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Abstract
We investigate the saturable absorption properties of Bi2Se3 in a bulk laser operating at 2 µm wavelength region. The Bi2Se3 saturable absorber (SA) is prepared with the liquid-phase exfoliation method, which gives a saturable input flux of 4.3 mJ/cm2, a modulation depth of ∼10%, and a non-saturable absorption of 10.2%. With the Bi2Se3 saturable absorber, a passive Q-witching Tm:YAG ceramic laser is realized with a shortest pulse duration of 355 ns, a single pulse energy of 6.76 µJ and peak power of 19 W. We believe that this is the first report on Bi2Se3 Q-switched 2 µm bulk laser.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
2 µm nanosecond pulsed lasers have wide applications in many fields such as: microsurgery, plastic material processing, and free-space telecommunications [1–3]. Moreover, these lasers are also ideal pumping sources for mid or far infrared optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs) [4,5]. Passive Q-switching fiber or bulk lasers based on SAs operating at 2 µm wavelength region are considered to be the primary path to such kind of laser pulses. Especially the passive Q-switching bulk laser is usually considered surpass the fiber laser with the good beam quality, the small beam-divergence angle, as well as the easy power and energy scaling ability. The passive Q-switching technique with SAs holds intrinsic advantages such as: no external driving devices, compactness, as well as flexibility. These 2 µm SAs includes: semiconductor saturable absorber mirrors (SESAMs) [6], low dimensional materials (graphene, carbon nanotubes (CNT), transition-metal dichalcogenides (WS2,MoS2 and MoSe2), et. al.) [7–12], and ions-doped bulk crystals (Cr2+:ZnSe, PbS-doped glass) [13,14]. However, they are all more or less having some drawbacks: SESAMs and ions-doped crystals suffer from narrow modulation bandwidth, long response time and complex fabricating process. Compared with SESAMs and ions-doped crystals, SAs based on low dimensional materials are featured with easy fabrication and broad modulation bandwidth. CNT SA can broaden its modulation bandwidth by incorporating different diameters of CNTs, while this somehow increases its scattering loss, thus resulting in a high laser threshold when employed for Q-switching operation [15]. Although graphene has an inherent broad modulation bandwidth, its low modulation depth prevents a shorter Q-switching pulse generation [16]. In the end, the transition-metal dichalcogenides, another popular 2 µm SA, also are bothered with a synthesis problem. Therefore, in order to improve the performances of 2 µm passive Q-switching pulse laser, the efforts on exploring novel SAs are still needed to be paid.
Recently, topological insulators (TIs) which belong to the Dirac material including Sb2Te3, Bi2Te3 and Bi2Se3, have drawn a lot of attentions due to the excellent photoelectronic properties [17–20]. The photoelectronic properties of TIs are determined by the insulating energy gaps exists in the bulk and gapless edges or the surface states on the sample boundary [17,21]. Just like graphene, TIs also possess an ultra-broad saturable absorption bandwidth. Take Bi2Se3 as an example, the absorption in its surface and edge makes it possess a strong saturation absorption in the wavelength region below 4.1 µm (0.3 eV). Moreover, Bi2Se3 is featured with a much higher modulation depth (up to 98%) compared with graphene, making it become an ideal SA for nanosecond laser pulse generation [22,23]. Especially, Bi2Se3 surpasses other TIs with a strong oxidation resistance. When manufactured as a SA, the oxidation resistance keeps Bi2Se3 from the contamination of the oxygen in the air, which is helpful for maintaining a stable chemical and optical property [24]. So far, Bi2Se3 SAs have been successfully employed in the fiber lasers for passive Q-switching or mode-locking in the spectral range from 1 µm to 3 µm [25–28], but the Bi2Se3 Q-switched bulk lasers are rarely reported. The existed few reports mainly concentrate on Yb or Nd doped bulk lasers based on the Bi2Se3 SA around 1 µm wavelength region. For example, a 1.6 µs pulse is achieved at 1043 nm in a Yb:KGW laser [29], a 720 ns pulse is obtained at 1077 nm and 1081 nm in a Nd:Lu2O3 laser [30], and a 433 ns pulse is realized at 1313 nm in a Nd:LiYF4 laser [31]. Other than that, no report is found on Bi2Se3 Q-switched bulk laser in 2 µm wavelength region.
