Open Access
Add to BibSonomy
Abstract
Both reduced graphene oxide (RGO) and gold (Au) nanoparticles have shown distinct nonlinear optical characteristics, such as saturable absorption, that are appropriate for generating ultrashort pulse lasers. Unfortunately, there are no studies of Q-switched waveguide lasers using RGO and RGO modified with Au nanoparticles (Au-RGO) as saturable absorbers. Here we perform the pump-probe experiments to investigate the nonlinear optical absorption properties of RGO and Au-RGO. Our results show that, compared with RGO, Au-RGO has advantages of a higher modulation depth, faster optical response and wavelength-independent nonlinear absorption, which demonstrates that nonlinear optical properties of Au-RGO have been enhanced owing to the Au nanoparticles. Moreover, the laser emission of Q-switched Nd:YAG channel waveguidewith Au-RGO as a saturable absorber has been obtained. In comparison with the waveguide laser system utilizing RGO, the Q-switched laser observed with Au-RGO exhibits a shorter pulse duration and a higher repetition frequency, indicating the potential application of Au-RGO as a cost-effective saturable absorber in the pulsed waveguide laser systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene, a flat monolayer of sp2 hybridized carbon atoms, was discovered less than two decades ago and has attracted tremendous attention in practical applications of energy storage systems, batteries, catalysis, sensors, solar cells and solid-state laser systems due to its excellent structural, electrical, magnetic and optical properties [1–5]. Particularly, benefiting from its broadband saturable absorption characteristics, the graphene has been demonstrated as an effective saturable absorber (SA) for generation of pulse lasers in various wavelength regions. These pulses include passively mode-locked lasers as well as Q-switched lasers, ranging from bulk, fiber to waveguide systems [6–11]. However, it is a challenge to fabricate large-scale and high-quality graphene regardless of using mechanical exfoliation and chemical vapor deposition. Accordingly, so much effort is gradually transferred to the reduction of graphene oxide to produce reduced graphene oxide (RGO) [12], which can be easily obtained by the Hummers and Offeman method [13,14]. In recent years, RGO, one of the most investigated derivatives of graphene, has received increasing attention because of its similar layered structure as graphene, simple preparation process in large quantities than perfect single-layer or few-layer graphene, large specific surface area, chemical stability and excellent nonlinear optical properties [15–17], which make it a promising replacement of graphene and an efficient SA for realization of laser pulses [18,19].
Owing to the admirable biocompatibility and stability at high temperature, noble-metal nanoparticles (e.g., Au, Ag) with surface plasmon resonance (SPR) have become one of the most widely used novel metal-based nanomaterials [20,21]. In particular, Au nanoparticles have been regular introduced into RGO systems to avoid the aggregation because SPR strongly depends on their environment, shape and size [22]. Thus, RGO can be regarded as an ideal carrier for Au nanoparticles, leading to an effective excitation of the surface plasmons as well as implementing intriguing applications in electrocatalysis, photodetectors and photocatalysts [23–25]. In addition, to improve its original optical absorption properties, RGO has also been modified by introducing Au nanostructures, resulting in an enhanced and multiple nonlinear optical response [26].
Over the years, superior lasing performances based on a waveguide structure as the gain media have been achieved due to the tight confinement of the pump and laser modes in an ultra-compact volume [27–29], which is important for construction of monolithically integrated light systems. Recently, various nanomaterials, such as carbon nanotubes [30], graphene [9], transition metal dichalcogenides [31], noble-metal nanoparticles [32], etc. have been used as saturable absorbers in the waveguide laser system to generate waveguide pulsed lasers owing to their strong light-material interactions and obvious saturable absorption properties. Note that, in the field of ultrafast pulsed lasers and nonlinear optics, although some investigation of nonlinear optical properties of RGO and gold nanoparticle-coated RGO (Au-RGO) have been reported [19], [26], [33–35], no researcher has yet achieved the output of Q-switched waveguide laser pulses with them as a SA.
