Open Access
Add to BibSonomy
Abstract
Wideband optical saturable absorbers (SA) are an ideal laser component for pulse laser generation in different wavelengths, which has attracted tremendous attention in recent years. Herein, a MoO3-x-based novel wideband optical SA has been demonstrated by utilizing it for ∼1/1.5/2 μm Q-switched pulses generation. After separately integrating the MoO3-x-SA into Yb-, Er- and Tm-doped fiber lasers, passively Q-switched pulses with pulse durations of ∼1.5/2.2/1.6 μs, repetition rates of several tens kilohertz at corresponding wavelengths of ∼1038/1562.9/1910 nm are obtained. Our work firstly reveals the remarkable wideband optical saturable absorption of the MoO3-x, which strongly implies the potential application in infrared photonics devices. It may also provide new opportunities for wideband optical SA based on oxygen-deficient metal oxides.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulse lasers have attracted a lot of research in recent years, since they have wide applications range from the military, industrial fabrication, communication to medical treatment [1–4]. Compared to the other pulse lasers, fiber laser with optical saturable absorber (SA) modulation is an effective and convenient way for pulse laser generation, which exhibits the advantages of compactness, simplicity, good beam quality and low cost. As the key component of such pulsed lasers, the optical SA has developed rapidly with much concerns.
In last few years, many research efforts were focused on the different functional optical SA materials, mainly including the SESAMs [5], carbon nanotubes (CNTs) [6], Two-dimension nanomaterials (graphene [7,8], topological insulators (TIs) [9,10], transition metal dichalcogenides (TMDs) [11–19], black phosphorus (BP) [20–23], topological semimetal [24], Mxene [25–27]), Transition metal monochalcogenides [28], heterojunction [29], nanosheets [30] and nanoparticles [31,32]. Among these SAs, they also suffer from respective deficiencies, e.g., the SESAM is limited by bandwidth and cost (a complicated design is needed for different operating wavelengths); the CNT also has a narrow absorption bandwidth, unless the combination of CNTs with different diameters; graphene suffers from the low modulation depth (∼1% per layer and less than 10% with multi-layer) [7]; The BP is easily to oxidized in natural condition; the sub-bandgap saturable absorption of the most reported TMDs was caused by the irregular crystallographic defects, and the repeatability is a challenge [33,34]. Therefore, there are still strong motivations of seeking for new SAs with better property and operability such as wideband operation, large modulation depth, ultrafast recover time, high damage threshold, easy-handling and low cost.
Molybdenum trioxide (MoO3) is a nontoxic and lowcost n-type semiconductor with wide bandgap, which was mainly studied in catalysis, capacitors, lithium-ion battery, and gas sensing [35,36]. Recently, the design of oxygen vacancies in MoO3 (MoO3-x) has been proved as an effective way to improve the properties of MoO3 such as electrocatalysis [37], charge storage [38], dyes adsorption [39], which has attracted a lot of attentions. Nevertheless, owing to the wide bandgap (∼3.1 eV) of MoO3, the nonlinear optical properties of MoO3-x have received relatively less attention, especially in infrared wavelength. In fact, the absorption bandwidth and intensity could be notably enhanced caused by the defect levels, which originates the vacancy defects [12]. In our previous work, by using the MoO3-x-SA in a double clad Tm-doped fiber laser, we obtained the Q-switched pulses with output power of ∼62.7 mW and pulse duration of ∼2 μs at ∼1992.1 nm [40]. The saturable absorption of the MoO3-x was mainly considered as the oxygen vacancies, since saturable absorption was not observed in the MoO3. Meanwhile, MoO3-x is a nonmetallic plasmonic nanocrystal [41]. The plasmon resonance will strengthen the interaction between the plasmonic nanostructures and pulse laser, which results in the enhancement of nonlinear optical effects in MoO3-x [42,43]. Furthermore, the plasmon resonance energies can cover from the near-infrared to mid-infrared spectral region, which indicates the potential application of wideband optical SA of MoO3-x.