In this paper, we firstly report the Q-switching property of Bi2Se3 in a 2 µm bulk laser. A high-quality Bi2Se3 SA is fabricated with the liquid-phase exfoliation method. With the Bi2Se3 SA, the pulse durations, repetition rates, single pulse energies, as well as peak powers of the Q-switched Tm:YAG ceramic laser are investigated experimentally. A 355 ns pulse with a single pulse energy of 6.76 µJ and peak power of 19 W is achieved in the Bi2Se3 Q-switched Tm:YAG ceramic laser.
2. Preparation and characterization of Bi2Se3 SA
The SA is fabricated by using commercially available Bi2Se3 powders (source: Six Carbon Technology, purity: >99.999%) with the liquid-phase exfoliation method. At the beginning, the powders are dissolved with the alcohol in a centrifuge tube and placed into an ultrasonic machine for 10 hours’ scattering. Followed by two times of 15 min long centrifugation with a speed of 4800 rpm/min, the prepared Bi2Se3 solution is divided into two layers. The Bi2Se3 SA is successfully prepared by dripping up-layer of the Bi2Se3 solution onto a piece of quartz glass and air-dried at an ambient temperature of 25°C for 20 hours (see the inset of Fig. 1(a)). We measured the absorption of the Bi2Se3 SA from 1.0 µm to 2.5 µm shown as Fig. 1(a). It is clear that the Bi2Se3 SA has a broad absorption bandwidth and the absorption is decreasing with the increasing of the wavelength. However the absorption rate of the Bi2Se3 SA is still above 24% around 2 µm wavelength region, indicating it is capable of being used in 2 µm wavelength region.
Fig. 1. (a) The absorption of Bi2Se3 SA. (b) The photograph of the fabricated Bi2Se3 SA on a quartz substrate. We circled the Bi2Se3 sample distributed on the substrate with a black curve. (c) SEM image of Bi2Se3 on the quartz substrate. (d) The measured nonlinear transmittance of the Bi2Se3 SA.
Figure 1(b) and Fig. 1(c) give the photograph for the fabricated Bi2Se3 SA on a quartz substrate and the scanning electron microscope (SEM) image, respectively. Figure 1(c) shows that the Bi2Se3 flakes are randomly distributed on the quartz on the scale of micrometer, indicating that most of the Bi2Se3 is in the bulk form. However, the surface states generated on the edges or surface also exists but relatively few [17]. Therefore, the linear and nonlinear optical absorption of the Bi2Se3 is a joint result of the bulk and surface states. The nonlinear absorption of the Bi2Se3 SA is measured based on a pair of balanced twin-detector which is shown in the inset of Fig. 1(d). An acousto-optically Q-switched Tm:YAP laser with a center wavelength of 1985nm, a pulse duration of 450 ns and pulse repetition frequency of 1 kHz is adopted as the pumping laser source. The measured result is fitted theoretically as Fig. 1(d) with the Eq. (1) [31]:
where ΔT, Ф, Фsat and Tns are the modulation depth, input pulse flux, saturated input pulse flux and the non-saturable absorption, respectively. The transmittance increases with the increasing of the input pulse flux and stays as a constant when the input pulse flux exceeds 15 mJ/cm2, which eventually determine a modulation depth of ∼10%, a saturable input flux of 4.3 mJ/cm2 and a non-saturable absorption loss of 10.2%. Compared with [25] and [32] which are also liquid-phase exfoliation prepared Bi2Se3 SAs, the modulation depth of 10% is larger than the values of 3.8% in [25] and 3.7% in [32], while the non-saturable absorption loss of 10.2% is smaller than the reported value of 15% in [25] and [32]. The smaller non-saturable absorption loss not only indicates the weak scattering from the surface and impurities in the Bi2Se3 material, but also indicates weak absorption of the employed SA substrate.3. Experiment results and discussion
In order to investigate the saturable absorption properties of the Bi2Se3 SA in 2 µm wavelength region, the experimental setup is arranged as shown in Fig. 2. We select a 3 mm×3 mm×6 mm Tm: YAG (5 at. %) ceramic as the gain medium due to its high thermal conductivity, high laser damage threshold, as well as stable chemical and optical properties. The pump source is a 793 nm fiber-coupled laser diode (LD), which is focused into the Tm: YAG ceramic with a 1:1 telescope. In order to effectively remove the generated heat, the laser crystal is water-cooled at 13 oC by a cooling system. The laser cavity is built with two plan mirrors (M1 and M2) with a total length of 2.7 cm. The input plan mirror M1 is highly reflective (HR) coated from 1820 nm to 2150 nm (reflectivity > 99.9%) and anti-reflection (AR) coated around 790 nm. The plan mirror M2 which is partially transmitted from 1820 nm to 2150 nm acts as an output coupler (OC). After M2, a filter is employed to block the pump laser. The Q-switched pulse profile is detected by an InGaAs PIN detector (EOT, ET-5000) and observed with an oscilloscope (Tektronix, DPO 4102B-L). The average output power is measured with a laser power meter (CNI, TS35).
