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Q-Switched Erbium-doped Fiber Laser Based on Silicon Nanosheets as Saturable Absorber
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Q-Switched Erbium-doped Fiber Laser Based on Silicon Nanosheets as Saturable Absorber
T
Guowei Liu, Yudong Lyu, Zongwen Li, Tiange Wu, Junjie Yuan, Xifu Yue, Huanian Zhang, Fang Zhang*, Shenggui Fu* No.266, Xincun West Road, Zibo, China
A R T IC LE I N F O
ABS TRA CT
Keywords: silicon nanosheets saturable absorber passively Q-switched fiber laser
Silicon nanosheets-polyvinyl alcohol (Si-PVA) film was prepared and employed as a SA in obtaining an erbium-doped Q-switched laser. To the best of our knowledge, this is the first time that few-layer silicon nanosheets has been used as a SA to achieve a Q-switched erbium-doped fiber laser. We have demonstrated passively Q-switched operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs with a repetition rate of 58.7 KHz. The results indicate that silicene owns excellent nonlinear properties and extensively potential applications in ultrafast photonics.
1. Introduction Over the past few decades, two-dimensional (2D) materials have become an ubiquitous tool widely employed in various applications, including electronics, optoelectronics and materials engineering. Since 2004, graphene as the first typical 2D material was found to receive an explosion of research interest, and it has caused a revolution in materials research due primarily to its 2D nature and to the linear dispersion of its band structure near the Dirac point [1,2]. Soon after, the study of two-dimensional materials has sprung up in the fields of materials science and nanotechnology as the results of its extraordinary characteristics. Inspired by the field of graphene research, other two-dimensional nano-materials with similar layered structure characteristics have drawn the attention of many researchers, such as topological insulators (Tis) [3,4], carbon nanotubes [5], transition metal dichalcogenides (TMDCs, e.g., MoS2, WS2, and SnS2) [6–9], black phosphorus (BP) [10], antimonene [11], and silicon nanosheets [12]. Recently, most 2D materials have been widely employed as saturable absorbers (SAs) for demonstrating pulsed fiber lasers [13,14] on account of their excellent nonlinear absorption (NLA) properties [15,16], especially its saturable absorption properties. The research of two-dimensional materials had a rapid development [17,18] since the graphene SA was first exhibited in 2009[19]. So far, various SA based on 2D materials have been utilized in fiber lasers [20,21]. Since 2009, the mode-locking operation in the Er3+-doped fiber lasers have been achieved by using the 2D materials, including graphene [19], TIs, TMDCs, BP. For the TI SAs, the maximum repetition rate of 2.95 GHz [22]. For the TMDC SAs, the minimum pulse width of 67 fs [23]. For the BP SAs, minimum pulse width of 128 fs [24]. Until now, the saturable absorption property of silicene has been confirmed [12], but there is almost no research on silicene and fiber lasers, which deeply restricts its application in optics. Here, the high-quality silicon nanosheets were exfoliated by the liquid phase exfoliation (LPE) method, Si-PVA film was successfully prepared and employed as a SA in obtaining an Er-doped Q-switched laser. Passive Q-switching operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs under a repetition rate of 58.7 KHz was
⁎
Corresponding authors. E-mail addresses: [email protected] (F. Zhang), [email protected] (S. Fu).
https://doi.org/10.1016/j.ijleo.2019.163692 Received 17 September 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Fig. 1. Preparation process of the Si-PVA film-type SAs.
