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Abstract
A saturable absorber (SA) based on molybdenum sulfoselenide (MoSSe) was fabricated using a combination of liquid exfoliation and laser deposition methods. A dual-wavelength passively Q-switched Er-doped fiber laser (EDFL) was fabricated by inserting the MoSSe-SA into a ring cavity. The maximum average output power was 23.4 mW. The pulse width decreased from 2.8 to 1.78 μs while the corresponding pulse repetition rate increased from 50 to 90 kHz. To the best of our knowledge, this is the first report on a passively Q-switched EDFL based on a MoSSe-SA operating at 1.5 μm. Our results indicate that MoSSe-like three-element transition-metal dichalcogenides could be a promising alternative optical modulators for pulsed fiber laser generation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the initial discovery of graphene, layered two-dimensional (2D) materials have been attracting increasing attention because of the many exotic properties that arise from the quantum confinement and surface effects in these materials. Among the available 2D materials, transition-metal dichalcogenides (TMDs), which are formed a sandwich structure with a transition metal layer (denoted by M; e.g., Mo, W, Re, Ti) between two chalcogen layers (denoted by X; e.g., S, Se, Te), have attracted considerable attention because of their excellent electrical and optical properties. TMDs have been widely applied in areas including field-effect transistors, valleytronics and photonic devices [1–6]. Breaking of the structural symmetry plays a crucial role in determining the electronic band structures of 2D TMDs, which are semiconductor materials with intrinsic in-plane asymmetries. It has been proposed theoretically that breaking of the out-of-plane mirror symmetry using external electric fields will have significant effects on the electronic and optical properties of the layered materials [7,8].
Recently, a new type of Janus-layered molybdenum sulfoselenide (MoSSe) composed of three layers of atoms (sulfur, molybdenum, and selenium, in order from top to bottom) was synthesized successfully by fully replacing one of the two Se (S) layers with S (Se) atoms within MoSe2 (MoS2) using a chemical vapor deposition method [9,10]. The electronic and optical properties of this material can be tuned effectively by controlling the alloy composition of the MoSxSe2−x. Additionally, structural defects such as point defects and grain boundaries are unavoidable during growth and exfoliation of the layered material formation and these defects can significantly affect the optical, electronic and mechanical properties of these ultrathin materials. The electronic and optical properties and tuning of the magnetism of MoSSe have been widely studied [11–15]. Due to the lack of the reflection symmetry, the MoSSe monolayer has large in-plane and vertical piezoelectric effect, making it a great material for nanoscale electronic and energy applications [16]. Moreover, owing to the difference in the electronegativity between Se and S atoms, the MoSSe monolayer has an intrinsic moment, resulting in the separation of holes and electrons causing enhanced photocatalytic activity [17]. The electronic and optical properties of the pristine MoSSe monolayer has recently been investigated by means of the first principles calculations based on the density functional theory (DFT) [18], which could bring exciting novel properties and great potential in nanoelectronic and optoelectronic devices. However, there are few reports on its saturable absorption properties.
In this paper, using a combination of liquid exfoliation and laser deposition methods, a high-quality saturable absorber (SA) based on MoSSe has been fabricated for the first time. By inserting a prepared MoSSe-SA into a ring cavity, we constructed the EDF-based Q-switched fiber lasers with dual synchronous wavelengths of 1532.2 and 1532.8 nm. The maximum output power and the shortest pulse width from these lasers were determined to be 23.4 mW and 1.78 μs, respectively. Our experimental results indicate that MoSSe-like three element TMDs could be promising alternative materials for optical modulators used in pulsed fiber laser generation.
2. Preparation and characterization of the MoSSe SA
The MoSSe-SA was fabricated using the combination of liquid exfoliation and laser deposition methods. First, the layered MoSSe material was prepared by the liquid exfoliation method. When compared with other synthesis methods, the liquid exfoliation method offers simple operation, low costs and good stability. A piece of crystal was clipped using tweezers and the ground into powder using a mortar. N-methylpyrrolidone (NMP) was used to dissolve this powder because of its outstanding organic solubility. The dispersion solution was then sonicated for 5 hours to dissolve the powder fully. After settling for 12 hours, the supernatant was collected and centrifuged at a rate of 4000 rpm for 20 min to remove the large sediment. Finally, the laser deposition method was applied to attach the resulting material to the end face of the fiber patch cord and the saturable absorber (SA) was then prepared successfully.
