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Abstract
Two-dimensional molecular crystals (2DMCs) are emerging ideal materials for future high-performance optoelectronic devices. People are constantly exploring new methods to solve the problem of difficult growth. Here, we design an improved chemical reaction-assisted vertical micro sublimation method based on vertical micro sublimation for the growth of two-dimensional Sb2O3 inorganic molecular crystals for the first time. The saturation absorption characteristics of the self-made Sb2O3 2DMCs were systematically tested, and the Q-switched laser output characteristics of Sb2O3 2DMCs at 1 µm were verified by using Nd: GYAP mixed crystal. The maximum average output power of 231 mW was achieved at 1080.6 nm, corresponding shortest pulse with a duration of 472 ns and maximum pulse repetition rate of 376 kHz. The maximum single pulse energy and the maximum peak power of the laser output were 0.614 µJ and 1.3 W, respectively. The growth of other 2DMCs would be motivated and its potential applications in the field of ultrafast photonics would be expanded with our findings.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Q-switching is one of the important techniques for generating short laser pulses which has important application prospects in industrial processing, laser communication, laser medicine, high-energy laser, remote sensing, scientific research, and so on [1–3]. Due to the high pulse energy and high peak power, a pulsed laser with a wavelength of around 1µm has many applications, such as free-space communication, nonlinear spectroscopy, biology, medicine, and military [4–6]. In the current methods that can achieve Q-switched pulsed laser output, the use of saturable absorbers (SA) as modulation devices has always been a research hotspot in the field of pulsed lasers. SA is a material that can reduce light absorption as light intensity increases [7–10]. Therefore, the SA can be used as a passive optical switch device, having the advantages of simple manufacturing method, low cost, compact structure, and broadband saturable absorption, which has attracted a lot of attention from researchers in recent years.
In recent years, two-dimensional (2D) materials, such as graphene, graphene oxide, black phosphorus (BP), topological insulators (TI), and transition metal dichalcogenides (TMDs) have been used as saturable absorbers. Meanwhile, Q-switched and mode-locked lasers with different laser gain media and different emission wavelengths have been successfully verified and applied [11–16]. Compared to the aforementioned SAs, two-dimensional molecular crystals (2DMCs) as a new family member of the 2D materials, have recently attracted extensive attention due to their intriguing properties including mall molecular thickness, long molecular order, diverse molecular structures, fewer defects, high charge transfer efficiency, and good flexibility [17–19]. However, the previous preparation methods of 2DMCs, including the chemical vapor deposition method (CVD), mechanical exfoliation method and solvothermal method, often have some common problems, such as complex experimental equipment processes, high environmental requirements, and long growth time [20–22]. At the same time, some of the methods are also easy to lead to weak anisotropy or the formation of one-dimensional rod-shaped crystals [23]. Therefore, growth difficulty has always been an important problem in the study of 2DMCs. Therefore, there is an urgent need for a customized, simple and low-cost two-dimensional material preparation method.
Herein, we successfully synthesized 2D inorganic bimolecular crystal Sb2O3 on mica substrate combining vertical micro sublimation (VMS) method and chemical reaction for the first time. In previous methods, the reaction temperature and reaction time of chemical vapor deposition are difficult to be accurately controlled, the materials obtained by mechanical stripping are difficult to form thin-layer structure, the operation steps of the solvothermal method are complex and it is difficult to prepare a few-layer structure. These problems have been solved by this method. Also, the micro-spacing distance keeps the sublimated molecules confined to a narrow space and ensures that the temperature difference between the source material and mica is very small, which plays a key role in reducing the thickness of the product and improving the controllability of the final product [23,24]. Therefore, more uniform and thickness controllable samples can be prepared, so as to reduce unnecessary losses in laser experiments and give better play to the saturable absorption function. Specifically, this method is simpler and more practical and is much cheaper than commercial saturable absorbers like graphene and SESAMs. As a result, it can be large-scale promoted and has potential for industrial applications.