obtained. Our result indicated that silicene with an excellent nonlinear saturable absorption property will has extensively wide ultrafast photonics and optoelectronic applications. 2. Sample preparation and characterization Silicene has excellent saturable absorption property, which we have reported before [12]. And it has been applied to solid-state lasers [25]. In our experiment, the few-layer silicene nanosheets was produced by the liquid phase exfoliation (LPE) method, which has been widely used to produce high-quality 2D nanomaterials from layered bulk crystals [26,27]. The preparation process of the film-type Si-PVA SA was shown in Fig. 1. Firstly, the Si crystals with purity of 99.999% (5 mg) were ground in agate mortar for 2 h. Ultra-pure ethanol was selected as the organic solvent and a small amount of ethanol (60 mL) was added during the grinding process to prevent oxidation of the sample. The Si powders were dispersed in ethanol with sonication (40 KHz and 300 W) for 2 h for better stripping of silicene nanosheets. The prepared dispersion was settled in a room temperature environment for 2 days for separated out the large Si deposited particles. The sample concentration was estimated to be 100 mg l-1. The prepared silicene dispersion and 4 wt.% PVA solution were mixed at the volume ratio of 1: 2. In order to effectively obtained the Si-PVA dispersion solution, the mixture was placed in the ultrasonic cleaner for 2.5 h. Afterwards, an appropriate amount of the Si-PVA dispersion solution (1 mL) was spin-coated on the inner surface of the petri dish, and the petri dish was settled in atmosphere for 1 day. Finally, we got a thin Si-PVA film, and took an appropriate size film on the end surface of the fiber optic jumper for making SAs. The morphology of the Si crystal and as-prepared silicene nanosheets were analyzed by a field-emission scanning electron microscope (SEM, Hitachi S-4800). Fig. 2(a) shows the SEM image of the Si crystal. As is shown, Si has an obvious stratification at the edge of crystal, which indicates the interlayer binding force of the sample is weak. So, Si can be effectively exfoliated by LPE or mechanical exfoliation. The surface appearance of silicene sample obtained from the dispersion is shown in Fig. 2(b). The silicon nanoflakes has an obvious layered structure, which is significantly different from the Si crystal. The thickness of the silicon nanosheets was measured by an atomic force microscopy (AFM, Dimension Icon, Veeco Instruments Inc.). An appropriate amount of asprepared silicene dispersion was deposited on a sapphire substrate and dried for 4 h before AFM measurement. Fig. 2(c) and (d) display the AFM image of the as-prepared silicon nanosheets and corresponding heights of sample. The heights are in the range of 3.9∼13.7 nm, which corresponding layer number is about 13∼44 layers [28]. The bandgap of bulk silicon is 1.1 eV. As the thickness of silicon nanosheets continues to decrease, its band gap continues to increase. When the thickness is reduced from 13 to 4 nm, the bandgap increases from 1.6 to 2.6 eV [28]. However, the actual number of layers might be less due to the unevaporated ethanol. Raman scattering is very sensitive to the lattice structure and crystal symmetry of microcrystalline materials [29]. A Raman spectrometer (LabRAM HR800, HORIBA Ltd) was used to further characterize the structure and quality of samples. Raman spectra of crystal Si and silicon nanosheets are shown in Fig. 3. The Raman spectrum of bulk silicon crystal consisted of one sharp peak at 521 cm-1 originating from the microcrystalline [30]. The Raman spectrum of silicon nanoflakes has an intense peak at 512 cm-1 with widened full width at half maximum (FWHM) and a visible shift (9 cm-1) to lower frequency. The Raman shift of 9 cm-1 from 521 cm-1 to 512 cm-1 further indicates that the nanosheets were efficiently exfoliated from Si crystal [31,32]. In addition, there is no amorphous surface oxidation and amorphous Si in the samples due to no peaks were detected at 300-450 cm-1 and 480 cm-1, which indicated that the high quality of silicon nanosheets [28]. Using Horizon mid-band OPO laser with tuning range of 192∼2750 nm (Continuum Inc, America), we measured the nonlinear absorption properties of silicon nanosheets at 1550 nm wavelength, which were coated on the sapphire substrate. The excited pulse width was 6 ns and the repetition frequency were 10 Hz. The result was shown in Fig. 3(b). We fitted the curve by the two-level 2
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Fig. 2. (a) SEM image of Si crystal, (b) SEM image of the as-prepared few-layer silicon nanosheets, (c) AFM image of silicene nanoflakes, and (d) corresponding heights of sample.
Fig. 3. (a)Raman spectra of silicon crystal and the prepared silicene nanosheets, (b) Transmittance versus incident power intensity for Si nanosheets dispersion at 1550 nm.
model,
⎞ ⎛ αs T=1−⎜ + αns ⎟ ⎟ ⎜1 + I Isat ⎠ ⎝
(1)
where T is the transmittance, αs is the saturable loss, αns is the non-saturable loss, I is the input intensity, and Isat is the saturation intensity. The fitted values of the modulation depth of the SA is 20.1%, and the saturation strength is 5.78 MW/cm2. The modulation depth of the SA is an important indicator. The comparison of modulation depth for different 2D materials was listed in Table 1. The values of modulation for LPE-prepared Si-PVA in this letter is 20.1%, which is a very high value for the PVAbased nanomaterial SAs. The higher values of modulation depth for Si-PVA indicated that the absorber has a strong ability to modulate the pulse intensity, and it could be exploited as potential material for mode-locked laser.