An atomic force microscope (AFM) was used to examine the surface morphology of the material and the results are as shown in Fig. 1
. The average thickness of the MoSSe sample was less than 9 nm, which was equivalent to approximately eight material layers (the thickness of single layer MoSSe is approximately 1.1 nm). Recently, Zhao et al. reported that the structural and electronic properties of MoSSe depend on the thicknesses of the layers. The MoSSe bandgap decreases monotonically because of the quantum confinement effect and the interactions between the interlayers. The corresponding bandgap for the tri-layer MoSSe is 0.743 eV. When the number of layers increases to 5–8 layers, the bandgap decreases to 0.69 eV, and the corresponding operating wavelength is 1797 nm [19]. When compared with MoSe2 (MoS2), the bandgap of the multilayer MoSSe structure decreases much more rapidly. These calculation results not only confirm the absorption properties of MoSSe at 1.55 μm (the MoSSe structure in our work contains approximately six to eight layers), but also show that MoSSe has strong potential for use as an SA in visible light and even in the near infrared band.A typical MoSSe Raman spectrum that was excited using a 633 nm laser source at room temperature is shown in Fig. 2(a)
. Three obvious peaks appeared at 190, 260, and 400 cm−1, which are characteristic of the Mo-S and Mo-Se bands. While Se substitution weakens the Mo-S bond strength in MoSSe, the Mo-S band is still in the dominant position [20]. We speculated that the difference may be the effects of the solution and the preparation methods used. Furthermore, to identify whether these materials synchronously contain both the S and Se elements, we measured the ratio of Se to S and the chemical states of Mo and S using energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The results of these measurements are shown in Fig. 2(b) and Fig. 3
Fig. 3 Energy-dispersive X-ray spectroscopy (EDS) image and corresponding S to Se atomic ratio of the MoSSe simple.
As shown in Fig. 2(b), the Mo 3d5/2 and 3d3/2 peaks were located at ~229.2 and ~232.3 eV, which correspond to the characteristics of the Mo4+ oxidation state in the hexagonal 2H phase and the higher state, respectively [21]. Additionally, the d electrons in the transition metals are involved in the bonding with the adsorbates, so we can modify the d-band electronic structure of molybdenum by varying the Se:S ratio to change the characteristics of MoSSe. The information from the EDS (shown in Fig. 3) not only proved that MoSSe contained the three elements simultaneously, but also showed that the ratio of Mo to S to Se was approximately 2.75:1:1.2.
A nonlinear polarization-rotating (NPR) ring mode-locking EDF laser with a width of 10 ps and a repetition rate of 13.1 MHz was used to measure the nonlinear absorption of the MoSSe-SAM at 1.55 μm. As shown in Fig. 4
, the modulation depth of the MoSSe-SA was calculated to be 0.64% based on the fitting curve, while the saturation intensity was determined to be ~2.7 MW/cm2. The measurement results indicated that MoSSe had the typical features of an SA.3. Experimental setup and results
A schematic diagram of the MoSSe-SA Q-switched fiber laser is shown in Fig. 5
Fig. 5 Configuration of pulsed EDFL based on MoSSe SA. WDM: wavelength division multiplexer; EDF: erbium-doped fiber; ISO: insensitive isolator; PC: polarization controller; OC: output coupler; SMF: single-mode fiber.
The details of the output characteristics are illustrated in Fig. 6
Fig. 6 (a) Increasing output power with various pump powers, where the red line is without the MoSSe SA and the black line is with the MoSSe SA. (b) Repetition rate and pulse duration versus various pump powers. (c) Calculated single pulse energy and peak power versus various pump powers. (d) Output optical spectrum.
