulse energy is about 0.06 µJ. On further increasing the pump power to over 996 mW, Q-switched laser became unstable, which may caused by the film damaged of the photothermal effect. To solve this problem, one effective way is to increase the interaction area of SA and input light by using the evanescent effect or choosing the appropriate film-forming agent [35–39]. At present, we just realized Q-switched laser at 2 µm wavelength based on the Cu1.8S NCs SA. In the future, we will try to realize mode-locking operation by using Cu1.8S NCs as SA.
In addition, long-term stability of the Q-switched lasers is a key element for practical applications. In order to confirm the long-term stability of Cu1.8S NCs as SA for Q-switched laser operation, we measured the central wavelength and the output power of the Q-switched laser every 10 min for 2 hours as shown in Fig. 8. Neither the central wavelength nor the output power was changed, indicated that it has good long-term stability.
4. Conclusion
In conclusion, we have successfully prepared and investigated the nonlinear optical properties of Cu1.8S NCs by using Z-scan and balanced twin detectors measurement technologies, and also employed the Cu1.8S NCs film as a SA for Q-switched laser generation at 2 µm wavelength experimentally. Stable passively Q-switching laser at 1975.16 nm was obtained for a threshold pump power of ∼931 mW, ∼3.4 µs pulse duration with a pulse repetition rate of ∼35.14 kHz were obtained for a pump power of ∼996 mW. The maximum average output power is about 1.97 mW and the corresponding pulse energy is about 0.06 µJ. Our results showed that Cu1.8S NCs is a promising SA for pulse laser generation at 2 µm wavelength.
Funding
National Natural Science Foundation of China (61605219, 61378004, 61527823, 61605058, 11774132, 11474132); Opened Fund of the State Key Laboratory on Integrated Optoelectronics and Tsinghua National Laboratory for Information Science and Technology Cross-discipline Foundation; Key Technology Research and Development Project of Jilin Province (20180201120GX); Major Science and Technology Tendering Project of Jilin Province (20170203012GX); Joint Foundation from Equipment Pre-research and Ministry of Education (6141A02022413); Youth Innovation Promotion Association of the Chinese Academy of Sciences.
References
1. Y. Wu, C. Wadia, W. Ma, B. Sadtler, and A. P. Alivisatos, “Synthesis and photovoltaic application of Copper (I) sulfide nanocrystals,” Nano Lett. 8(8), 2551–2555 (2008). [CrossRef]
2. Y. X. Zhao, H. C. Pan, Y. B. Lou, X. F. Qiu, J. J. Zhu, and C. Burda, “Plasmonic Cu2-xS nanocrystals: optical and structural properties of copper-deficient copper (I) sulfides,” J. Am. Chem. Soc. 131(12), 4253–4261 (2009). [CrossRef]
3. S. H. Wang, A. Riedinger, H. B. Li, C. H. Fu, H. Y. Liu, L. L. Li, T. L. Liu, L. F. Tan, M. J. Barthel, G. Pugliese, F. D. Donato, M. S. D’Abbusco, X. W. Meng, L. Manna, H. Meng, and T. Pellegrino, “Plasmonic copper sulfide nanocrystals exhibiting near-infrared photothermal and photodynamic therapeutic effects,” ACS Nano 9(2), 1788–1800 (2015). [CrossRef]
4. I. Kriege, C. Y. Jiang, J. Rodriguez-Fernandez, R. D. Schaller, D. V. Talapin, E. Como, and J. Feldmann, “Tuning the excitonic and plasmonic properties of copper chalcogenide nanocrystals,” J. Am. Chem. Soc. 134(3), 1583–1590 (2012). [CrossRef]
5. T. X. Wei, Y. F. Liu, W. J. Dong, Y. Zhang, C. Y. Huang, Y. Sun, X. Chen, and N. Dai, “Surface-dependent localized surface plasmon resonances in CuS nanodisks,” ACS Appl. Mater. Interfaces 5(21), 10473–10477 (2013). [CrossRef]