The saturable absorption properties of the self-made Sb2O3 2DMCs were systematically characterized. The optical modulations of Sb2O3 2DMCs in the generation of pulsed lasers were achieved on the b-cut Nd: GYAP (Nd: Gd0.1Y0.9AlO3) laser device for the first time, to the best of our knowledge. The 2D Sb2O3 molecular crystal is a two-level system. When the photon energy of the material is similar to the bandgap energy, the electrons from the valence band are excited into the conduction band [25]. Then the states in the valence band are depleted and the final states in the conduction band are partially occupied. Due to the Pauli blocking process, no two electrons can fill the same state. Therefore, further electron excitation is prevented and photon absorption stops, resulting in the saturated absorption [26].The maximum average output power of 231 mW was achieved at 1080.6 nm, corresponding shortest pulse with a duration of 472 ns and maximum pulse repetition rate of 376 kHz. The laser output with the maximum single pulse energy and the maximum peak power was 0.61 µJ and 1.30 W, respectively. Compared with the other 2D saturable absorbers, the preparation method of 2D-Sb2O3 is more simple and economical. It is easy to control the number of layers and growth morphology, and can make the sample have a better uniformity. At the same time, as an oxide, it has stable chemical properties, low environmental requirements and easy preservation. Further, compared with other 2D oxide saturable absorbers, the sample also has higher average output power, relatively narrow pulse width and higher repetition frequency in 1 µm band, as shown in Table 1, which shows extraordinary advantages. We believe that our work will provide an important reference for the growth of more 2DMCs and the potential applications of optical modulation-dependent nonlinear optical devices.
Table 1. The properties of some of 2D-Oxide as saturable absorbers
2. Preparation and characterization of the Sb2O3
In this paper, 2D Sb2O3 crystal was successfully prepared on mica substrate by combining the VMS method and chemical reaction for the first time. Details of the fabrication process are illustrated in Fig. 1. (a). In the process, hydrophilic SbCl3·xH2O was selected as the precursor, and a mica (KMg3(AlSi3O10)F2) was fixed above the precursor as the substrate. The distance between mica and reactant is about 5 cm so that the whole device can react in a small space. When the temperature of the heat stage was heated to the expected temperature of 320 °C, the whole device was placed in the center of the heat stage and heated to 400 °C. Finally, 2D Sb2O3 crystal was successfully grown on mica substrate. In the whole reaction process, the ratio of InCl3 to reactant is 1:20, and the whole reaction goes through the following reactions [30]:
The reaction can be divided into two steps:
A scanning electron microscope (SEM) was used to characterize the morphology of 2D Sb2O3 crystal samples. It can be seen from Fig. 1. that the Sb2O3 crystals are mainly hexagonal two-dimensional crystals. However, as Fig. 1. (b) shows that there are also some sectorial, branched Sb2O3 micro-structures, which are composed of long columnar micro-tubules in the prepared samples. This phenomenon may be since the Gibbs free energy of the newly formed Sb2O3 molecules after the temperature rise is higher than the Gibbs free energy of the molecules that have formed crystals. Therefore, the new molecules gather around the original crystals and form this micro-structure along the fastest growth direction [31]. Besides, it can be seen that the 2D Sb2O3 crystal appears to be flat distributed on the surface of the mica substrate, and the crystal size is relatively uniform. The 2D Sb2O3 crystal distribution in the film is dispersed, and the mica substrate is not completely covered, so the thickness of the film is equal to that of a 2D Sb2O3 crystal. Figure 1. (d) and Fig. 1. (e) show the XPS spectrum and fine spectrum of our sample, which proves that the component of the two-dimensional crystal we successfully grown is Sb2O3. A small amount of impurities of In and Cl comes from the passivator. The Raman spectrum of Sb2O3 film was measured for further characterizing by Raman spectrometer excited by 532 nm laser source with the power of 12.1 µW and beam diameter of 4 µm which is shown in Fig. 1. (d). The two main characteristic peaks represent the vibration modes of F2g and Ag, and they match well with the Raman spectrum results of the standard sample, which proves that our product is Sb2O3. We also observed Raman red-shift due to the size-induced phonon confinement effect and surface relaxation because of crystal size and dimension [32]. The linear optical transmission curve of Sb2O3 crystal in the wavelength range of 600–1200 nm was measured by UV-vis-NIR spectrophotometer (UV-3150, Shimadzu, Japan). As shown in Fig. 1. (e), the transmission spectrum of pure mica is recorded as a reference, which shows that it has a relatively flat transmittance curve with a 90.80% transmission at 1080 nm, while the transmittance of mica with Sb2O3 film is 84.94% at 1080 nm. Considering the loss of mica substrate, we calculated that the linear optical loss of Sb2O3 film is about 5.86%.