3
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Table 1 Comparison of the Modulation depth for Different 2D Materials. Sample Bismuthene HfS2 MoTe2 MoTe2 Si-PVA
Fabrication
Wavelength
Modulation depth
Reference
Sonochemical exfoliation Liquid exfoliation MSD CVD LPE
1561 nm 1562 nm 1572.4 nm 1935 nm 1550 nm
5.6% 15.7% 25.5% 5.7% 20.1%
[33] [34] [35] [36] This letter
3. Experimental details Er-doped fiber laser based on Si-PVA is shown in Fig. 4. A 976 nm laser diode (LD) was used for pumping, which the maximum pump power is 400 mW. The pump light was coupled into the laser cavity via a 980/1550 nm wave division multiplexer (WDM). A piece of 0.4 m long erbium-doped fiber (EDF, Liekki Er110-4/125) with a group velocity dispersion (GVD) of 12 ps2/km at 1550 nm served as the laser gain medium. A polarization independent isolator (PI-ISO) was used to ensure that the unidirectional operation of the ring cavity, and two polarization controller (PC) were used for adjusting the polarization state of the cavity. Two intra-cavity PCs make it easier to tune the cavity birefringence. A 20:80 optical coupler (OC) was used to output the signal. Intra-cavity passive components are all made by single mode fibers (SMFs) and connected by SMFs with GVD of -22 ps2/km at 1550 nm. The total length of the ring cavity was about 11 m, and the net dispersion of the fiber laser was calculated to be -0.2284 ps2. 4. Experimental results and discussions In the experiment, as we enlarged the pump power, we monitored laser output using a combination of oscilloscope and photodetector. We found that a stable Q-switched pulse was established at the pump power of only 41.5 mW by adjusting the PCs in the cavity. When the incident pump power continuously increases, the Q-switching operation can still be maintained. The stable Qswitched phenomenon was observed with the maximum pump power of 164 mW. When the pump power reached 165 mW, we found that the Q-switch pulse began to become unstable. This kind of unstable pulse was observed in some other passively Q-switched fiber lasers [37,38]. In order to verify whether Si-PVA-based SA has thermal damage, we decreased the pump power. When the power is gradually reduced from 165 mW to 0, the stable Q-switching operation appeared again at the pump power from 41.5 to 164 mW, which indicated that the Si-PVA-based SA was intact. Therefore, we believe that the possible cause of unstable Q-switch operation is SA oversaturation. Fig. 5 shows some experimental results of the passively Q-switched fiber laser. As shown in Fig. 5(a), the stable trains under three different pump powers at 62, 102, and 164 mW were observed, under the fixed cavity polarization setting, the pulse repetition frequency increases with increasing pump power., while the pulse string still maintains uniform intensity distribution without obvious fluctuation. Fig. 5(b) shows the Q-switched output optical spectrum with the central wavelength of 1567.1 nm, and the 3 dB spectral bandwidth of ∼0.369 nm. Fig. 5(c) displays seven pulse envelopes, and the shortest pulse width of 2.32 μs with repetition rate of 58.7 KHz under an input pump power of 164 mW. These experimental results show a high passive Q-switching performance of the fiber laser feasible by Si-PVA-based SA. In Fig. 6, we summarized the relations between the repetition rate of the Q-switched pulses, pulse width of the Q-switched pulses, average output power, and peak pulse power with respect to different input pomp power. The pulse repetition rate and pulse width are shown in Fig. 6(a). As the pump power is gradually increased from 41.5 mW to 164 mW, pulse repetition frequency increases from 11.1 KHz to 58.7 KHz, and pulse width decreases from 5.47 μs to 2.32 μs. As expected, the pulse repetition frequency of a passive Qswitched laser is proportional to the pump power, and the pulse duration is inversely proportional to the pump power [39]. Fig. 6(b) shows the average output power and peak pulse power as a function of input pump power. When the incident pump power varied from 41.5 mW to 164 mW, the average output power increases linearly from 0.17 mW to 0.89 mW. With the increase of pump power,
Fig. 4. Experimental setup of Si-PVA EDF laser. 4
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Fig. 5. Experimental results of the Si-PVA-based Q-switched fiber laser. (a) Typical pulse trains under three different incident pump powers at 62, 102, and164 mW. (b) Output optical spectrum and (c) seven pulse envelopes.
Fig. 6. (a) Repetition rate and pulse width of the input laser corresponding to different pump powers. (b) Average power and peak pulse power of the input laser versus pump power.
the peak pulse power increases sharply to the vicinity of 6.3 mW and then tends to be gentle. Further possibilities of scaling of output power and shortening of laser pulses can be achieved by shortening the laser cavity length [40] or better preparation process of Si-SA. Moreover, the pulse duration could be further reduced by improving the modulation depth of the Si-SA, because pulse duration is expected to be inversely proportional to the modulation depth of the SA [41]. However, we all know that due to the thermal effect of laser operation on two-dimensional materials, the increase of power and decrease of pulse width are limited. A 60 minutes evolution process of the pulse is shown in Fig. 7, which indicates the stability of Q-switched laser. We think that the possible cause of this phenomenon is the saturation of SA.