In the present paper, the wideband saturable absorption of the MoO3-x in infrared spectral region was revealed by using it as optical SA for Q-switched pulse generation. After separately integrating the MoO3-x-SA into the Yb-, Er- and Tm-doped ring cavity fiber laser, Q-switched operations at central wavelength of ∼1038 nm, ∼1563 nm and ∼1910 nm were successfully observed. Our experimental results cover the three key infrared wavelengths (∼1.0/1.5/2.0 μm), which reveals the remarkable wideband saturable absorption of MoO3-x.
2. Fabrication and characterization of the oxygen-deficient MoO3-x
The MoO3-x was synthesized by Mo powder through the solvothermal method [39]. Firstly, 1 mmol Mo power (Aladdin, 99.9%) mixed with ∼1.5 mL H2O2 (Aladdin, 30wt%) and ∼28.5 mL ethanol solution (Aladdin, AR). After ∼30 min stirring, the mixed solution was sealed and heated at 160℃ for 12 hours. Then, the sediment was collected by centrifugation (10000 rpm for 30 min) and rinsed with ethanol three times. Finally, MoO3-x powder was obtained after drying the sediment under vacuum condition, as shown in Fig. 1(a) (inset is the ethanol solution of MoO3-x). Figure 1(b) shows the scanning electron microscopy (SEM) image of the MoO3-x, in which a cluster structure with nanosheets was observed. The detail analysis of the MoO3-x, including Raman spectra, Electron spin resonance (ESR) spectra, and X-ray photoelectron spectroscopy (XPS) can be seen in our previous work [40].
Fig. 1. (a) The MoO3-x powder (inset: photograph of MoO3-x ethanol solution), (b) SEM image of MoO3-x.
For the convenience of integrating MoO3-x into fiber laser, the MoO3-x was embedded into PVA film. Briefly, the MoO3-x ethanol solution with concentration of ∼4 mg/mL was prepared by an hour ultrasonic dispersing. Then the solution was centrifuged at a rate of 1000 rpm for 5 minutes. Subsequently, the supernatant was extracted and mixed with PVA aqueous solution (∼8wt %) with volume ration of ∼1:1. After ∼2 hours mechanical agitating and an hour ultrasonic dispersing, the dispersed mixed-solution was poured into a polystyrene cell for drying (at 50℃ vacuum oven for ∼24 hours). The obtained MoO3-x-PVA film was exhibited in Fig. 2 with typical thickness of ∼35 μm. Then, the film was cut into pieces with area of a few square millimeters for integrating into fiber laser, as can be seen in Fig. 2(c) (MoO3-x-PVA film on the single-mode fiber (SMF) facet, the black circle is the cladding and core of the SMF). In addition, the linear transmission spectra of MoO3-x-PVA film and pure PVA film were measured with spectral range of 1000-2500 nm by an optical spectrometer (PerkinElmer Lambda 950), as shown in Fig. 3(a). The transmittance of the MoO3-x-PVA film at wavelengths of ∼1038/1563/1910 nm (the three operating wavelengths of Q-switched fiber lasers in our experiments) are ∼45.2%/47.7%/47.2%, respectively. We also measured the nonlinear absorption of MoO3-x-PVA film at 1.5 μm by a home-made mode-locked fiber laser with seed pulse duration of ∼0.3 ps, as shown in Fig. 3(b). The modulation depth of the MoO3-x-SA at 1.5 μm is ∼3.9% with the fitting curve. Moreover, according to previously measured results, the modulation depth of MoO3-x-SA at 2 μm is ∼15.7% [40] and the modulation depth of MoO3-x nanosheets at 1 μm is ∼35% [41].
Fig. 2. (a) MoO3-x-PVA film, (b) The thickness of the film, (c) Optical micrograph of fiber facet with MoO3-x-PVA film (the black circle is the fiber core and cladding).
Fig. 3. (a) Linear transmission spectra of MoO3-x-PVA film & pure PVA film, (b) Measured nonlinear curve of MoO3-x-PVA film at 1.5 μm.