In this work, RGO and Au-RGO were prepared and a series of experiments were performed to characterize their morphology and optical properties. The linear absorption intensity of Au-RGO was 1.5 times larger than RGO in the spectral range of 200–1100 nm. The nonlinear absorption of Au-RGO has also been enhanced by Au-nanoparticles modification, which offers the possibility to optimize the saturable absorption characteristics of RGO. In addition, the Q-switched waveguide lasers were generated from the Nd:YAG channel waveguide with RGO and Au-RGO as saturable absorbers, respectively. The results show that maximum output power and minimum pulse duration were 61 mW and 27.9 ns for Au-RGO, while maximum output power and minimum pulse duration obtained from RGO system at the same absorbed power were only about 39 mW and 38.7 ns, indicating that Au-RGO is an effective SA in the integrated waveguide platform.
2. Experiment in details
2.1 Preparation and characterization of RGO and Au-RGO
RGO and Au-RGO powders were provided by XFNANO Materials Company (Nanjing, Jiangsu, China). RGO dispersions were fabricated in the following three steps: Firstly, 250 mg of RGO powder was poured into 2.5 mg/mL PVP (polyvinyl pyrrolidone) aqueous solution which was used as a surfactant. Then, RGO aqueous solution was ultrasonically agitated for 1 hour to achieve dispersion. After that, the dispersed solution of RGO was centrifuged with the velocity of 2000 rpm (20 s) to remove sedimentation of the non-exfoliated large RGO clusters. Au-RGO dispersions were obtained by the identical procedure.
To characterize RGO and Au-RGO, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were performed. The linear absorption spectra were recorded under the wavelength ranging from 200 nm to 1100 nm using a U4100 UV-visible-IR spectrophotometer. The nonlinear optical properties were investigated by ultrafast pump-probe spectroscopy based on a pump pulse with a peak fluence of 200 µJ/cm2 (100 fs, 400 nm) and wide broadband probed pulse (450–750 nm). During the measurement, the pump pulse was used to photoexcite carries and a subsequent probe pulse to measure the time resolved differential transmission changes at different probe photon energies. By varying the time between pump and probe pulses, the time resolved differential transmission spectra were recorded, which can retrieve the transmittance recovery timescale of RGO and Au-RGO, respectively. All measurements described above were performed at room temperature.
2.2 Waveguide laser setup
The experimental configuration of Q-switched waveguide laser is shown in Fig. 1(a). A Ti:sapphire laser (Coherent MBR 110) was used as the pumping source, which delivers linearly polarized continuous light with a central wavelength of 810 nm. The resonant cavity consisted of a Nd:YAG channel waveguide (length 9 mm), an input mirror (the reflectivity of 99.9% at 1064 nm) and an output mirror (the reflectivity of 99.9% at 810 nm). Nd:YAG channel waveguide shown in Fig. 1(b), a semicircle columnar structure with the width of 20 µm (depth: 3.8 µm), was fabricated by ion irradiation and femtosecond laser writing technique. Details of the fabrication of Nd:YAG channel waveguide have been reported in [36]. 800 µL of RGO and Au-RGO dispersions were dropped on the surface of Nd:YAG waveguide respectively and dried at room temperature to form films. The atomic force microscopy (AFM) is carried out to measure the thickness of RGO and Au-RGO films on the surface of Nd:YAG waveguide. With RGO film or Au-RGO film as the SA, the pump light was coupled into the waveguide using a plano-convex lens (focal length 20 mm). The generated laser from the resonant cavity was collected with a microscope objective (N.A. = 0.4) and monitored by a digital oscilloscope, an optical spectrum analyzer, a CCD and a power meter.
Fig. 1. (a) Schematic setup of Q-switched channel waveguide laser. (b) The end micrograph of channel waveguide.