5. Conclusion In summary, the high-quality silicene nanosheets were exfoliated by LPE method, Si-PVA film was successfully prepared and we demonstrate that a passively Q-switched all-fiber laser at 1567.1 nm by the Si-PVA-based SA experiment. According to previous reports, this is the first time that few layers silicon nanosheets has been used as a SA to achieve a Q-switched erbium-doped fiber laser. At a repetition rate of 58.7 KHz, a passive Q-switching operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs was obtained. Our result indicated that silicon nanosheets with an excellent nonlinear saturable absorption property will have good foreground in ultrafast photonics and optoelectronic applications. And it may provide a guidance for the application of silicon nanosheets in fiber lasers. 5
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Fig. 7. Long-term performance of Q-switching within 60 min.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 11704227, 61505109, 11704226), Natural Science Foundation of Shandong Province (ZR2016AB05, ZR2017MA051). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M. Katsnelson, I. Grigorieva, S. Dubonos, A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197. [3] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang, S.-C. Zhang, Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface, Nature physics 5 (2009) 438. [4] J.E. Moore, The birth of topological insulators, Nature 464 (2010) 194. [5] M.A. Solodyankin, E.D. Obraztsova, A.S. Lobach, A.I. Chernov, A.V. Tausenev, V.I. Konov, E.M. Dianov, Mode-locked 1.93 μm thulium fiber laser with a carbon nanotube absorber, Opt. let. 33 (2008) 1336–1338. [6] C. Ataca, H. Sahin, S.J.T. Ciraci, Stable, single-layer MX2 transition-metal oxides and dichalcogenides in a honeycomb-like structure, J. Phys. Chem.C 116 (2012) 8983–8999. [7] K. Niu, Q. Chen, R. Sun, B. Man, H. Zhang, Passively Q-switched erbium-doped fiber laser based on SnS2 saturable absorber, Opt. Mat. Express 7 (2017) 3934–3943. [8] J. Li, Y. Zhao, Q. Chen, K. Niu, R. Sun, H. Zhang, Passively mode-locked ytterbium-doped fiber laser based on SnS2 as saturable absorber, IEEE Photonics J. 9 (2017) 1–7. [9] K. Niu, R. Sun, Q. Chen, B. Man, H. Zhang, Passively mode-locked Er-doped fiber laser based on SnS2 nanosheets as a saturable absorber, Photonics Research 6 (2018) 72–76. [10] H.O. Churchill, P. Jarillo-Herrero, Two-dimensional crystals: Phosphorus joins the family, Nature nanotechnology 9 (2014) 330. [11] F. Zhang, M. Wang, Z. Wang, K. Han, X. Liu, X.J. Xu, Excellent nonlinear absorption properties of β-antimonene nanosheets, J. Mat. Chem. C 6 (2018) 2848–2853. [12] F. Zhang, M. Wang, Z. Wang, K. Han, X. Liu, X. Xu, Nonlinear absorption properties of silicene nanosheets, Nanotechnology 29 (2018) 225701. [13] H. Zhang, D. Tang, R. Knize, L. Zhao, Q. Bao, K.P. Loh, Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser, Applied Physics Letters 96 (2010) 111112. [14] Z. Qin, G. Xie, H. Zhang, C. Zhao, P. Yuan, S. Wen, L. Qian, Black phosphorus as saturable absorber for the Q-switched Er: ZBLAN fiber laser at 2.8 μm, Opt. Express 23 (2015) 24713–24718. [15] Z. Guo, H. Zhang, S. Lu, Z. Wang, S. Tang, J. Shao, Z. Sun, H. Xie, H. Wang, X.F. Yu, From black phosphorus to phosphorene: basic solvent exfoliation, evolution of Raman scattering, and applications to ultrafast photonics, Advanced Functional Materials 25 (2015) 6996–7002. [16] F. Zhang, S. Han, Y. Liu, Z. Wang, X. Xu, Dependence of the saturable absorption of graphene upon excitation photon energy, Applied Physics Letters 106 (2015) 091102. [17] Z. Sun, T. Hasan, A. Ferrari, Ultrafast lasers mode-locked by nanotubes and graphene, Physica E: Low-dimensional Systems and Nanostructures 44 (2012) 1082–1091.