A typical oscilloscope image of a Q-switched pulse train and the shortest pulse profile at the incident pump power of 350 mW are shown in Fig. 7(a) and (b)
, respectively. The pulse seemed to be asymmetrical, which may have been caused by the nonlinear absorption characteristics of MoSSe. The intracavity photon density was low when the Q-switched pulse began to build. At this time, the single photon took the lead, so the initial pulse rise time was slow. However, as the number of photons in the cavity began to accumulate, the internal energy increased. While the photon energy was less than the difference between two energy levels, the atoms were able to absorb two photons simultaneously to gather a specific amount of energy and complete a transmission, which corresponded to a sudden drop in transmittance. This macroscopic manifestation was the fast falling edge of the Q-pulse [23]. Additionally, the pulse interval became increasingly small, which meant that the repetition rate rose with increasing pump power. When the pump power reached 350 mW, the shortest pulse length of 1.78 μs was observed. The spectra are shown in Fig. 8Fig. 8 RF spectrum at pump power of 300 mW up to 700 kHz; the inset shows the spectrum up to 200 kHz.
Table 1. Typical Q-Switched Fiber Laser Characteristics in 1.5 µm Wavelength Band
4. Conclusion
In conclusion, a stable dual-wavelength passively Q-switched fiber laser based on a novel TMD SA (MoSSe) was demonstrated in this work. Stable Q-switched operation was achieved at the dual synchronous wavelengths of 1532.2 nm and 1532.8 nm. When the pump power was increased from 167 to 350 mW, the repetition rate also increased from 50 to 90 kHz, while the pulse duration decreased from 2.81 to 1.78 μs. The maximum energy and peak power of the resulting optical pulses were 257 nJ and 145 mW, respectively. Overall, the Q-switched operation shows good long-term stability, which demonstrates that MoSSe can be considered as a potential candidate SA material for pulsed fiber laser fabrication.
Funding
Key R&D program of Shandong province (No: 2017CXCC0808); Development Program for Public Welfare in Shandong (No: 2017GGX20134); Young Scholars Program of Shandong University (Grant No: 2017WLJH48).
References
1. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2.,” Nano Lett. 10(4), 1271–1275 (2010). [CrossRef] [PubMed]
2. Y. Yu, S.-Y. Huang, Y. Li, S. N. Steinmann, W. Yang, and L. Cao, “Layer-dependent electrocatalysis of MoS2 for hydrogen evolution,” Nano Lett. 14(2), 553–558 (2014). [CrossRef] [PubMed]
3. J. A. Wilson and A. D. Yoffe, “The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties,” Adv. Phys. 18(73), 193–335 (1969). [CrossRef]
4. 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]
5. S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, “Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.,” Nano Lett. 12(11), 5576–5580 (2012). [CrossRef] [PubMed]
6. M. W. Lin, C. Ling, Y. Zhang, H. J. Yoon, M. M.-C. Cheng, L. A. Agapito, N. Kioussis, N. Widjaja, and Z. Zhou, “Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors,” Nanotechnology 22(26), 265201 (2011). [CrossRef] [PubMed]