At the beginning, the average output powers of the Tm:YAG ceramic laser are measured in both continuous-wave (cw) and passive Q-switching (PQS) regimes with different output couplers (OCs), which are shown as Fig. 3. The output powers increase with the increasing of absorbed pump power and transmissions of OCs (${T_{\textrm{oc}}}$) in both cw and PQS regimes. In order to quantitatively analyze the power performances, we record the slope efficiencies, optical-to-optical conversion efficiencies, maximum output powers, as well as the power stabilities for Toc of 1%, 3% and 5% respectively (see in Table 1). Among these three cases, the laser with Toc = 5% gives the best output performance, i.e., highest slope and optical-to-optical conversion efficiencies, maximum output power, as well as a stable laser output. The selected OCs with transmittance of 1%, 3% and 5% are a compromise between the obtained laser slope efficiency and Q-switching pulse duration. Although a higher laser slope efficiency can be achieved with a higher transmittance OC, the obtained Q-switching pulse duration is also expected to be much broader [33]. Due to the introduced loss of the Bi2Se3 SA, the laser thresholds in PQS regime are slightly higher compared with that in cw regime. However, the inserted Bi2Se3 SA doesn’t deteriorate the beam quality of the 2 µm laser. The M2 factor are measured to be around 2.0 in both two regimes with the knife-edge method. The beam diameter inside the laser crystal is estimated to be 112 µm, which is very close to the pumping diameter of 105 µm, indicating a nearly perfect mode matching. Moreover, the beam diameter on the saturable absorber is estimated to be around 190 µm, which can be slightly varied by changing the position of the SA along the axis of the laser cavity. The emission wavelength of the Tm:YAG laser is measured with a spectrometer (Wavescan USB, APE), showing a central peak located at 2015nm for both cw and PQS operation.
Fig. 3. (a) Average output powers versus the absorbed pump power with different ${T_{\textrm{oc}}}$ in (a) cw regime and (b) PQS regime.
Table 1. Laser performance in cw and PQS regime.
Figure 4 gives the pulse repetition rate and the pulse duration versus the absorbed pump power for different OCs. The pulse repetition rate increases with the increasing of the absorbed pump power, while the pulse duration shows a contradictory trend which decreases as the enhancement of the absorbed pump power. Under the maximum absorbed pump power of 8.32 W, a highest pulse repetition rate of 148.5 kHz is achieved with Toc = 1%, and a minimum pulse duration of 355 ns is obtained with Toc = 5%. Since a large modulation depth is more favorable for generating a short Q-switching pulse, the Q-switching pulse duration is expected to be further narrowed with a larger modulation depth Bi2Se3 SA, which could be achieved either by using the polyol method [23,34,35] or by fabricating appropriate thickness of Bi2Se3 SA [22]. We believe our SA is quite homogeneous since we don’t see any obvious changes of the pulse duration when the SA is moved transversely in the laser cavity within the stable operation range of the PQS laser [34]. Moreover, the homogeneous of the SA also can be verified by directly measuring the nonlinear transmittances on different positions of the Bi2Se3 SA. All of these measurements result in a similar modulation depth. However, we can’t get a Q-switching operation on some positions of the SA when it is moved transversely in the laser cavity, this maybe an indication that the Bi2Se3 is not uniformly distributed on the substrate.