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7
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Q-Switched Erbium-doped Fiber Laser Based on Silicon Nanosheets as Saturable Absorber
T
Guowei Liu, Yudong Lyu, Zongwen Li, Tiange Wu, Junjie Yuan, Xifu Yue, Huanian Zhang, Fang Zhang*, Shenggui Fu* No.266, Xincun West Road, Zibo, China
A R T IC LE I N F O
ABS TRA CT
Keywords: silicon nanosheets saturable absorber passively Q-switched fiber laser
Silicon nanosheets-polyvinyl alcohol (Si-PVA) film was prepared and employed as a SA in obtaining an erbium-doped Q-switched laser. To the best of our knowledge, this is the first time that few-layer silicon nanosheets has been used as a SA to achieve a Q-switched erbium-doped fiber laser. We have demonstrated passively Q-switched operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs with a repetition rate of 58.7 KHz. The results indicate that silicene owns excellent nonlinear properties and extensively potential applications in ultrafast photonics.
1. Introduction Over the past few decades, two-dimensional (2D) materials have become an ubiquitous tool widely employed in various applications, including electronics, optoelectronics and materials engineering. Since 2004, graphene as the first typical 2D material was found to receive an explosion of research interest, and it has caused a revolution in materials research due primarily to its 2D nature and to the linear dispersion of its band structure near the Dirac point [1,2]. Soon after, the study of two-dimensional materials has sprung up in the fields of materials science and nanotechnology as the results of its extraordinary characteristics. Inspired by the field of graphene research, other two-dimensional nano-materials with similar layered structure characteristics have drawn the attention of many researchers, such as topological insulators (Tis) [3,4], carbon nanotubes [5], transition metal dichalcogenides (TMDCs, e.g., MoS2, WS2, and SnS2) [6–9], black phosphorus (BP) [10], antimonene [11], and silicon nanosheets [12]. Recently, most 2D materials have been widely employed as saturable absorbers (SAs) for demonstrating pulsed fiber lasers [13,14] on account of their excellent nonlinear absorption (NLA) properties [15,16], especially its saturable absorption properties. The research of two-dimensional materials had a rapid development [17,18] since the graphene SA was first exhibited in 2009[19]. So far, various SA based on 2D materials have been utilized in fiber lasers [20,21]. Since 2009, the mode-locking operation in the Er3+-doped fiber lasers have been achieved by using the 2D materials, including graphene [19], TIs, TMDCs, BP. For the TI SAs, the maximum repetition rate of 2.95 GHz [22]. For the TMDC SAs, the minimum pulse width of 67 fs [23]. For the BP SAs, minimum pulse width of 128 fs [24]. Until now, the saturable absorption property of silicene has been confirmed [12], but there is almost no research on silicene and fiber lasers, which deeply restricts its application in optics. Here, the high-quality silicon nanosheets were exfoliated by the liquid phase exfoliation (LPE) method, Si-PVA film was successfully prepared and employed as a SA in obtaining an Er-doped Q-switched laser. Passive Q-switching operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs under a repetition rate of 58.7 KHz was
⁎
Corresponding authors. E-mail addresses: [email protected] (F. Zhang), [email protected] (S. Fu).
https://doi.org/10.1016/j.ijleo.2019.163692 Received 17 September 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 202 (2020) 163692
G. Liu, et al.
Fig. 1. Preparation process of the Si-PVA film-type SAs.