3. Results and discussion
3.1 Morphology and optical properties of RGO and Au-RGO
The TEM and HRTEM images in Fig. 2 show the morphology of RGO and Au-RGO. As shown in Fig. 2(a), the RGO sheets possess wrinkled and folded morphology with a layered structure. Similarly, Fig. 2(b) displays the Au-RGO sheets are wrinkled and layered, which means that they have a similar morphology and size to that RGO sheets. Figure 2(c) shows a large number of black spots with an oval-shaped structure with the size in range from 3 nm to 25 nm (red circle region), which represents the formation of quasi-spherical Au nanoparticles dispersed on RGO sheets. Further HRTEM characterization in Fig. 2(d) shows the crystalline phase of Au, which can be attributed to the (111) face and (200) face of Au with the atomic distances around 0.201 nm (200) and 0.234 nm (111), respectively.
Fig. 2. Low-magnification TEM images of RGO (a) and Au-RGO (b). (c), (d) The representative HRTEM images of Au-RGO.
The XRD was further employed to characterize the presence of Au on RGO and the results are presented in Fig. 3(a). RGO possesses a broad and main diffraction peak at around 25 degrees, indicating an interlayer distance of 3.53 Å [37]. In comparison, the Au-RGO sample shows not only a broad feature peak at 25 degrees but also multiple sharp peaks assigned to the (111) and (200) faces of Au [38]. The strongest peak position locates at 38.3 degrees, corresponding to the Au (111) with interlayer distance of 0.235 nm, which is consistent with the HRTEM results and verifies that Au nanoparticles are successfully modified inside RGO. In addition, the elements in Au-RGO sheets can be seen by STEM and EDX analysis. By measuring the red rectangle in Fig. 3(b), the EDX images in Fig. 3(c) was obtained and clearly confirm the coexistence of Au nanoparticle, carbon and oxygen elements with the uniform distribution, which reveal that Au nanoparticles have been combined with RGO forming Au-RGO nanocomposites.
Fig. 3. (a) XRD patterns of RGO and Au-RGO. (b) High angle annular dark-field STEM image of Au-RGO. (c) Corresponding EDX element maps in red rectangle of (b).
Figure 4(a) shows few-layer RGO and Au-RGO films on a quartz substrate, which was used to study their linear and nonlinear optical properties. From Fig. 4(b), we can see that the UV-visible-IR linear absorption spectra of RGO and Au-RGO possess similar peak position and broadband absorption but different peak intensity. For RGO, the absorption peak intensity of 231 nm is stronger than the absorption peak of 270 nm, while Au-RGO shows the opposite trend, suggesting that RGO has been further reduced because of the replacement certain oxygen functional groups with Au nanoparticles. Furthermore, optical absorption of Au-RGO can be strengthened significantly compared with RGO without Au (about 1.5 times), indicating an enhanced broadband absorption effect. Note that, from the absorption spectrum of Au-RGO, no obvious SPR peak of Au located at nearly 550 nm [39] is observed because the film of Au-RGO is too thin, which means the low amount of Au nanoparticles in our Au-RGO sample.
Fig. 4. (a) Photograph of RGO and Au-RGO films. (b) Linear optical absorption spectra of RGO and Au-RGO films.
By measuring the differential transmittance change as a function of probe delay of RGO and Au-RGO, it is possible to observe their transmittance recovery timescale. Figure 5(a) shows the measured time resolved differential transmission spectra of RGO and Au-RGO films with a 532 nm probe light. We see that both RGO and Au-RGO possess a rapid recovery time on scale of a few hundred femtoseconds and a slower one on scale of a few picoseconds, which can be attributed to carrier thermalization and electron-hole recombination, respectively [26], [40]. Nevertheless, the relaxation of Au-RGO (red line: 0.341 ps) is faster than corresponding RGO sample (blue line: 0.493 ps). Moreover, the peak amplitude of Au-RGO is higher than RGO. A higher peak amplitude and a faster recovery time demonstrate that Au-RGO possesses more absorption losses before saturable absorption which is called modulation depth and a faster optical response process, indicating that the modification of RGO with Au gives rise to an enhanced nonlinear optical absorption [41]. In order to investigate how the nonlinear properties are affected by the probe wavelength, we record the probe wavelength-dependent transient differential transmission spectra, as displayed in Fig. 5(b) and 5(c). The recovery time of RGO film at different probe wavelengths are the same, while the peak amplitude varies with the probe wavelength, implying that the nonlinear absorption effect of RGO depends on wavelength. For Au-RGO, the recovery time and signal intensity are almost unchanged with the wavelength of probe light, which demonstrates that compared with the RGO sample, the nonlinear absorption of Au-RGO sample is less dependent on the probe wavelength.