3. MoO3-x as wide-band SA for Q-switched operations
In this section, to verify the wideband saturable absorption of MoO3-x, we have separately constructed three ring cavity fiber lasers (the Yb-, Er- and Tm-doped fiber lasers) for Q-switched operations with MoO3-x-SA, which cover the mainly infrared wavelengths in spectral regime of ∼1 μm to ∼2 μm.
3.1 Passively Q-switched Yb-doped fiber laser
The experimental setup of MoO3-x-SA based passively Q-switched Yb-doped fiber (YDF) laser is shown in Fig. 4. A section of ∼2 m long YDF (SM-YSF-LO-HP) served as the gain-fiber, which was pumped by a ∼976 nm LD through a 980/1064 nm wavelength division multiplexer (WDM). Behind the gain fiber, a polarization independent isolator was used for ensuring the unidirectional operation of the laser. The MoO3-x-film was sandwiched between two fiber patch cords with inset loss of ∼3.6 dB, placed behind the output coupler, which was functioned as SA for Q-switched pulse generation. About 50% of the cavity energy was outputted by a 3dB fiber fused coupler. Thus, a simple ring cavity resonator was established with total length of ∼9 m.
Fig. 4. (a) Schematic diagram of passively Q-switched Yb-doped fiber laser, (b) Q-switched pulse trains at different pump powers
While the pump power exceeded the threshold of ∼102.6 mW, stable Q-switched pulses were observed, as shown in Fig. 4(b)(i). The waveforms were measured by a photodetector (EOT-5000F) together with an oscilloscope (Tektronix, DPO7104C). Figure 4(b) exhibits the Q-switched pulse trains at pump powers of ∼102.6/111.2/120.6/128.4 mW, respectively. With increasing the pump power, the pulses have a narrower pulse duration and higher repetition frequency, which are in consistent with the characteristics of Q-switched pulse. Nevertheless, further increasing the pump power, the Q-switched pulses will become unstable since the pulse energy is close to the damage threshold of the SA. The typical Q-switched pulse properties were recorded at maximum pump power of ∼128.4 mW, as exhibited in Fig. 5. The Q-switched fiber laser operated at central wavelength of ∼1038 nm with minimum pulse duration of ∼1.5 μs, as can be seen in Figs. 5(a) and 5(b). The radio-frequency (RF) spectra of the Q-switched pulses with different frequency ranges have been measured by a spectrum analyzer (Agilent, E 4407B), as shown in Fig. 5(c). A moderate signal-noise ratio (SNR) of ∼30 dB at frequency of ∼67.9 kHz and a smooth wideband RF spectrum [Fig. 5(c) inset] imply the acceptable stability of the Q-switched operation. Moreover, the output properties of the Q-switched YDF laser with pump power varying from ∼102.6 mW to ∼128.4 mW are shown in Fig. 6. As the pump power increasing, the pulse duration reduces from ∼4.8 μs to ∼1.5 μs while the pulse repetition rate increases from ∼38.2 kHz to ∼67.9 kHz. Figure 6(b) depicts the evolution of average output power and pulse energy with pump power, in which the maximum output power and pulse energy of ∼9.2 mW and ∼0.14 μJ was obtained, respectively.
Fig. 5. Output characteristics of Yb-doped Q-switched fiber laser at pump power of ∼128.4 mW; (a) output spectrum; (b) pulse envelope; (c) RF spectra.
Fig. 6. (a) Pulse duration and repetition rate versus pump power; (b) Average output power and pulse energy versus pump power.
3.2 Passively Q-switched Er-doped fiber laser
For revealing the saturable absorption of MoO3-x in 1.5 μm waveband, the MoO3-x film was served as SA and integrated into a ring cavity Er-doped fiber (EDF) laser. Figure 7(a) plots the schematic diagram of the passively Q-switched EDF laser. The ring cavity resonator was constructed by a 980/1550 nm WDM, a section of ∼1.4 m long EDF (LIEKKIE r80-8/125), a fiber fused coupler, a piece of MoO3-x film (sandwiched between two fiber patch cords with insert loss of ∼3.3 dB) and a polarization independent isolator. The gain fiber was pumped by a 976 nm LD and about 45% of the cavity energy was extracted by a coupler. The total cavity length is ∼ 6 m.