Fig. 1. (a) The fabrication process for 2D Sb2O3 crystal; (b) SEM image of the 2D Sb2O3 crystal; (c) A typical SEM image of hexagonal 2D Sb2O3 crystal; (d) XPS spectrum of 2D Sb2O3; (e)XPS fine spectrum; (f) Raman spectrum; (g) Linear optical transmission.
3. Experimental results and discussion
3.1 Experimental setup
In order to investigate the saturation absorption characteristics of the Sb2O3 saturable absorber, we used a Q-switched laser composed of Nd: GYAP disordered laser crystal grown by the Czochralski (Cz) method and Sb2O3 SA to verify [33,34]. The laser experimental setup is shown in Fig. 2. The pump source was a fiber-coupled laser diode emitting at 808 nm with a core diameter of 400 µm and numerical aperture (NA) was 0.22. The pumping beam was focused by an optical imaging system (1:1 image module) and was imaged into the Nd: GYAP disordered laser crystal with a spot radius of 200 µm. The Nd: GYAP disordered laser crystal, as the laser gain medium, was cut and polished along the b-axis with a Nd3+ doping concentration of 1 at. % and the dimension of 4×4×5 mm3. The Nd: GYAP disordered laser crystal for the laser experiment was uncoated. To efficiently reduce the thermal lensing effect, the Nd: GYAP disordered laser crystal was wrapped in indium foil and placed in a copper block with circulating cooling water at 17 °C for heat dissipation. The laser resonant cavity with a length of 34 mm consisted of two plane mirrors. The input mirror M1 was coated for high reflectance from 1050 nm to 1100 nm and high transmittance from 800 nm to 820 nm. The output coupler M2 was coated with 10% or 20% transmittance from 1050 nm to 1100 nm. For Q-switching operation, the Sb2O3 SA was inserted near the output coupler. A 1000 nm long-wavelength pass filter (Thorlabs, FEL 1000) was placed at the rear end of the output coupler to block the pump light. The Nd: GYAP which obtains the large FWHM at 1083 nm and long fluorescence lifetime can achieve a maximum output power up to 8.12 W under the absorbed power of 24.12 W, which is much higher than the laser output of some other Nd3+-doped disordered crystals [33,34]. It also has good thermal conductivity, which is conducive to the crucial control of the temperature stability of LD pump output wavelength and improve the stability of the laser. At the same time, as a disordered crystal, it can broaden the absorption and emission spectrum which is conducive to the further study of mode-locking and tunable broadband lasers based on the study of Q-switching.
3.2 CW operation and passively Q-switched operation
Firstly, the laser performances in the continuous wave (CW) regime without the Sb2O3 SA were investigated. In order to enable the Sb2O3 SA to be inserted into the cavity, the cavity length of the laser resonator was 34 mm. The pumped power absorption efficiency of the Nd: GYAP crystal was ∼77% [34]. The linear relationship between the CW output power and absorbed pump power is shown in Fig. 3 when the transmission rates of output coupler M2 were 10% and 20%, respectively. As shown in Fig. 3, the threshold for the CW operation were 0.605 W and 0.759 W for T = 10% and 20%, respectively. After linear fitting of the CW output power curve, the corresponding slope efficiency were 31.23% and 25.99%, respectively. For output coupler M2 with 10% transmittance, the maximum CW output power obtained was 0.92 W at the absorbed pump power of 1.172 W, which was higher than 0.693 W with the 20% transmittance output coupler M2. The corresponding optical to optical efficiency were 25.7% and 19.4%, respectively.