Figure 5 gives the typical Q-switching laser pulse profiles and the pulse trains for each OC under different absorbed pump powers. Q-switching operations are observed when the absorbed pump powers are slightly above the laser thresholds (see the first row in Fig. 5). Due to the low photon intensity inside the laser cavity near the laser thresholds, the SA could not be bleached in time, thus leading to a relatively unstable Q-switching operation. Further enhancing the absorbed pump power will greatly increase the intracavity photon intensity and subsequently shorten the bleaching time of the SA, which results in a short Q-switching pulse and a stable Q-switching pulse train (see the second and third row in Fig. 5). When the pumping power is enhanced to above 8.32 W, the accumulated heat inside the SA greatly deteriorates its optical properties, leading to an unstable Q-switching operation. Therefore, we didn’t further increase the pump power. The single pulse energy and pulse peak power are calculated as shown in Fig. 6. Both of them increase with the increasing of the absorbed pump power for each OC. The largest single pulse energy of 6.8 µJ and highest peak power of 19.1 W are obtained at the maximum absorbed pump power of 8.32 W with Toc = 5%. We successfully repeat the experiment with the same SA prepared one month ago, indicating a good chemical stability of the Bi2Se3 SA.
We compare the laser performance of Bi2Se3 Q-switching bulk laser with other low dimensional material SA Q-switching bulk lasers in 2 µm wavelength region, which is summarized in Table 2. It is shown that the pulse duration of the Bi2Se3 passive Q-switching laser is relatively shorter compared with the other Q-switching 2 µm bulk laser based on low dimensional material SAs. Our Q-switching pulse duration can be further shortened by using a Bi2Se3 SA with a larger modulation depth or employing a much compacted laser cavity [34]. From Table 2, we can also safely conclude that the Bi2Se3 passive Q-switching 2 µm laser surpass most of the passive Q-switching 2 µm lasers in the output power, the single pulse energy and the peak power.
4. Conclusion
In this paper, a passively Q-switched Tm:YAG ceramic laser with a central wavelength of 2015nm is realized by using Bi2Se3 as a saturable absorber. Stable pulse train with a pulse repetition rate of 124.5 kHz and a pulse duration of 355 ns is achieved when Toc = 5%, corresponding to a single pulse energy of 6.8 µJ and peak power of 19.1 W. The maximum repetition rate of 148.5 kHz with the single pulse duration of 416 ns is obtained at Toc = 1%. To the best our knowledge, this is the first report on a Bi2Se3 Q-switched 2 µm bulk laser. A narrower pulse duration with a higher peak power can be expected with a compact Q-switching laser cavity and a larger modulation depth of Bi2Se3 SA. Moreover, 2 µm picosecond or femtosecond laser pulses are also expected from a Bi2Se3 based mode-locking bulk laser.
Funding
National Natural Science Foundation of China (61775119); Natural Science Foundation of Shandong Province (ZR2018MF033); National Basic Research Program of China (973 Program) (2018YFB1107403-2); Foundation of President of China Academy of Engineering Physics (YZJJLX2018005); Taishan Young Scholar Program of Shandong Province and Qilu Young Scholar Program of Shandong University (tsqn201812010).
Disclosures
The authors declare no conflicts of interest.
References
1. M. Ith, M. Frenz, and H. P. Weber, “Scattering and thermal lensing of 2.12-µm laser radiation in biological tissue,” Appl. Opt. 40(13), 2216–2223 (2001). [CrossRef]
2. S. T. Hendow and S. A. Shakir, “Structuring materials with nanosecond laser pulses,” Opt. Express 18(10), 10188–10199 (2010). [CrossRef]
3. H. Lubatschowski and A. Heisterkamp, “Ophthalmic Applications,” in Femtosecond Technology for Technical and Medical Applications, TAP.96, 187–203 (Springer Verlag Publishing, 2004).