Fig. 7. (a) Schematic diagram of Q-switched Er-doped fiber laser, (b) Q-switched pulse trains at different pump powers
When the MoO3-x film was inserted into the cavity, Q-switched operation was observed at pump power of ∼63.2 mW. The typical Q-switched pulse trains at pump power of ∼63.2/87.9/105.9/120.8 mW were measured separately, as shown in Fig. 7(b). In addition, we also measured the output characteristics of the passively Q-switched EDF laser, as exhibited in Fig. 8 and Fig. 9, respectively. Figure 8 illustrates the Q-switched properties at the maximum pump power of ∼120.8 mW. The Q-switched laser operated at ∼1562.9 nm with 3 dB bandwidth of ∼1 nm. Under the pump power of ∼120.8 mW, we have achieved the minimum pulse duration of ∼2.2 μs, as shown in Fig. 8(b). The SNR of ∼48 dB at frequency of ∼45.4 kHz [shown in Fig. 8(c)] indicates that the laser operated with a relatively good stability. Figures 9(a) and (b) exhibit the functions of pump power versus pulse repetition rate, pulse duration, output power and pulse energy respectively. With increasing the pump power from ∼63.2 mW to ∼120.8 mW, the pulse duration was reduced from ∼8.2 μs to ∼2.2 μs, the pulse repetition rate and output power were increased from ∼29 kHz to ∼45.4 kHz and ∼5.5 mW to 12.2 mW, respectively. Moreover, the maximum pulse energy of ∼0.29 μJ was calculated.
Fig. 8. Output characteristics of Er-doped Q-switched fiber laser at pump power of ∼120.8 mW, (a) output spectrum; (b) single pulse envelope; (c) RF spectra.
Fig. 9. (a) Pulse duration and repetition rate versus pump power; (b) Average output power and pulse energy versus pump power.
3.3 Passively Q-switched Tm-doped fiber laser
Finally, a passively Q-switched Tm-doped fiber (TDF) laser was constructed for further proofing the wideband saturable absorption of the MoO3-x. Similar with the EDF laser [Fig. 7(a)], a simple and compact ring cavity was employed, as plotted in Fig. 10(a). The ring cavity has the total length of ∼7.5 m which includes a piece of ∼3 m TDF (SM-TSF-9/125, Nufern). A 1560 nm fiber laser was used as pump source which was coupled into the TDF by a 1550/2000 nm WDM. A 2 μm polarization independent isolator was used for ensuring the unidirectional propagation of the laser. A coupler with output ratio of ∼23% was used for laser outputting, which was put before the MoO3-x-SA. The MoO3-x-PVA film also sandwiched between fiber patch cords as Q-switcher with insert loss of ∼4.1 dB.
Fig. 10. (a) Schematic diagram of Q-switched Tm-doped fiber laser, (b) Evolution of Q-switched pulse trains at different pump power.
Q-switched operation of the TDF laser was initialized when the pump power reached ∼212 mW. Figure 10(b) exhibits the evolution of Q-switched pulse trains with pump power increasing from ∼212 mW to ∼358 mW. The details of the Q-switched properties were recorded at maximum pump power ∼358 mW, as exhibited in Fig. 11. The Q-switched TDF laser has a central wavelength of ∼1910nm, as shown in Fig. 11(a). The dips in the optical spectrum were mainly considered as the results of water absorption lines [22]. The pulse properties were given in Fig. 11(b). The Q-switched pulses have a pulse duration of ∼1.6 μs. The SNR of ∼38 dB at frequency of ∼60.1 kHz and the absence of spectral modulation in harmonic RF spectrum [shown in Fig. 11(c) inset] further verify the relatively stable Q-switched operation. As the pump power increased from ∼212 mW to ∼358 mW, the pulse duration could be reduced from ∼8.2 μs to ∼1.6 μs and the repetition rate increased from ∼24.3 kHz to ∼60.1 kHz, as shown in Fig. 12. The maximum average output power of ∼7.9 mW and maximum pulse energy of ∼0.15 μJ were obtained.