6. C. Zhang, S. Z. Jiang, Y. Y. Huo, A. H. Liu, S. C. Xu, X. Y. Liu, Z. C. Sun, Y. Y. Xu, Z. Li, and B. Y. Man, “SERS detection of R6G based on a novel graphene oxide/silver nanoparticles/silicon pyramid arrays structure,” Opt. Express 23(19), 24811–24821 (2015). [CrossRef]
7. J. Yu, Y. Guo, H. J. Wang, S. Su, C. Zhang, B. Y. Man, and F. C. Lei, “Quasi optical cavity of hierarchical ZnO nanosheets@Ag nanoravines with synergy of near- and far-field effects for in situ Raman detection,” J. Phys. Chem. Lett. 10(13), 3676–3680 (2019). [CrossRef]
8. C. Zhang, C. H. Li, J. Yu, S. Z. Jiang, S. C. Xu, C. Yang, Y. J. Liu, X. G. Gao, A. H. Liu, and B. Y. Man, “SERS activated platform with three-dimensional hot spots and tunable nanometer gap,” Sens. Actuators, B 258, 163–171 (2018). [CrossRef]
9. J. H. Xu, C. H. Li, H. P. Si, X. F. Zhao, L. Wang, S. Z. Jiang, D. M. Wei, J. Yu, X. W. Xiu, and C. Zhang, “3D SERS substrate based on Au-Ag bi-metal nanoparticles/MoS2 hybrid with pyramid structure,” Opt. Express 26(17), 21546–21557 (2018). [CrossRef]
10. L. Brus, “Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule raman spectroscopy,” Acc. Chem. Res. 41(12), 1742–1749 (2008). [CrossRef]
11. Y. Z. Chu, E. Schonbrun, T. Yang, and K. B. Crozier, “Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays,” Appl. Phys. Lett. 93(18), 181108 (2008). [CrossRef]
12. S. Link and M. A. El-Sayed, “Size and temperature dependence of the plasmin absorption of colloidal gold nanoparticles,” J. Phys. Chem. B 103(21), 4212–4217 (1999). [CrossRef]
13. S. K. Ghosh and T. Pal, “Interparticle coupling effect on the surface plasmon resonance of gold nanoparticles: from theory to applications,” Chem. Rev. 107(11), 4797–4862 (2007). [CrossRef]
14. D. E. Gomez, K. C. Vernon, P. Mulvaney, and T. J. Davis, “Surface plasmon mediated strong exciton-photon coupling in semiconductor nanocrystals,” Nano Lett. 10(1), 274–278 (2010). [CrossRef]
15. J. E. Millstone, S. Park, K. L. Shuford, L. D. Qin, G. C. Schatz, and C. A. Mirkin, “Observation of a Quadrupole plasmon mode for a colloidal solution of gold nanoprisms,” J. Am. Chem. Soc. 127(15), 5312–5313 (2005). [CrossRef]
16. Y. Xie, A. Riedinger, M. Prato, A. Casu, A. Genovese, P. Guardia, S. Sottini, C. Sangregorio, K. Miszta, S. Ghosh, T. Pellegrino, and L. Manna, “Copper sulfide nanocrystals with tunable composition by reduction of covellite nanocrystals with Cu+ ions,” J. Am. Chem. Soc. 135(46), 17630–17637 (2013). [CrossRef]
17. X. L. Wang and M. T. Swihart, “Controlling the size, shape, phase, band gap, and localized surface plasmon resonance of Cu2-xS and CuxInyS nanocrystals,” Chem. Mater. 27(5), 1786–1791 (2015). [CrossRef]
18. F. Wang, Q. Li, L. Lin, H. Peng, Z. Liu, and D. Xu, “Monodisperse copper chalcogenide nanocrystals: controllable synthesis and the pinning of plasmonic resonance absorption,” J. Am. Chem. Soc. 137(37), 12006–12012 (2015). [CrossRef]
19. A. Comin and L. Manna, “New materials for tunable plasmonic colloidal nanocrystals,” Chem. Soc. Rev. 43(11), 3957–3975 (2014). [CrossRef]
20. Q. B. Guo, Y. H. Yao, Z. C. Luo, Z. P. Qin, G. Q. Xie, M. Liu, J. Kang, S. Zhang, G. Bi, X. F. Liu, and J. R. Qiu, “Universal near-infrared and mid-infrared optical modulation for ultrafast pulse generation enabled by colloidal plasmonic semiconductor nanocrystals,” ACS Nano 10(10), 9463–9469 (2016). [CrossRef]
21. M. Y. Liu, D. L. Zhou, Z. X. Jia, Z. R. Li, N. Li, S. Q. Li, Z. Kang, J. Yi, C. J. Zhao, G. S. Qin, H. W. Song, and W. P. Qin, “Plasmonic Cu1.8S nanocrystals as saturable absorbers for passively Q-switched erbium-doped fiber lasers,” J. Mater. Chem. C 5(16), 4034–4039 (2017). [CrossRef]