In order to better achieve passive Q-switched output, the mica substrate with Sb2O3 SA was inserted in the cavity near the output coupler M2 with 10% transmittance. As shown in Fig. 3, the relationship between the average output power of the Q-switched laser and the absorbed pumped power was investigated. When the Sb2O3 SA was inserted into the resonator, the loss in the cavity increases, which leads to the increase of laser threshold and the decrease of slope efficiency. The Nd: GYAP laser entered into Q-switched pulsed operation using T = 10% transmittance output coupler M2 when the absorbed pump power exceeded about 2.498 W. The highest Q-switching average output power of 231 mW was achieved when the absorbed pump power was at 5.477 W, corresponding to a linear slope efficiency of about 7.76%.
The laser pulses were detected by a fast InGaAs photodetector (Thorlabs, DET08C/M) connected to an Agilent digital oscilloscope (DSO90604A, 6 GHz). Figure 4 shows the relationship between the pulse width (full width at half maximum, FWHM), repetition rate, single pulse energy, and peak power as a function of absorption pump power. The pulse duration decreases as the power of the absorbed pump increases. When the absorbed pump power increased from 2.498 W to 5.477 W, the duration of the pulse decreased from 734 ns to 472 ns. The pulse repetition rate is increased with the increase of the absorbed pump power. As the pump power increased from 2.498 W to 5.477 W, the pulse repetition rate increased from 137 kHz to 376 kHz. According to these pulse characteristics, we can also approximately calculate the single pulse energy and peak power. Both the single pulse energy and peak power increase linearly with the increase of absorbed pump power. When the maximum absorbed pump power is 5.477 W, the maximum single pulse is 0.61 µJ, and the maximum peak power is 1.30 W. As shown in Fig. 5, the temporal pulse train and single pulse profile were measured under maximum absorbed pump power of 5.477 W. The shortest pulse duration of 472 ns was obtained and the corresponding repetition rate was 376 kHz. When the pump power continues to increase, theoretically, the average output power can continue to increase and the pulse width can be reduced and the repetition frequency can be increased. However, since the SA did not add a heat sink, the pulse sequence will become unstable when it operates at higher power.
Fig. 4. Dependences of pulse width, repetition rate, single pulse energy, and peak power on absorbed pump power.
To eliminate the influence of the mica sheet on the experiment, we inserted a blank mica sheet at the position of Sb2O3-SA for an experiment. It shows that when a mica sheet is inserted, a series of irregular waveforms will be generated. With the increase of absorbed pump power, the pulse width will decrease and the repetition frequency will increase to a certain extent. Due to its irregular waveform, it can only be roughly measured that when the absorption pump power increases from 2.498W to 5.477W, the pulse width decreases from ∼1.8 µs to ∼600 ns, and the repetition frequency increases from ∼100kHz to ∼450kHz.
The laser emission spectra in CW and Q-switching regimes were monitored by an optical spectrum analyzer (Zolix, Omni-λ300) and are shown in the insert of Fig. 6. For the CW operation, the output spectrum for CW regime had two emission peaks located at the central wavelength of 1073.1 nm and 1079.7 nm. Under the Q-switching regime, the central emission wavelength was red-shifted to be 1080.6 nm and the emission at 1073.1 nm was weakened. At the same time, the FWHM of the emission spectrum at 1080.6 nm was narrowed to be 0.9 nm (CW: FWHM = 1.2 nm). Such change is mainly due to the insertion loss caused by the Sb2O3 SA, which increases the cavity loss and prevents the oscillation of the low gain laser mode. The beam quality and two-dimensional beam pattern of the Nd: GYAP laser in the Q-switching regime were examined by using a laser beam analyzer (Spiricon, Inc. M2−200 s). As shown in Fig. 7, the beam quality factors Mx2 and My2 were determined to be 1.91 and 1.89, respectively.