obtained. Our result indicated that silicene with an excellent nonlinear saturable absorption property will has extensively wide ultrafast photonics and optoelectronic applications. 2. Sample preparation and characterization Silicene has excellent saturable absorption property, which we have reported before [12]. And it has been applied to solid-state lasers [25]. In our experiment, the few-layer silicene nanosheets was produced by the liquid phase exfoliation (LPE) method, which has been widely used to produce high-quality 2D nanomaterials from layered bulk crystals [26,27]. The preparation process of the film-type Si-PVA SA was shown in Fig. 1. Firstly, the Si crystals with purity of 99.999% (5 mg) were ground in agate mortar for 2 h. Ultra-pure ethanol was selected as the organic solvent and a small amount of ethanol (60 mL) was added during the grinding process to prevent oxidation of the sample. The Si powders were dispersed in ethanol with sonication (40 KHz and 300 W) for 2 h for better stripping of silicene nanosheets. The prepared dispersion was settled in a room temperature environment for 2 days for separated out the large Si deposited particles. The sample concentration was estimated to be 100 mg l-1. The prepared silicene dispersion and 4 wt.% PVA solution were mixed at the volume ratio of 1: 2. In order to effectively obtained the Si-PVA dispersion solution, the mixture was placed in the ultrasonic cleaner for 2.5 h. Afterwards, an appropriate amount of the Si-PVA dispersion solution (1 mL) was spin-coated on the inner surface of the petri dish, and the petri dish was settled in atmosphere for 1 day. Finally, we got a thin Si-PVA film, and took an appropriate size film on the end surface of the fiber optic jumper for making SAs. The morphology of the Si crystal and as-prepared silicene nanosheets were analyzed by a field-emission scanning electron microscope (SEM, Hitachi S-4800). Fig. 2(a) shows the SEM image of the Si crystal. As is shown, Si has an obvious stratification at the edge of crystal, which indicates the interlayer binding force of the sample is weak. So, Si can be effectively exfoliated by LPE or mechanical exfoliation. The surface appearance of silicene sample obtained from the dispersion is shown in Fig. 2(b). The silicon nanoflakes has an obvious layered structure, which is significantly different from the Si crystal. The thickness of the silicon nanosheets was measured by an atomic force microscopy (AFM, Dimension Icon, Veeco Instruments Inc.). An appropriate amount of asprepared silicene dispersion was deposited on a sapphire substrate and dried for 4 h before AFM measurement. Fig. 2(c) and (d) display the AFM image of the as-prepared silicon nanosheets and corresponding heights of sample. The heights are in the range of 3.9∼13.7 nm, which corresponding layer number is about 13∼44 layers [28]. The bandgap of bulk silicon is 1.1 eV. As the thickness of silicon nanosheets continues to decrease, its band gap continues to increase. When the thickness is reduced from 13 to 4 nm, the bandgap increases from 1.6 to 2.6 eV [28]. However, the actual number of layers might be less due to the unevaporated ethanol. Raman scattering is very sensitive to the lattice structure and crystal symmetry of microcrystalline materials [29]. A Raman spectrometer (LabRAM HR800, HORIBA Ltd) was used to further characterize the structure and quality of samples. Raman spectra of crystal Si and silicon nanosheets are shown in Fig. 3. The Raman spectrum of bulk silicon crystal consisted of one sharp peak at 521 cm-1 originating from the microcrystalline [30]. The Raman spectrum of silicon nanoflakes has an intense peak at 512 cm-1 with widened full width at half maximum (FWHM) and a visible shift (9 cm-1) to lower frequency. The Raman shift of 9 cm-1 from 521 cm-1 to 512 cm-1 further indicates that the nanosheets were efficiently exfoliated from Si crystal [31,32]. In addition, there is no amorphous surface oxidation and amorphous Si in the samples due to no peaks were detected at 300-450 cm-1 and 480 cm-1, which indicated that the high quality of silicon nanosheets [28]. Using Horizon mid-band OPO laser with tuning range of 192∼2750 nm (Continuum Inc, America), we measured the nonlinear absorption properties of silicon nanosheets at 1550 nm wavelength, which were coated on the sapphire substrate. The excited pulse width was 6 ns and the repetition frequency were 10 Hz. The result was shown in Fig. 3(b). We fitted the curve by the two-level 2
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Fig. 2. (a) SEM image of Si crystal, (b) SEM image of the as-prepared few-layer silicon nanosheets, (c) AFM image of silicene nanoflakes, and (d) corresponding heights of sample.
Fig. 3. (a)Raman spectra of silicon crystal and the prepared silicene nanosheets, (b) Transmittance versus incident power intensity for Si nanosheets dispersion at 1550 nm.
model,
⎞ ⎛ αs T=1−⎜ + αns ⎟ ⎟ ⎜1 + I Isat ⎠ ⎝
(1)
where T is the transmittance, αs is the saturable loss, αns is the non-saturable loss, I is the input intensity, and Isat is the saturation intensity. The fitted values of the modulation depth of the SA is 20.1%, and the saturation strength is 5.78 MW/cm2. The modulation depth of the SA is an important indicator. The comparison of modulation depth for different 2D materials was listed in Table 1. The values of modulation for LPE-prepared Si-PVA in this letter is 20.1%, which is a very high value for the PVAbased nanomaterial SAs. The higher values of modulation depth for Si-PVA indicated that the absorber has a strong ability to modulate the pulse intensity, and it could be exploited as potential material for mode-locked laser.