Fig. 11. Output characteristics of Tm-doped Q-switched fiber laser at pump power of ∼120.8 mW, (a) output spectrum; (b) single pulse envelope; (c) RF spectra.
Fig. 12. (a) Pulse duration and repetition rate versus pump power; (b) Average output power and pulse energy versus pump power.
For more clearly illustrating the passively Q-switching properties of the MoO3-x wideband SA, Table 1 shows a comparison of wideband passively Q-switched fiber lasers based on other SAs. Compared with the previously reported results based on MoS2 and MoSe2, in our work, the obtained pulse durations were shorter than the corresponding results of the other two wideband SAs based Q-switched pulses. Meanwhile, the MoO3-x-SA based wideband Q-switched fiber lasers exhibited a more evenly output properties during the ∼900 nm wavelength range (the corresponding minimum pulse durations are ∼1.5/2.2/1.6 μs and average output powers are ∼9.2/12.2/7.9 mW at the three laser wavelengths). In addition, in our experiments, all the fiber lasers are operated without polarization controller, which indicates a polarization insensitive property of the wideband MoO3-x-SA. The excellent Q-switched performance of MoO3-x in ∼900 nm wavelength range is the direct evidence of wideband saturable absorption of MoO3-x, which may have potential applications for future infrared photonics devices.
Table 1. Comparison of passively Q-switched Yb-, Er- and Tm-doped fiber lasers with other wideband SAs
The sub-bandgap wideband saturable absorption of the MoO3-x-SA was mainly contributed from the oxygen vacancies, since the MoO3 with bandgap value of ∼3.1 eV does not exhibit the saturable absorption in infrared wavelength. Moreover, the plasmon resonance may strengthen the wideband nonlinear absorptions [42]. Compared with the other wideband optical SA based on the random edge defects, the MoO3-x-SA with vacancy defects has a better repeatability.
Moreover, a tendency of decline of pulse energy was observed with high pump power, as shown in Figs. 6(b), 9(b) and 12(b). The reason for such a phenomenon was considered as the performance degradation of PVA film instead of the MoO3-x. Firstly, the MoO3-x has a better temperature resistance than the PVA film [39]. In addition, the thermal dispersion performance of PVA film is bad. In fact, as we know, the pulse will be unstable and disappear gradually when the pump power approaches the damage threshold. However, we noted that the pulse can be reappeared with further increasing the pump power. Such a phenomenon further proves the health of MoO3-x, and the damaged PVA film causes the large loss. According to maximum pulse energy, a damage threshold of the SA was roughly calculated with value of ∼1.3 μJ/cm2.
During the experiment, no stable passively mode-locked operations were observed. The possible reasons are as following: 1) For standard single mode fiber mode-locked laser, a polarization controller (PC) was needed to optimize the polarization states and initialize the mode-locked operation. But no PCs were used in our experiments. 2) Besides the balance of dispersion and nonlinear effects, the small cavity round-trip period (short cavity length) and large cavity loss are unfavorable for passive mode-locking [44]. The typical cavity length in our experiments is less than ∼9 m and total loss larger than ∼5.2 dB, which limit the mode-locked operation.
4. Conclusion
We have experimentally demonstrated a novel ultra-wideband optical SA-MoO3-x, which was synthesized by controlling the oxygen vacancies in MoO3. To reveal the wideband optical saturable absorption, the passively Q-switched performances based on MoO3-x-PVA film were separately illustrated in three typical infrared fiber lasers (YDF, EDF and TDF lasers) with bandwidth of ∼900 nm. Based on the MoO3-x-SA, Q-switched pulses with pulse durations of ∼1.5/2.2/1.6 μs and average output powers of ∼9.2/12.2/7.9 mW at corresponding central wavelengths of ∼1038/1562.9/1910nm were obtained, respectively. This is, to our knowledge, the first demonstration of ultra-wideband Q-switching based on MoO3-x. The comparative performance indicates that the MoO3-x may have potential photonic applications. Furthermore, the saturable absorption of MoO3-x in mid-infrared wavelength (∼3 μm) could be expected.