Fig. 6. The laser emission spectra of Nd: GYAP laser in CW and Q-switching regimes. Insert: 2D profile of laser beam.
Fig. 7. The beam quality and two-dimensional beam pattern of the Nd: GYAP laser in Q-switching regime.
4. Conclusion
In summary, we have successfully grown high-quality two-dimensional Sb2O3 inorganic molecular crystals by a chemical reaction-assisted vertical micro sublimation method for the first time. The results show that the synthetic method is simpler and more practical. Moreover, the as-grown Sb2O3 2DMCs were employed to achieve a generation of pulsed lasers in the Nd: GYAP solid-state bulk laser for the first time, to the best of our knowledge. The maximum average output power of 231 mW was achieved at 1080.6 nm, corresponding shortest pulse with a duration of 472 ns and maximum pulse repetition rate of 376 kHz. The laser output with the maximum single pulse energy and the maximum peak power were 0.61 µJ and 1.30 W, respectively. Therefore, the experimental results show that the 2D Sb2O3 crystal device has the advantages of easy preparation, low cost and low environmental requirements, and can be applied in the low-cost and compact solid-state Q-switched laser to achieve high power and high repetition rate compact pulsed laser output. Our results would motivate the synthesis of more 2DMCs and expand its potential applications in the field of ultrafast photonics.
Funding
National Natural Science Foundation of China (51702124, 51872307, 51972149, 61935010); Key-Area Research and Development Program of Guangdong Province (2020B090922006); Guangzhou Science and Technology project (201904010385, 201904010094); Fundamental Research Funds for the Central Universities (21620445).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. 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]
2. B.M. Walsh, J.M. McMahon, W.C. Edwards, N.P. Barnes, R.W. Equall, and R.L. Hutcheson, “Spectroscopic characterization of Nd: Y2O3: application toward a differential absorption lidar system for remote sensing of ozone,” J. Opt. Soc. Am. B 19(12), 2893–2903 (2002). [CrossRef]
3. K. Meiners-Hagen, R. Schödel, F. Pollinger, and A. Abou-Zeid, “Multi-wavelength interferometry for length measurements using diode lasers, Meas,” Sci. Rev. 9, 16 (2009). [CrossRef]
4. J. Ma, W. Jiang, C. Shen, and S. Yuan, “Passively Q-switched Yb: YAG laser based on a MoSe2 saturable absorber,” Appl. Opt. 57(8), 1958–1961 (2018). [CrossRef]
5. M. W. Wright and G. C. Valley, “Yb-doped fiber amplifier for deep-space optical communications,” J. Lightwave Technol. 23(3), 1369–1374 (2005). [CrossRef]
6. S. N. Son, J.-J. Song, J. U. Kang, and C.-S. Kim, “Simultaneous second harmonic generation of multiple wavelength laser outputs for medical sensing,” Sensors 11(6), 6125–6130 (2011). [CrossRef]
7. X. Li, X. Yu, Z. Sun, Z. Yan, B. Sun, Y. Cheng, X. Yu, Y. Zhang, and Q. J. Wang, “High-power graphene mode-locked Tm/Ho co-doped fiber laser with evanescent field interaction,” Sci. Rep. 5(1), 16624 (2015). [CrossRef]