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Table 1 Comparison of the Modulation depth for Different 2D Materials. Sample Bismuthene HfS2 MoTe2 MoTe2 Si-PVA
Fabrication
Wavelength
Modulation depth
Reference
Sonochemical exfoliation Liquid exfoliation MSD CVD LPE
1561 nm 1562 nm 1572.4 nm 1935 nm 1550 nm
5.6% 15.7% 25.5% 5.7% 20.1%
[33] [34] [35] [36] This letter
3. Experimental details Er-doped fiber laser based on Si-PVA is shown in Fig. 4. A 976 nm laser diode (LD) was used for pumping, which the maximum pump power is 400 mW. The pump light was coupled into the laser cavity via a 980/1550 nm wave division multiplexer (WDM). A piece of 0.4 m long erbium-doped fiber (EDF, Liekki Er110-4/125) with a group velocity dispersion (GVD) of 12 ps2/km at 1550 nm served as the laser gain medium. A polarization independent isolator (PI-ISO) was used to ensure that the unidirectional operation of the ring cavity, and two polarization controller (PC) were used for adjusting the polarization state of the cavity. Two intra-cavity PCs make it easier to tune the cavity birefringence. A 20:80 optical coupler (OC) was used to output the signal. Intra-cavity passive components are all made by single mode fibers (SMFs) and connected by SMFs with GVD of -22 ps2/km at 1550 nm. The total length of the ring cavity was about 11 m, and the net dispersion of the fiber laser was calculated to be -0.2284 ps2. 4. Experimental results and discussions In the experiment, as we enlarged the pump power, we monitored laser output using a combination of oscilloscope and photodetector. We found that a stable Q-switched pulse was established at the pump power of only 41.5 mW by adjusting the PCs in the cavity. When the incident pump power continuously increases, the Q-switching operation can still be maintained. The stable Qswitched phenomenon was observed with the maximum pump power of 164 mW. When the pump power reached 165 mW, we found that the Q-switch pulse began to become unstable. This kind of unstable pulse was observed in some other passively Q-switched fiber lasers [37,38]. In order to verify whether Si-PVA-based SA has thermal damage, we decreased the pump power. When the power is gradually reduced from 165 mW to 0, the stable Q-switching operation appeared again at the pump power from 41.5 to 164 mW, which indicated that the Si-PVA-based SA was intact. Therefore, we believe that the possible cause of unstable Q-switch operation is SA oversaturation. Fig. 5 shows some experimental results of the passively Q-switched fiber laser. As shown in Fig. 5(a), the stable trains under three different pump powers at 62, 102, and 164 mW were observed, under the fixed cavity polarization setting, the pulse repetition frequency increases with increasing pump power., while the pulse string still maintains uniform intensity distribution without obvious fluctuation. Fig. 5(b) shows the Q-switched output optical spectrum with the central wavelength of 1567.1 nm, and the 3 dB spectral bandwidth of ∼0.369 nm. Fig. 5(c) displays seven pulse envelopes, and the shortest pulse width of 2.32 μs with repetition rate of 58.7 KHz under an input pump power of 164 mW. These experimental results show a high passive Q-switching performance of the fiber laser feasible by Si-PVA-based SA. In Fig. 6, we summarized the relations between the repetition rate of the Q-switched pulses, pulse width of the Q-switched pulses, average output power, and peak pulse power with respect to different input pomp power. The pulse repetition rate and pulse width are shown in Fig. 6(a). As the pump power is gradually increased from 41.5 mW to 164 mW, pulse repetition frequency increases from 11.1 KHz to 58.7 KHz, and pulse width decreases from 5.47 μs to 2.32 μs. As expected, the pulse repetition frequency of a passive Qswitched laser is proportional to the pump power, and the pulse duration is inversely proportional to the pump power [39]. Fig. 6(b) shows the average output power and peak pulse power as a function of input pump power. When the incident pump power varied from 41.5 mW to 164 mW, the average output power increases linearly from 0.17 mW to 0.89 mW. With the increase of pump power,
Fig. 4. Experimental setup of Si-PVA EDF laser. 4
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Fig. 5. Experimental results of the Si-PVA-based Q-switched fiber laser. (a) Typical pulse trains under three different incident pump powers at 62, 102, and164 mW. (b) Output optical spectrum and (c) seven pulse envelopes.
Fig. 6. (a) Repetition rate and pulse width of the input laser corresponding to different pump powers. (b) Average power and peak pulse power of the input laser versus pump power.
the peak pulse power increases sharply to the vicinity of 6.3 mW and then tends to be gentle. Further possibilities of scaling of output power and shortening of laser pulses can be achieved by shortening the laser cavity length [40] or better preparation process of Si-SA. Moreover, the pulse duration could be further reduced by improving the modulation depth of the Si-SA, because pulse duration is expected to be inversely proportional to the modulation depth of the SA [41]. However, we all know that due to the thermal effect of laser operation on two-dimensional materials, the increase of power and decrease of pulse width are limited. A 60 minutes evolution process of the pulse is shown in Fig. 7, which indicates the stability of Q-switched laser. We think that the possible cause of this phenomenon is the saturation of SA.