Fig. 5. (a) Transient differential transmission spectra of RGO and Au-RGO films measured with a 532 nm probe. (b), (c) Transient differential transmission spectra of RGO and Au-RGO measured with a 510 nm, 560 nm, 590 nm, 630 nm probe, respectively.
The conclusion as shown in Fig. 5 can be further confirm by the time resolved differential transmission spectra of RGO and Au-RGO films in range of 500 nm to 640 nm probe wavelength in Fig. 6. By comparing Fig. 6(a) with Fig. 6(b), we can observe that the nonlinear absorption of Au-RGO is more independent with the probe wavelength and its signal intensity is higher than corresponding RGO sample in all probe wavelength regions. These observations verify that RGO and Au-RGO possess nonlinear absorption in a broadband wavelength region, but the nonlinear absorption effect and the charge transfer effect of Au-RGO are enhanced by Au nanoparticles, which makes it a possible prominent SA for the application of a shorter pulses.
Fig. 6. (a), (b) Transient differential transmission spectra of RGO and Au-RGO films recorded with probe wavelengths ranging from 500 nm to 640 nm.
3.2 Q-switched waveguide lasers
Figure 7(a) and (b) show the AFM images of RGO and Au-RGO on the surface of Nd:YAG waveguide with the heights of 145 ± 20 nm and 173 ± 30 nm, respectively.
Fig. 7. (a), (b) AFM images of RGO and Au-RGO films on surface of Nd:YAG waveguide, including the thickness profiles.
The stable Q-switched waveguide lasers based on RGO and Au-RGO as a SA were obtained at an output coupling ratio of 70%. The laser oscillation wavelength is 1064 nm in both laser systems as shown in Fig. 8(a). The inset of Fig. 8(a) depicts the mode of Q-switched laser based on RGO SA under the pumping power of 250 mW, which demonstrates the single-mode waveguide laser. What needs to be clear is that when using Au-RGO as a SA, the observed waveguide laser mode is also a single mode. In order to avoid duplication, we don’t show this result. The typical pulse sequences of Q-switched waveguide lasers observed at the maximum absorbed power (765 mW) are displayed in Fig. 8(b). Q-switched waveguide laser based on Au-RGO SA possesses a lower pulse duration of 27.9 ns.
Fig. 8. Q-switched waveguide laser characterization: (a) Spectrum of the output laser (0.4 nm resolution). Inset shows the corresponding laser mode at the wavelength of 1064 nm. (b) Pulse profiles.
The diagrams of the output power, duration, repetition rate and pulse energy as a function of the absorbed laser power are depicted in Fig. 9(a), (b), (c) and (d). In case of RGO, the maximum output power is 39 mW under the absorbed power of 756.9 mw, leading to a slope coefficient of about 7%. While the obtained maximum Q-switched laser output power from Au-RGO laser system is around 61 mW, corresponding the slope coefficient of 9%. In Fig. 9(b), one can see that the pulse duration of Au-RGO laser system changes from 62.5 ns to 27.9 ns with increasing absorbed power. The corresponding repetition rate(Fig. 9(c)) increases when the absorbed power increases. A maximum frequency of 7.2 MHz was obtained with a duration of 27.9 ns for Au-RGO, while a relatively long pulse duration (38.7 ns) and a low repetition frequency (5.4 MHz) were obtained in RGO. As plotted in Fig. 9(d), a maximum pulse energy is 8.5 nJ for Au-RGO and there is a similar value for RGO system. These results suggest that Q-switched waveguide pulsed lasers based on Au-RGO exhibit high performances.
Fig. 9. Q-switched waveguide laser performance (a) Output power. (b) Pulse duration. (c) Repetition rate. (d) Pulse energy.