8. C. Wang, L. Wang, X. Li, W. Luo, T. Feng, Y. Zhang, P. Guo, and Y. Ge, “Few-layer bismuthene for femtosecond soliton molecules generation in Er-doped fiber laser,” Nanotechnology 30(2), 025204 (2019). [CrossRef]
9. G.-R. Lin and Y. Lai, “A novel phase-noise-suppressed and delay-time-tunable mode-locked erbium-doped fiber laser,” Opt. Commun. 212(1-3), 169–175 (2002). [CrossRef]
10. L. Lu, W. Wang, L. Wu, X. Jiang, Y. Xiang, J. Li, D. Fan, and H. Zhang, “All-optical switching of two continuous waves in few layer bismuthene based on spatial cross-phase modulation,” ACS Photonics 4(11), 2852–2861 (2017). [CrossRef]
11. F. Lou, R. Zhao, J. He, Z. Jia, X. Su, Z. Wang, J. Hou, and B. Zhang, “Nanosecond-pulsed, dual-wavelength, passively Q-switched ytterbium-doped bulk laser based on few-layer MoS2 saturable absorber,” Photonics Res. 3(2), A25–A29 (2015). [CrossRef]
12. P. Li, G. Zhang, H. Zhang, C. Zhao, J. Chi, Z. Zhao, C. Yang, H. Hu, and Y. Yao, “Q-switched mode-locked Nd: YVO4 laser by topological insulator Bi2Te3 Saturable Absorber,” IEEE Photonics Technol. Lett. 26(19), 1912–1915 (2014). [CrossRef]
13. Y.-J. Sun, C.-K. Lee, J.-L. Xu, Z.-J. Zhu, Y.-Q. Wang, S.-F. Gao, H.-P. Xia, Z.-Y. You, and C.-Y. Tu, “Passively Q-switched tri-wavelength Yb3+: GdAl3(BO3)4 solid-state laser with topological insulator Bi2Te3 as saturable absorber,” Photonics Res. 3(3), A97–A101 (2015). [CrossRef]
14. X. Sun, H. Nie, J. He, R. Zhao, X. Su, Y. Wang, B. Zhang, R. Wang, and K. Yang, “Passively mode-locked 1.34 µm bulk laser based on few-layer black phosphorus saturable absorber,” Opt. Express 25(17), 20025–20032 (2017). [CrossRef]
15. M. Fan, T. Li, S. Zhao, G. Li, H. Ma, X. Gao, C. Kränkel, and G. Huber, “Watt-level passively Q-switched Er: Lu2O3 laser at 2.84 µm using MoS2,” Opt. Lett. 41(3), 540–543 (2016). [CrossRef]
16. Z. Wang, R. Zhao, J. He, B. Zhang, J. Ning, Y. Wang, X. Su, J. Hou, F. Lou, and K. Yang, “Multi-layered black phosphorus as saturable absorber for pulsed Cr: ZnSe laser at 2.4 µm,” Opt. Express 24(2), 1598–1603 (2016). [CrossRef]
17. J.-K. Sun, Y. I. Sobolev, W. Zhang, Q. Zhuang, and B. A. Grzybowski, “Enhancing crystal growth using polyelectrolyte solutions and shear flow,” Nature 579(7797), 73–79 (2020). [CrossRef]
18. C.-H. Lee, L. Liu, C. Bejger, A. Turkiewicz, T. Goko, C. J. Arguello, B. A. Frandsen, S. C. Cheung, T. Medina, and T. J. Munsie, “Ferromagnetic ordering in superatomic solids,” J. Am. Chem. Soc. 136(48), 16926–16931 (2014). [CrossRef]
19. X. Qiu, V. Ivasyshyn, L. Qiu, M. Enache, J. Dong, S. Rousseva, G. Portale, M. Stöhr, J. C. Hummelen, and R. C. Chiechi, “Thiol-free self-assembled oligoethylene glycols enable robust air-stable molecular electronics,” Nat. Mater. 19(3), 330–337 (2020). [CrossRef]
20. A. Jones, K. Mistry, M. Kao, A. Shahin, M. Yavuz, and K. P. Musselman, “In-situ spatial and temporal electrical characterization of ZnO thin films deposited by atmospheric pressure chemical vapour deposition on flexible polymer substrates,” Sci. Rep. 10(1), 1–10 (2020). [CrossRef]