4. E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, “A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator,” Opt. Express 18(25), 26475–26483 (2010). [CrossRef]
5. E. Lippert, G. Rustad, G. Arisholm, and K. Stenersen, “High power and efficient long Wave IR ZnGeP2 parametric oscillator,” Opt. Express 16(18), 13878–13884 (2008). [CrossRef]
6. Y. Mashiko, E. Fujita, and M. Tokurakawa, “Tunable noise-like pulse generation in mode-locked Tm fiber laser with a SESAM,” Opt. Express 24(23), 26515–26520 (2016). [CrossRef]
7. W. Cai, S. Z. Jiang, S. C. Xu, Y. Q. Li, J. Liu, C. Li, L. H. Zheng, L. B. Su, and J. Xu, “Graphene saturable absorber for diode pumped Yb:Sc2SiO5 mode-locked laser,” Opt. Laser Technol. 65, 1–4 (2015). [CrossRef]
8. H. Yu, L. Zhang, Y. Wang, S. Yan, W. Sun, J. Li, Y. Tsang, and X. Lin, “Sub-100 ns solid-state Q-switched with double wall carbon nanotubes,” Opt. Commun. 306(1), 128–130 (2013). [CrossRef]
9. C. Luan, K. Yang, J. Zhao, S. Zhao, L. Song, T. Li, H. Chu, J. Qiao, C. Wang, Z. Li, S. Jiang, B. Man, and L. Zheng, “WS2 as a saturable absorber for Q-switched 2 micron lasers,” Opt. Lett. 41(16), 3783–3786 (2016). [CrossRef]
10. J. M. Serres, P. Loiko, X. Mateos, H. Yu, H. Zhang, Y. Chen, V. Petrov, U. Griebner, K. Yumashev, M. Aguilo, and F. Diaz, “MoS2 saturable absorber for passive Q-switching of Yb and Tm microchip lasers,” Opt. Mater. Express 6(10), 3262–3273 (2016). [CrossRef]
11. Z. Luo, Y. Li, M. Zhong, Y. Huang, X. Wan, J. Peng, and J. Weng, “Nonlinear optical absorption of few-layer molybdenum diselenide (MoSe2) for passively mode-locked soliton fiber laser [Invited],” Photonics Res. 3(3), A79–A86 (2015). [CrossRef]
12. T. Jiang, K. Yin, C. Wang, J. You, H. Ouyang, R. Miao, C. Zhang, K. Wei, H. Li, H. Chen, R. Zhang, X. Zheng, Z. Xu, X. Cheng, and H. Zhang, “Ultrafast fiber lasers mode-locked by two-dimensional materials: review and prospect,” Photonics Res. 8(1), 78–90 (2020). [CrossRef]
13. Y. Tang, Y. Yang, J. Xu, and Y. Hang, “Passive Q-switching of shortlength Tm3+-doped silica fiber lasers by polycrystalline Cr2+:ZnSe microchips,” Opt. Commun. 281(22), 5588–5591 (2008). [CrossRef]
14. M. S. Gaponenko, I. A. Denisov, V. E. Kisel, A. M. Malyarevich, A. A. Zhilin, A. A. Onushchenko, N. V. Kuleshov, and K. V. Yumashev, “Diode-pumped Tm:KY(WO4)2 laser passively Q-switched with PbS-doped glass,” Appl. Phys. B 93(4), 787–791 (2008). [CrossRef]
15. A. Martinez and Z. Sun, “Nanotube and graphene saturable absorbers for fibre lasers,” Nat. Photonics 7(11), 842–845 (2013). [CrossRef]
16. C. A. Zaugg, Z. Sun, V. J. Wittwer, D. Popa, S. Milana, T. S. Kulmala, R. S. Sundaram, M. Mangold, O. D. Sieber, M. Golling, Y. Lee, J. H. Ahn, A. C. Ferrari, and U. Keller, “Ultrafast and widely tunable vertical external-cavity surface-emitting laser, mode-locked by a graphene integrated distributed Bragg reflector,” Opt. Express 21(25), 31548–31559 (2013). [CrossRef]
17. H. Zhang, C. Liu, X. Qi, X. Dai, Z. Fang, and S. Zhang, “Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface,” Nat. Phys. 5(6), 438–442 (2009). [CrossRef]
18. J. Zhao, Z. Xu, Y. Zang, Y. Gong, X. Zheng, K. He, X. Cheng, and T. Jiang, “Thickness-dependent carrier and phonon dynamics of topological insulator Bi2Te3 thin films,” Opt. Express 25(13), 14635–14643 (2017). [CrossRef]