7. M. Y. Han, B. Özyilmaz, Y. Zhang, and P. Kim, “Energy band-gap engineering of graphene nanoribbons,” Phys. Rev. Lett. 98(20), 206805 (2007). [CrossRef] [PubMed]
8. R. Balog, B. Jørgensen, L. Nilsson, M. Andersen, E. Rienks, M. Bianchi, M. Fanetti, E. Laegsgaard, A. Baraldi, S. Lizzit, Z. Sljivancanin, F. Besenbacher, B. Hammer, T. G. Pedersen, P. Hofmann, and L. Hornekaer, “Bandgap opening in graphene induced by patterned hydrogen adsorption,” Nat. Mater. 9(4), 315–319 (2010). [CrossRef] [PubMed]
9. J. Mann, Q. Ma, P. M. Odenthal, M. Isarraraz, D. Le, E. Preciado, D. Barroso, K. Yamaguchi, G. von Son Palacio, A. Nguyen, T. Tran, M. Wurch, A. Nguyen, V. Klee, S. Bobek, D. Sun, T. F. Heinz, T. S. Rahman, R. Kawakami, and L. Bartels, “2-Dimensional transition metal dichalcogenides with tunable direct band gaps: MoS2 (1–x) Se2x monolayers,” Adv. Mater. 26(9), 1399–1404 (2014). [CrossRef] [PubMed]
10. A.-Y. Lu, H. Zhu, J. Xiao, C.-P. Chuu, Y. Han, M.-H. Chiu, C.-C. Cheng, C.-W. Yang, K.-H. Wei, Y. Yang, Y. Wang, D. Sokaras, D. Nordlund, P. Yang, D. A. Muller, M.-Y. Chou, X. Zhang, and L.-J. Li, “Janus monolayers of transition metal dichalcogenides,” Nat. Nanotechnol. 12(8), 744–749 (2017). [CrossRef] [PubMed]
11. K. D. Pham, N. N. Hieu, H. V. Phuc, B. D. Hoi, V. V. Ilysov, B. Amin, and C. V. Nguyen, “First principles study of the electronic properties and Schottky barrier in vertically stacked graphene on the Janus MoSeS under electric field,” Comput. Mater. Sci. 153, 438–444 (2018). [CrossRef]
12. Y. Wen, M. Xia, and S. Zhang, “Bandgap engineering of Janus MoSSe monolayer implemented by Se vacancy,” Comput. Mater. Sci. 152, 20–27 (2018). [CrossRef]
13. R. Peng, Y. Ma, S. Zhang, B. Huang, and Y. Dai, “Valley Polarization in Janus Single-Layer MoSSe via Magnetic Doping,” J. Phys. Chem. Lett. 9(13), 3612–3617 (2018). [CrossRef] [PubMed]
14. M. Wang, Y. Pang, D. Y. Liu, S. H. Zheng, and Q. L. Song, “Tuning magnetism by strain and external electric field in zigzag Janus MoSSe nanoribbons,” Comput. Mater. Sci. 146, 240–247 (2018). [CrossRef]
15. Z. Guan, S. Ni, and S. Hu, “Tunable Electronic and Optical Properties of Monolayer and Multilayer Janus MoSSe as a Photocatalyst for Solar Water Splitting: A First-Principles Study,” Phys Chem. C 122(11), 6209–6216 (2018). [CrossRef]
16. L. Dong, J. Lou, and V. B. Shenoy, “Large in-plane and vertical piezoelectricity in Janus transition metal dichalchogenides,” ACS Nano 11(8), 8242–8248 (2017). [CrossRef] [PubMed]
17. Y. Ji, M. Yang, H. Lin, T. Hou, L. Wang, Y. Li, and S.-T. Lee, “Janus Structures of Transition Metal Dichalcogenides as the Heterojunction Photocatalysts for Water Splitting,” PhysChemComm 122(5), 3123–3129 (2018).
18. F. Li, W. Wei, P. Zhao, B. Huang, and Y. Dai, “Electronic and Optical Properties of Pristine and Vertical and Lateral Heterostructures of Janus MoSSe and WSSe,” J. Phys. Chem. Lett. 8(23), 5959–5965 (2017). [CrossRef] [PubMed]
19. Z. Guan, S. Ni, and S. Hu, “Tunable Electronic and Optical Properties of Monolayer and Multilayer Janus MoSSe as a Photocatalyst for Solar Water Splitting: A First-Principles Study,” PhysChemComm 122(11), 6209–6216 (2018).