Funding
National Natural Science Foundation of China (61575129, 61805156).
Acknowledgments
We thank Prof. P. Yan, C. Guo and J. Wang for providing the necessary fiber components, we also thank Dr. Z. Jin for fruitful discussions.
Disclosures
The authors declare no conflicts of interest.
References
1. K. C. Phillips, H. H. Gandhi, E. Mazur, and S. K. Sundaram, “Ultrafast laser processing of materials: a review,” Adv. Opt. Photonics 7(4), 684 (2015). [CrossRef]
2. M. Malinauskas, A. Žukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: from science to industry,” Light: Sci. Appl. 5(8), e16133 (2016). [CrossRef]
3. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, 2 µm Laser Sources and Their Possible Applications, Frontiers in Guided Wave Optics and Optoelectronics (IntechOpen, 2010).
4. D. Sebbag, A. Korenfeld, and S. Noach, “Passively Q-switched 2 μm Tm-doped laser for medical applications,” in Advanced Solid-State Lasers Congress (2015), paper AF2A.7.
5. T. Hakulinen and O. G. Okhotnikov, “8 ns fiber laser Q switched by the resonant saturable absorber mirror,” Opt. Lett. 32(18), 2677–2679 (2007). [CrossRef]
6. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, “Wideband-tuneable, nanotube mode-locked, fibre laser,” Nat. Nanotechnol. 3(12), 738–742 (2008). [CrossRef]
7. A. Martinez and Z. Sun, “Nanotube and graphene saturable absorbers for the fibre lasers,” Nat. Photonics 7(11), 842–845 (2013). [CrossRef]
8. Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic layer graphene as saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
9. J. Sotor, G. Sobon, W. Macherzynski, P. Paletko, and K. M. Abramski, “Mode-locking in Er-doped fiber laser based on mechanically exfoliated Sb2Te3 saturable absorber,” Opt. Mater. Express 4(1), 1 (2014). [CrossRef]
10. Z. Luo, C. Liu, Y. Huang, D. Wu, J. Wu, H. Xu, Z. Cai, Z. Lin, L. Sun, and J. Weng, “Topological-Insulator Passively Q-Switched Double-Clad Fiber Laser at 2 μm Wavelength,” IEEE J. Sel. Top. Quantum Electron. 21(1), 204 (2013). [CrossRef]
11. H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P. Loh, “Molybdenum disulfide (MoS2) as a broadband saturable absorber for ultra-fast photonics,” Opt. Express 22(6), 7249–7260 (2014). [CrossRef]
12. S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, and J. Wang, “Broadband Few-Layer MoS2 Saturable Absorbers,” Adv. Mater. 26(21), 3538–3544 (2014). [CrossRef]
13. 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 fibre lasers Q-switched by a broadband few-layer MoS2 saturable absorber,” J. Lightwave Technol. 32(24), 4679–4686 (2014). [CrossRef]
14. 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 Tm-doped fiber lasers,” Opt. Express 23(15), 20051–20061 (2015). [CrossRef]
15. D. Mao, B. Du, D. Yang, S. Zhang, Y. Wang, W. Zhang, X. She, H. Cheng, H. Zeng, and J. Zhao, “Nonlinear Saturable Absorption of Liquid-Exfoliated Molybdenum/Tungsten Ditelluride Nanosheets,” Small 12(11), 1489–1497 (2016). [CrossRef]
16. Z. Xie, F. Zhang, Z. Liang, T. Fan, Z. Li, X. Jiang, H. Chen, J. Li, and H. Zhang, “Revealing of the ultrafast third-order nonlinear optical response and enabled photonic application in two-dimensional tin sulfide,” Photonics Res. 7(5), 494–502 (2019). [CrossRef]
17. K. Niu, R. Sun, Q. Chen, B. Man, and H. Zhang, “Passively mode-locked Er-doped fiber laser based on SnS2 nanosheets as a saturable absorber,” Photonics Res. 6(2), 72–76 (2018). [CrossRef]
18. H. Zhang, P. Ma, M. Zhu, W. Zhang, G. Wang, and S. Fu, “Palladium selenide as a broadband saturable absorber for ultra-fast photonics,” Nanophotonics 9(8), 2557–2567 (2020). [CrossRef]
19. N. Xu, H. Wang, H. Zhang, L. Guo, X. Shang, S. Jiang, and D. Li, “Palladium diselenide as a direct absorption saturable absorber for ultrafast mode-locked operations: from all anomalous dispersion to all normal dispersion,” Nanophotonics (2020) https://doi.org/10.1515/nanoph-2019-0545.