19. R. Miao, Y. Hu, H. Ouyang, Y. Tang, C. Zhang, J. You, X. Zheng, Z. Xu, X. Cheng, and T. Jiang, “A polarized nonlinear optical response in a topological insulator Bi2Se3–Au nanoantenna hybrid-structure for all-optical switching,” Nanoscale 11(31), 14598–14606 (2019). [CrossRef]
20. T. Jiang, R. Miao, J. Zhao, Z. Xu, T. Zhou, K. Wei, J. You, X. Zheng, Z. Wang, and X. Cheng, “Electron–phonon coupling in topological insulator Bi2Se3 thin films with different substrates,” Chin. Opt. Lett. 17(2), 020005 (2019). [CrossRef]
21. Y. I. Jhon, J. Lee, Y. M. Jhon, and J. Lee, “Topological Insulators for Mode-locking of 2-µm Fiber Lasers,” IEEE J. Sel. Top. Quantum Electron. 24(5), 1–10 (2018). [CrossRef]
22. J. Zhang, T. Jiang, T. Zhou, H. Ouyang, C. Zhang, Z. Xin, Z. Wang, and X. Cheng, “Saturated absorption of different layered Bi2Se3 films in the resonance zone,” Photonics Res. 6(10), C8–C14 (2018). [CrossRef]
23. C. J. Zhao, Y. Zou, Y. Chen, Z. Wang, S. Lu, H. Zhang, S. C. Wen, and D. Y. Tang, “Wavelength-tunable picosecond soliton fiber laser with topological insulator: Bi2Se3 as a mode locker,” Opt. Express 20(25), 27888–27895 (2012). [CrossRef]
24. L. V. Yashina, J. Sánchez-Barriga, M. R. Scholz, A. A. Volykhov, A. P. Sirotina, V. Neudachina, M. Tamm, A. Varykhalov, D. Marchenko, and O. Rader, “Negligible surface reactivity of topological insulators Bi2Se3 and Bi2Te3 towards oxygen and water,” ACS Nano 7(6), 5181–5191 (2013). [CrossRef]
25. Z. Luo, Y. Huang, J. Weng, H. Cheng, Z. Lin, B. Xu, Z. Cai, and H. Xu, “1.06 µm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2Se3 as a saturable absorber,” Opt. Express 21(24), 29516–29522 (2013). [CrossRef]
26. K. Yin, R. Zhu, B. Zhang, G. Liu, P. Zhou, and J. Hou, “300 W-level, wavelength-widely-tunable, all-fiber integrated thulium-doped fiber laser,” Opt. Express 24(10), 11085–11090 (2016). [CrossRef]
27. W. Li, K. Wang, T. Du, J. Zou, Y. Li, and Z. Luo, “Miniaturized Mid-Infrared All-Fiber Laser Passively Q-Switched by Topological Insulator Bi2Se3,” ACP conference. (Oct. 26-29, 2018, Hangzhou, China).
28. R. Miao, M. Tong, K. Yin, H. Ouyang, Z. Wang, X. Zheng, X. Cheng, and T. Jiang, “Soliton mode-locked fiber laser with high-quality MBE-grown Bi2Se3 film,” Chin. Opt. Lett. 17(7), 071403 (2019). [CrossRef]
29. M. Hu, J. Liu, J. Tian, Z. Dou, and Y. Song, “Generation of Q-switched pulse by Bi2Se3 topological insulator in Yb:KGW laser,” Laser Phys. Lett. 11(11), 115806 (2014). [CrossRef]
30. B. Wang, H. Yu, H. Zhang, C. Zhao, S. Wen, H. Zhang, and J. Wang, “Topological Insulator Simultaneously Q-Switched Dual-Wavelength Nd : Lu2O3 Laser,” IEEE Photonics J. 6(3), 1–7 (2014). [CrossRef]
31. B. Xu, Y. Wang, J. Peng, Z. Luo, H. Xu, Z. Cai, and J. Weng, “Topological insulator Bi2Se3 based Q-switched Nd:LiYF4 nanosecond laser at 1313 nm,” Opt. Express 23(6), 7674–7680 (2015). [CrossRef]
32. Z. Luo, C. Liu, Y. Huang, D. Wu, J. Wu, Z. Lin, L. Sun, and J. Weng, “Topological-Insulator Passively Q-Switched Double-Clad Fiber Laser at 2 µm Wavelength,” IEEE J. Sel. Top. Quantum Electron. 20(5), 1–8 (2014). [CrossRef]