21. M. Yi and Z. Shen, “A review on mechanical exfoliation for the scalable production of graphene,” J. Mater. Chem. A 3(22), 11700–11715 (2015). [CrossRef]
22. H. Bai, X. Li, F. Guo, B. Zhang, Q. Yang, L. Zhang, Y. Song, and Y. Huang, “Comparative study on the growth mechanism of multi-shaped PbS via hydro- and solvothermal methods,” Opt. Mater. Express 9(2), 932–943 (2019). [CrossRef]
23. X. Feng, Z. Sun, K. Pei, W. Han, F. Wang, P. Luo, J. Su, N. Zuo, G. Liu, and H. Li, “2D inorganic bimolecular crystals with strong in-plane anisotropy for second-order nonlinear optics,” Adv. Mater. 32(32), 2003146 (2020). [CrossRef]
24. L Tang, T Li, Y Luo, S Feng, Z Cai, H Zhang, B Liu, and HM Cheng, “Vertical chemical vapor deposition growth of highly uniform 2D transition metal dichalcogenides,” ACS Nano 14(4), 4646–4653 (2020). [CrossRef]
25. S. Zhang, N. Dong, N. McEvoy, M. O’Brien, S. Winters, N. C. Berner, C. Yim, Y. Li, X. Zhang, Z. Chen, L. Zhang, G. S. Duesberg, and J. Wang, “Direct observation of degenerate two-photon absorption and its saturation in WS2 and MoS2 monolayer and few-layer films,” ACS Nano 9(7), 7142–7150 (2015). [CrossRef]
26. R. N. Zitter, “Saturated optical absorption through band filling in semiconductors,” Appl. Phys. Lett. 14(2), 73–74 (1969). [CrossRef]
27. X. Feng, J. Liu, W. Yang, X. Yu, S. Jiang, T. Ning, and J. Liu, “Broadband indium tin oxide nanowire arrays as saturable absorbers for solid-state lasers,” Opt. Express 28(2), 1554–1560 (2020). [CrossRef]
28. D. Mao, X. Cui, Z. He, H. Lu, W. Zhang, L. Wang, Q. Zhuang, S. Hua, T. Mei, and J. Zhao, “Broadband polarization-insensitive saturable absorption of Fe2O3 nanoparticles,” Nanoscale 10(45), 21219–21224 (2018). [CrossRef]
29. H. Ahmad, H. S. Albaqawi, N. Yusoff, S. A. Reduan, and C. W. Yi, “Reduced graphene oxide-silver nanoparticles for optical pulse generation in ytterbium- and erbium-doped fiber lasers,” Sci Rep 10(1), 9408 (2020). [CrossRef]
30. W. Han, P. Huang, L. Li, F. Wang, P. Luo, K. Liu, X. Zhou, H. Li, X. Zhang, and Y. Cui, “Two-dimensional inorganic molecular crystals,” Nat. Commun. 10(1), 1–10 (2019). [CrossRef]
31. L. Song, S. Zhang, and Q. Wei, “Perfect, sectorial, branched Sb2O3 microstructures consisting of prolate microtubes: controllable seeded growth synthesis and optical properties,” Cryst. Growth Des. 12(2), 764–770 (2012). [CrossRef]
32. C. Yang and S. Li, “Size-dependent Raman red shifts of semiconductor nanocrystals,” J. Phys. Chem. B 112(45), 14193–14197 (2008). [CrossRef]
33. D. Li, Q. Liu, P. Zhang, H. Zhou, S. Zhu, Y. Zhang, Z. Li, H. Yin, Z. Chen, and Y. Hang, “Crystal growth, optical properties and laser performance of new mixed Nd3+ doped Gd0.1Y0.9AlO3 crystal,” J. Alloys Compd. 789, 664–669 (2019). [CrossRef]
34. H. Zhou, S. Zhu, Z. Li, H. Yin, P. Zhang, Z. Chen, S. Fu, Q. Zhang, and Q. Lv, “Investigation on 1.0 and 1.3 µm laser performance of Nd3+: GYAP crystal,” Opt. Laser Technol. 119, 105601 (2019). [CrossRef]