20. K. Park, J. Lee, Y. T. Lee, W. K. Choi, J. H. Lee, and Y. W. Song, “Black phosphorus saturable absorber for ultrafast mode-locked pulse laser via evanescent field interaction,” Ann. Phys. 527(11-12), 770–776 (2015). [CrossRef]
21. X. Wang and S. Lan, “Optical properties of black phosphorus,” Adv. Opt. Photonics 8(4), 618 (2016). [CrossRef]
22. J. Sotor, G. Sobon, M. Kowalczyk, W. Macherzynski, P. Paletko, and K. M. Abramski, “Ultrafast thulium-doped fiber laser mode locked with black phosphorus,” Opt. Lett. 40(16), 3885–3888 (2015). [CrossRef]
23. C. Zhang, Y. Chen, T. Fan, Y. Ge, C. Zhao, H. Zhang, and S. Wen, “Sub-hundred nanosecond pulse generation from a black phosphorus saturable Q-switched Er-doped fiber laser,” Opt. Express 28(4), 4708–4716 (2020). [CrossRef]
24. C. Zhu, F. Wang, Y. Meng, X. Yuan, F. Xiu, H. Luo, Y. Wang, J. Li, X. Lv, L. He, Y. Xu, J. Liu, C. Zhang, Y. Shi, R. Zhang, and S. Zhu, “A robust and tuneable mid-infrared optical switch enabled by bulk Dirac fermions,” Nat. Commun. 8(1), 14111 (2017). [CrossRef]
25. Y. I. Jhon, J. Koo, B. Anasori, M. Seo, J. H. Lee, Y. Gogotsi, and Y. M. Jhon, “Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers,” Adv. Mater. 29(40), 1702496 (2017). [CrossRef]
26. X. Jiang, S. Liu, W. Liang, S. Luo, Z. He, Y. Ge, H. Wang, R. Cao, F. Zhang, Q. Wen, J. Li, Q. Bao, D. Fan, and H. Zhang, “Broadband Nonlinear Photonics in Few-Layer MXene Ti3C2Tx (T = F, O, or OH),” Laser Photonics Rev. 12(2), 1700229 (2018). [CrossRef]
27. N. Xu, P. Ma, S. Fu, X. Shang, and H. Zhang, “Tellurene-based saturable absorber to demonstrate large-energy dissipative soliton and noise-like pulse generations,” Nanophotonics 9(9), 2783–2795 (2020). [CrossRef]
28. Y. I. Jhon, J. Lee, M. Seo, J. H. Lee, and Y. M. Jhon, “van der Waals Layered Tin Selenide as Highly Nonlinear Ultrafast Saturable Absorber,” Adv. Opt. Mater. 7, 1801745 (2019). [CrossRef]
29. G. Zhao, J. Hou, Y. Z. Wu, J. L. He, and X. P. Hao, “Preparation of 2D MoS2/Graphene Heterostructure through a Monolayer Intercalation Method and its Application as an Optical Modulator in Pulsed Laser Generation,” Adv. Opt. Mater. 3(7), 937–942 (2015). [CrossRef]
30. C. Xing, Z. Xie, Z. Liang, W. Liang, T. Fan, J. S. Ponraj, S. C. Dhanabalan, D. Fan, and H. Zhang, “2D Nonlayered Selenium Nanosheets: Facile Synthesis, Photoluminescence, and Ultrafast Photonics,” Adv. Opt. Mater. 5(24), 1700884 (2017). [CrossRef]
31. H. Ahmad, M. Z. Samion, A. Muhamad, A. S. Sharbirin, and M. F. Ismail, “Passively Q-switched thulium-doped fiber laser with silver-nanoparticle film as the saturable absorber for operation at 2.0 µm,” Laser Phys. Lett. 13(12), 126201 (2016). [CrossRef]