20. B. Konkena, J. Masa, W. Xia, M. Muhler, and W. Schuhmann, “MoSSe@ reduced graphene oxide nanocomposite heterostructures as efficient and stable electrocatalysts for the hydrogen evolution reaction,” Nano Energy 29, 46–53 (2016). [CrossRef]
21. Q. Gong, L. Cheng, C. Liu, M. Zhang, Q. Feng, H. Ye, M. Zeng, L. Xie, Z. Liu, and Y. Li, “Ultrathin MoS2 (1–x) Se2x Alloy Nanoflakes For Electrocatalytic Hydrogen Evolution Reaction,” ACS Cata 5(4), 2213–2219 (2015). [CrossRef]
22. G. Wang, S. Zhang, X. Zhang, L. Zhang, Y. Cheng, D. Fox, H. Zhang, J. N. Coleman, W. J. Blau, and J. Wang, “Tunable nonlinear refractive index of two-dimensional MoS2, WS2, and MoSe2 nanosheet dispersions,” Photon. Res. 3(2), A51–A55 (2015). [CrossRef]
23. R. Zhao, J. He, X. Su, Y. Wang, X. Sun, H. Nie, B. Zhang, and K. Yang, “Tunable High-Power Q-Switched Fiber Laser Based on BP-PVA Saturable Absorber,” IEEE J. Sel. Top. Quantum Electron. 24(3), 1–5 (2018). [CrossRef]
24. M. Zhang, G. Hu, G. 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] [PubMed]
25. L. Li, Y. Wang, Z. F. Wang, X. Wang, and G. Yang, “High energy Er-doped Q-switched fiber laser with WS2 saturable absorber,” Opt. Commun. 406, 80–84 (2018). [CrossRef]
26. N. Dong, Y. Li, S. Zhang, N. McEvoy, X. Zhang, Y. Cui, L. Zhang, G. S. Duesberg, and J. Wang, “Dispersion of nonlinear refractive index in layered WS2 and WSe2 semiconductor films induced by two-photon absorption,” Opt. Lett. 41(17), 3936–3939 (2016). [CrossRef] [PubMed]
27. B. Chen, X. Zhang, C. Guo, K. Wu, J. Chen, and J. Wang, “Tungsten diselenide Q-switched erbium-doped fiber laser,” Opt. Eng. 55(8), 081306 (2016). [CrossRef]
28. H. Ahmad, M. Suthaskumar, Z. C. Tiu, A. Zarei, and S. W. Harun, “Q-switched Erbium-doped fiber laser using MoSe2 as saturable absorber,” Opt. Laser Technol. 79, 20–23 (2016). [CrossRef]
29. D. Wu, Z. Cai, Y. Zhong, J. Peng, Y. Cheng, J. Weng, Z. Luo, and H. Xu, “Compact passive Q-switching Pr3+-doped ZBLAN fiber laser with black phosphorus-based saturable absorber,” IEEE J. Sel. Top. Quantum Electron. 23(1), 7–12 (2017). [CrossRef]
30. J. Ren, S. Wang, Z. Cheng, H. Yu, H. Zhang, Y. Chen, L. Mei, and P. Wang, “Passively Q-switched nanosecond erbium-doped fiber laser with MoS(2) saturable absorber,” Opt. Express 23(5), 5607–5613 (2015). [CrossRef] [PubMed]
31. Y. Huang, Z. Luo, Y. Li, M. Zhong, B. Xu, K. Che, H. Xu, Z. Cai, J. Peng, and J. Weng, “Widely-tunable, passively Q-switched erbium-doped fiber laser with few-layer MoS2 saturable absorber,” Opt. Express 22(21), 25258–25266 (2014). [CrossRef] [PubMed]
32. 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,” Light Technol. 32(24), 4679 (2014). [CrossRef]
33. Z.-C. Luo, F.-Z. Wang, H. Liu, M. Liu, R. Tang, A.-P. Luo, and W.-C. Xu, “Pulsed erbium-doped fiber laser by a few-layer molybdenum disulfide saturable absorber: from Q-switching to mode-locking,” Opt. Eng. 55(8), 081308 (2016). [CrossRef]
34. Z. Jun, O. Hao, Z. Xin, Y. Jie, R. Chen, Z. Tong, Y. Sui, L. Yu, X. Chen, and J. Tian, “Ultrafast saturable absorption of MoS 2 nanosheets under different pulse-width excitation conditions,” Opt. Lett. 43(2), 243–246 (2018).
35. J. Wang, C. Dou, L. Chen, H. Yan, L. Meng, J. Zhu, and Z. Wei, “High energy passively Q-switched Er-doped fiber laser based on Mo0.5W0.5S2 saturable absorber,” Opt. Mater. Express 8(2), 324–331 (2018). [CrossRef]