33. J. Qiao, S. Zhao, K. Yang, W. Song, W. Qiao, C. Wu, J. Zhao, G. Li, D. Li, H. Liu, and C. Lee, “High-quality 2-µm Q-switched pulsed solid-state lasers using spin-coating-coreduction approach synthesized Bi2Te3 topological insulators,” Photonics Res. 6(4), 314–320 (2018). [CrossRef]
34. G. J. Spühler, R. Paschotta, R. Fluck, B. Braun, M. Moser, G. Zhang, E. Gini, and U. J. Keller, “Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers,” J. Opt. Soc. Am. B 16(3), 376–388 (1999). [CrossRef]
35. J. Zhang, Z. P. Peng, A. Soni, Y. Y. Zhao, Y. Xiong, B. Peng, J. B. Wang, M. S. Dresselhaus, and Q. H. Xiong, “Raman spectroscopy of Few-Quintuple Layer Topological insulator Bi2Se3 nanoplatelets,” Nano Lett. 11(6), 2407–2414 (2011). [CrossRef]
36. Z. Chu, J. Liu, Z. Guo, and H. Zhang, “2 µm passively Q-switched laser based on black phosphorus,” Opt. Mater. Express 6(7), 2374–2379 (2016). [CrossRef]
37. X. Liu, K. Yang, S. Zhao, T. Li, W. Qiao, H. Zhang, B. Zhang, J. He, J. Bian, L. Zheng, L. Su, and J. Xu, “High-power passively Q-switched 2 µm all-solidstate laser based on a Bi2Te3 saturable absorber,” Photonics Res. 5(5), 461–466 (2017). [CrossRef]
38. P. Gao, H. Huang, X. Wang, H. Liu, J. Huang, W. Weng, S. Dai, J. Li, and W. Lin, “Passively Q-switched solid-state Tm:YAG laser using topological insulator Bi2Te3 as a saturable absorber,” Appl. Opt. 57(9), 2020–2024 (2018). [CrossRef]
39. B. Yan, B. Zhang, H. Nie, H. Wang, G. Li, X. Sun, R. Wang, N. Lin, and J. He, “High-power passively Q-switched 2.0 µm allsolid-state laser based on a MoTe2 saturable absorber,” Opt. Express 26(14), 18505–18512 (2018). [CrossRef]
40. L. Cao, W. Tang, S. Zhao, Y. Li, X. Zhang, N. Qi, and D. Li, “2 µm Passively Q-switched all-solid-state laser based on WSe2 saturable absorber,” Opt. Laser Technol. 113, 72–76 (2019). [CrossRef]
41. Y. Shen, L. Li, X. Duan, R. Lan, L. Zhou, W. Xie, and X. Yang, “High-beam-quality operation of a 2 µm passively Q-switched solid-state laser based on a boron nitride saturable absorber,” Appl. Opt. 58(10), 2546–2550 (2019). [CrossRef]
42. L. Chen, X. Li, H. Zhang, and W. Xia, “Passively Q-switched 1.989 µm all-solid-state laser based on a WTe2 saturable absorber,” Appl. Opt. 57(35), 10239–10242 (2018). [CrossRef]
43. C. Zhang, J. Liu, X. Fan, Q. Peng, X. Guo, D. Jiang, X. Qian, and L. Su, “Compact passive Q-switching of a diode-pumped Tm,Y:CaF2 laser near 2 µm,” Opt. Laser Technol. 103, 89–92 (2018). [CrossRef]
44. H. Huang, M. Li, P. Liu, L. Jin, H. Wang, and D. Shen, “Gold nanorods as the saturable absorber for a diode-pumped nanosecond Q-switched 2 µm solid-state laser,” Opt. Lett. 41(12), 2700–2703 (2016). [CrossRef]
45. X. Liu, Q. Yang, C. Zuo, Y. Cao, X. Lun, P. Wang, and X. Wang, “2-µm passive Q-switched Tm:YAP laser with SnSe2 absorber,” Opt. Eng. 57(12), 126105 (2018). [CrossRef]
46. X. Zhang, H. Chu, Y. Li, S. Zhao, and D. Li, “Diameter-selected single-walled carbon nanotubes for the passive Q-switching operation at 2 µm,” Opt. Mater. 100, 109627 (2020). [CrossRef]
47. J. Hou, B. Zhang, J. He, Z. Wang, F. Lou, J. Ning, R. Zhao, and X. Su, “Passively Q-switched 2 µm Tm:YAP laser based on graphene saturable absorber mirror,” Appl. Opt. 53(22), 4968–4971 (2014). [CrossRef]