22. J. Ma, G. Q. Xie, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene mode-locked femtosecond laser at 2 µm wavelength,” Opt. Lett. 37(11), 2085–2087 (2012). [CrossRef]
23. G. Q. Xie, J. Ma, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene saturable absorber for Q-switching and mode locking at 2 µm wavelength,” Opt. Mater. Express 2(6), 878–883 (2012). [CrossRef]
24. Y. Wang, S. Alam, E. D. Obraztsova, A. S. Pozharov, S. Y. Set, and S. Yamashita, “Generation of stretched pulses and dissipative solitons at 2 µm from an all-fiber mode-locked laser using carbon nanotube saturable absorbers,” Opt. Lett. 41(16), 3864–3867 (2016). [CrossRef]
25. Z. Q. Luo, Y. Z. Huang, M. Zhong, Y. Y. Li, J. Y. Wu, B. Xu, H. Y. Xu, Z. P. 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]
26. Z. Kang, M. Y. Liu, X. J. Gao, N. Li, S. Y. Yin, G. S. Qin, and W. P. Qin, “Mode-locked thulium-doped fiber laser at 1982nm by using a gold nanorods saturable absorber,” Laser Phys. Lett. 12(4), 045105 (2015). [CrossRef]
27. X. H. Wang, J. L. Xu, S. F. Gao, Y. Y. Liu, Z. Y. You, and C. Y. Tu, “A 2 micron passively Q-switched bulk state pulsed laser based on WS2,” RSC Adv. 7(75), 47565–47569 (2017). [CrossRef]
28. B. X. Li, Y. Xie, and Y. Xue, “Controllable synthesis of CuS nanostructures from self-assembled precursors with biomolecule assistance,” J. Phys. Chem. C 111(33), 12181–12187 (2007). [CrossRef]
29. L. Liu, H. Z. Zhong, Z. L. Bai, T. Zhang, W. P. Fu, L. J. Shi, H. Y. Xie, L. G. Deng, and B. S. Zou, “Controllable transformation from rhombohedral Cu1.8S nanocrystals to hexagonal CuS clusters: Phase- and composition-dependent plasmonic properties,” Chem. Mater. 25(23), 4828–4834 (2013). [CrossRef]
30. M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]
31. R. I. Woodward, R. C. T. Howe, T. H. Runcorn, G. Hu, F. Torrisi, E. J. R. Kelleher, and T. Hasan, “Wideband saturable absorption in few-layer molybdenum diselenide (MoSe2) for Q-switching Yb-, Er- and T,-doped fiber lasers,” Opt. Express 23(15), 20051–20061 (2015). [CrossRef]
32. J. Liu, J. Xu, and P. Wang, “Graphene-based passively Q-switched 2 µm thulium-doped fiber laser,” Opt. Commun. 285(24), 5319–5322 (2012). [CrossRef]
33. W. Shi, E. B. Petersen, D. T. Nguyen, Z. D. Yao, A. Chavez-Pirson, N. Peyghambarian, and J. R. Yu, “220 µJ monolithic single-frequency Q-switched fiber laser at 2 µm by using highly Tm-doped germinate fibers,” Opt. Lett. 36(18), 3575–3577 (2011). [CrossRef]
34. A. F. El-Sherif and T. A. King, “High-peak-power operation of a Q-switched Tm3+-doped silica fiber laser operating near 2 µm,” Opt. Lett. 28(1), 22–24 (2003). [CrossRef]
35. D. F. Fan, C. B. Mou, X. K. Bai, S. F. Wang, N. Chen, and X. L. Zeng, “Passively Q-switched erbium-doped fiber laser using evanescent field interaction with gold nanosphere based saturable absorber,” Opt. Express 22(15), 18537–18542 (2014). [CrossRef]
36. J. Du, Q. K. Wang, G. B. Jiang, C. W. Xu, C. J. Zhao, Y. J. Xiang, Y. Chen, S. C. Wen, and H. Zhang, “Ytterbium-doped fiber laser passively mode locked by few-layer molybdenum disulfide saturable absorber functioned with evanescent field interaction,” Sci. Rep. 4(1), 6346 (2015). [CrossRef]
37. M. Jung, J. Koo, Y. M. Chang, P. Debnath, Y. W. Song, and J. H. Lee, “An all fiberized, 1.89 µm Q-switched laser employing carbon nanotube evanescent field interaction,” Laser Phys. Lett. 9(9), 669–673 (2012). [CrossRef]
38. Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interaction,” Appl. Phys. Lett. 96(5), 051122 (2010). [CrossRef]
39. M. Zhang, G. H. Hu, G. Q. Hu, R. C. T. Howe, L. Chen, Z. Zheng, and T. Hasan, “Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber,” Sci. Rep. 5(1), 17482 (2015). [CrossRef]