4. Conclusion
In summary, we studied the optical properties of RGO and Au-RGO as well as employed them as saturable absorbers to obtain Q-switched waveguide pulsed lasers. Enhanced broadband linear and nonlinear absorption of Au-RGO have been observed, suggesting that Au-RGO seems to be a promising modulator in the field of ultrafast pulsed lasers. Both RGO and Au-RGO supported Q-switched operation in presented waveguide laser configuration. The maximum laser power generated from Au-RGO laser system was 61 mw with 27.9 ns duration and 7.2 MHz rate, which generates optimizing cavity and high-quality Q-switched pulsed laser compared with RGO laser system, making Au-RGO exciting opportunities for future integrated photonic applications.
Funding
National Natural Science Foundation of China (61674096, 91850110).
Disclosures
The authors declare no conflicts of interest.
References
1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef]
2. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, “Two-dimensional gas of massless Dirac fermions in graphene,” Nature 438(7065), 197–200 (2005). [CrossRef]
3. Y. Zhang, J. W. Tan, H. L. Stormer, and P. Kim, “Experimental observation of the quantum Hall effect and Berry’s phase in graphene,” Nature 438(7065), 201–204 (2005). [CrossRef]
4. P. Blake, E. W. Hill, A. H. C. Neto, K. S. Novoselov, D. Jiang, R. Yang, T. J. Booth, and A. K. Geim, “Making graphene visible,” Appl. Phys. Lett. 91(6), 063124 (2007). [CrossRef]
5. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). [CrossRef]
6. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
7. A. Muti, F. Canbaz, M. Tonelli, J. E. Bae, F. Rotermund, V. Petrov, and A. Sennaroglu, “Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 µm,” Opt. Lett. 45(3), 656–659 (2020). [CrossRef]
8. D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, and A. C. Ferrari, “Graphene Q-switched, tunable fiber laser,” Appl. Phys. Lett. 98(7), 073106 (2011). [CrossRef]
9. Y. Tan, C. Cheng, S. Akhmadaliev, S. Q. Zhou, and F. Chen, “Nd:YAG waveguide laser Q-switched by evanescent-field interaction with graphene,” Opt. Express 22(8), 9101–9106 (2014). [CrossRef]
10. H. Liu, C. Cheng, C. Romero, J. R. V. de Aldana, and F. Chen, “Graphene-based Y-branch laser in femtosecond laser written Nd:YAG waveguides,” Opt. Express 23(8), 9730–9735 (2015). [CrossRef]
11. E. Kifle, P. Loiko, J. R. V. de Aldana, C. Romero, A. Ródenas, V. Zakharov, A. Veniaminov, H. H. Yu, H. J. Zhang, Y. X. Chen, M. Aguiló, F. Díaz, U. Griebner, V. Petrov, and X. Mateos, “Fs-laser-written thulium waveguide lasers Q-switched by graphene and MoS2,” Opt. Express 27(6), 8745–8755 (2019). [CrossRef]
12. A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B. Shenoy, “Structural evolution during the reduction of chemically derived graphene oxide,” Nat. Chem. 2(7), 581–587 (2010). [CrossRef]
13. W. S. Hummers and R. E. Offeman, “Preparation of graphitic oxide,” J. Am. Chem. Soc. 80(6), 1339 (1958). [CrossRef]
14. C. Mattevi, G. Eda, S. Agnoli, S. Miller, K. A. Mkhoyan, O. Celik, D. Mastrogiovanni, G. Granozzi, E. Garfunkel, and M. Chhowalla, “Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films,” Adv. Funct. Mater. 19(16), 2577–2583 (2009). [CrossRef]