5. Conclusion In summary, the high-quality silicene nanosheets were exfoliated by LPE method, Si-PVA film was successfully prepared and we demonstrate that a passively Q-switched all-fiber laser at 1567.1 nm by the Si-PVA-based SA experiment. According to previous reports, this is the first time that few layers silicon nanosheets has been used as a SA to achieve a Q-switched erbium-doped fiber laser. At a repetition rate of 58.7 KHz, a passive Q-switching operation with an average output power of 0.89 mW, a peak pulse power of 6.52 mW, and a pulse width of 2.32 μs was obtained. Our result indicated that silicon nanosheets with an excellent nonlinear saturable absorption property will have good foreground in ultrafast photonics and optoelectronic applications. And it may provide a guidance for the application of silicon nanosheets in fiber lasers. 5
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Fig. 7. Long-term performance of Q-switching within 60 min.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 11704227, 61505109, 11704226), Natural Science Foundation of Shandong Province (ZR2016AB05, ZR2017MA051). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M. Katsnelson, I. Grigorieva, S. Dubonos, A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197. [3] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang, S.-C. Zhang, Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface, Nature physics 5 (2009) 438. [4] J.E. Moore, The birth of topological insulators, Nature 464 (2010) 194. [5] M.A. Solodyankin, E.D. Obraztsova, A.S. Lobach, A.I. Chernov, A.V. Tausenev, V.I. Konov, E.M. Dianov, Mode-locked 1.93 μm thulium fiber laser with a carbon nanotube absorber, Opt. let. 33 (2008) 1336–1338. [6] C. Ataca, H. Sahin, S.J.T. Ciraci, Stable, single-layer MX2 transition-metal oxides and dichalcogenides in a honeycomb-like structure, J. Phys. Chem.C 116 (2012) 8983–8999. [7] K. Niu, Q. Chen, R. Sun, B. Man, H. Zhang, Passively Q-switched erbium-doped fiber laser based on SnS2 saturable absorber, Opt. Mat. Express 7 (2017) 3934–3943. [8] J. Li, Y. Zhao, Q. Chen, K. Niu, R. Sun, H. Zhang, Passively mode-locked ytterbium-doped fiber laser based on SnS2 as saturable absorber, IEEE Photonics J. 9 (2017) 1–7. [9] K. Niu, R. Sun, Q. Chen, B. Man, H. Zhang, Passively mode-locked Er-doped fiber laser based on SnS2 nanosheets as a saturable absorber, Photonics Research 6 (2018) 72–76. [10] H.O. Churchill, P. Jarillo-Herrero, Two-dimensional crystals: Phosphorus joins the family, Nature nanotechnology 9 (2014) 330. [11] F. Zhang, M. Wang, Z. Wang, K. Han, X. Liu, X.J. Xu, Excellent nonlinear absorption properties of β-antimonene nanosheets, J. Mat. Chem. C 6 (2018) 2848–2853. [12] F. Zhang, M. Wang, Z. Wang, K. Han, X. Liu, X. Xu, Nonlinear absorption properties of silicene nanosheets, Nanotechnology 29 (2018) 225701. [13] H. Zhang, D. Tang, R. Knize, L. Zhao, Q. Bao, K.P. Loh, Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser, Applied Physics Letters 96 (2010) 111112. [14] Z. Qin, G. Xie, H. Zhang, C. Zhao, P. Yuan, S. Wen, L. Qian, Black phosphorus as saturable absorber for the Q-switched Er: ZBLAN fiber laser at 2.8 μm, Opt. Express 23 (2015) 24713–24718. [15] Z. Guo, H. Zhang, S. Lu, Z. Wang, S. Tang, J. Shao, Z. Sun, H. Xie, H. Wang, X.F. Yu, From black phosphorus to phosphorene: basic solvent exfoliation, evolution of Raman scattering, and applications to ultrafast photonics, Advanced Functional Materials 25 (2015) 6996–7002. [16] F. Zhang, S. Han, Y. Liu, Z. Wang, X. Xu, Dependence of the saturable absorption of graphene upon excitation photon energy, Applied Physics Letters 106 (2015) 091102. [17] Z. Sun, T. Hasan, A. Ferrari, Ultrafast lasers mode-locked by nanotubes and graphene, Physica E: Low-dimensional Systems and Nanostructures 44 (2012) 1082–1091.
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