32. N. Ming, S. Tao, W. Yang, Q. Chen, and H. Zhang, “Mode-locked Er-doped fiber laser based on PbS/CdS core/shell quantum dots as saturable absorber,” Opt. Express 26(7), 9017–9026 (2018). [CrossRef]
33. R. I. Woodward and E. J. Kelleher, “2D Saturable Absorbers for Fibre Lasers,” Appl. Sci. 5(4), 1440–1456 (2015). [CrossRef]
34. C. Ma, C. Wang, B. Gao, J. Adams, G. Wu, and H. Zhang, “Recent progress in ultrafast lasers based on 2D materials as a saturable absorber,” Appl. Phys. Rev. 6(4), 041304 (2019). [CrossRef]
35. T. Brezesinski, J. Wang, S. H. Tolbert, and B. Dunn, “Ordered mesoporous α-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors,” Nat. Mater. 9(2), 146–151 (2010). [CrossRef]
36. Y. Shi, B. Guo, S. A. Corr, Q. Shi, Y. S. Hu, K. R. Heier, L. Chen, R. Seshadri, and G. D. Stucky, “Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity,” Nano Lett. 9(12), 4215–4220 (2009). [CrossRef]
37. Z. Luo, R. Miao, T. D. Huan, I. M. Mosa, A. S. Poyraz, W. Zhong, J. E. Cloud, D. A. Kriz, S. Thanneeru, J. He, Y. Zhang, R. Ramprasad, and S. L. Suib, “Mesoporous MoO3-x Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions,” Adv. Energy Mater. 6(16), 1600528 (2016). [CrossRef]
38. H.-S. Kim, J. B. Cook, H. Lin, J. S. Ko, S. H. Tolbert, V. Ozolins, and B. Dunn, “Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x,” Nat. Mater. 16(4), 454–460 (2017). [CrossRef]
39. S. Huang, Y. Long, H. Yi, Z. Yang, L. Pang, Z. Jin, Q. Liao, L. Zhang, Y. Zhang, Y. Chen, H. Cui, J. Lu, X. Peng, H. Liang, S. Ruan, and Y.-J. Zeng, “Multifunctional molybdenum oxide for solar-driven water evaporation and charged dyes adsorption,” Appl. Surf. Sci. 491, 328–334 (2019). [CrossRef]
40. M. Wang, S. Huang, Y.-J. Zeng, J. Yang, J. Pei, and S. Ruan, “Passively Q-switched thulium-doped fiber laser based on oxygen vacancy MoO3-x saturable absorber,” Opt. Mater. Express 9(11), 4429–4437 (2019). [CrossRef]
41. W. Liu, Q. Xu, W. Cui, C. Zhu, and Y. Qi, “CO2-Assisted Fabrication of Two-Dimensional Amorphous Molybdenum Oxide Nanosheets for Enhanced Plasmon Resonances,” Angew. Chem., Int. Ed. 56(6), 1600–1604 (2017). [CrossRef]
42. W. Wang, W. Yue, Z. Liu, T. Shi, J. Du, Y. Leng, R. Wei, Y. Ye, C. Liu, X. Liu, and J. Qiu, “Ultrafast Nonlinear Optical Response in Plasmonic 2D Molybdenum Oxide Nanosheets for Mode-Locked Pulse Generation,” Adv. Opt. Mater. 6(17), 1700948 (2018). [CrossRef]
43. M. Kauranen and A. V. Zayats, “Nonlinear plasmonics,” Nat. Photonics 6(11), 737–748 (2012). [CrossRef]
44. U. Keller and K. J. Weingarten, “Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 2(3), 435–453 (1996). [CrossRef]