15. C. Gomez-Navarro, R. Thomas Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard, and K. Kern, “Electronic transport properties of individual chemically reduced graphene oxide sheets,” Nano Lett. 7(11), 3499–3503 (2007). [CrossRef]
16. U. N. Maiti, J. Lim, K. E. Lee, W. J. Lee, and S. O. Kim, “Three-dimensional shape engineered, interfacial gelation of reduced graphene oxide for high rate, large capacity super capacitors,” Adv. Mater. 26(4), 615–619 (2014). [CrossRef]
17. S. D. Pan, L. Cui, J. Q. Liu, B. Teng, J. H. Liu, and X. H. Ge, “Passively Q-switched mode-locking Nd:GdVO4 laser with a chemically reduced graphene oxide saturable absorber,” Opt. Mater. 38, 42–45 (2014). [CrossRef]
18. W. D. Tan, C. Y. Su, R. J. Knize, G. Q. Xie, L. J. Li, and D. Y. Tang, “Mode locking of ceramic Nd:yttrium aluminum garnet with graphene as a saturable absorber,” Appl. Phys. Lett. 96(3), 031106 (2010). [CrossRef]
19. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, K. Librant, M. Zdrojek, L. Lipinska, and K. M. Abramski, “Linearly polarized, Q-switched Er-doped fiber laser based on reduced graphene oxide saturable absorber,” Appl. Phys. Lett. 101(24), 241106 (2012). [CrossRef]
20. C. Pang, R. Li, Z. Q. Li, N. N. Dong, F. Ren, J. Wang, and F. Chen, “A novel hierarchical nanostructure for enhanced optical nonlinearity based on scattering mechanism,” Small 16(39), 2003172 (2020). [CrossRef]
21. P. Poizot, B. Humbert, C. P. Ewels, J.-Y. Mevellec, N. Stephant, and J. Simonet, “Facile route to gold-graphene electrodes by exfoliation of natural graphite under electrochemical conditions,” Carbon 107, 823–830 (2016). [CrossRef]
22. Y. Q. Guo, X. Y. Sun, Y. Liu, W. Wang, H. X. Qiu, and J. P. Gao, “One pot preparation of reduced graphene oxide (RGO) or Au (Ag) nanoparticle-RGO hybrids using chitosan as a reducing and stabilizing agent and their use in methanol electrooxidation,” Carbon 50(7), 2513–2523 (2012). [CrossRef]
23. S. J. Li, M. M. Shi, B. R. Wulan, J. M. Yan, and Q. Jiang, “Amorphizing of Au nanoparticles by CeOx-RGO hybrid support towards highly efficient electrocatalyst for N2 reduction under ambient conditions,” Adv. Mater. 29(33), 1700001 (2017). [CrossRef]
24. M. Reddeppa, S. B. Mitta, T. Chandrakalavachi, B. G. Park, G. Murali, R. Jeyalakshmi, S. G. Kim, S. H. Park, and M. D. Kim, “Solution-processed Au@rGO/GaN nanorods hybrid-structure for self-powered UV, visible photodetector and CO gas sensors,” Curr. Appl. Phys. 19(8), 938–945 (2019). [CrossRef]
25. A. Kumar, K. L. Reddy, S. Kumar, A. Kumar, V. Sharma, and V. Krisnan, “Rational design and development of lanthanide-doped NaYF4@CdS-Au-RGO as quaternary plasmonic photocatalysts for harnessing visible-near-infrared broadband spectrum,” ACS Appl. Mater. Interfaces 10(18), 15565–15581 (2018). [CrossRef]
26. S. R. Bongu, P. B. Bisht, T. V. Thu, and A. Sandhu, “Multiple nonlinear optical response of gold decorated-reduced graphene oxide-nanocomposite for photonic applications,” Journal of Atomic, Molecular Condensate & Nano Physics 2(3), 207–214 (2015).
27. A. Kumar and S. Aditya, “Performance of S-bends for integrated-optic waveguides,” Microw. Opt. Technol. Lett. 19(4), 289–292 (1998). [CrossRef]
28. L. N. Ma, Y. Tan, S. X. Wang, S. Akhmadaliev, S. Q. Zhou, H. H. Yu, H. J. Zhang, and F. Chen, “Continuous-wave and Q-switched Yb:YSGG waveguide laser,” J. Lightwave Technol. 35(13), 2642–2645 (2017). [CrossRef]
29. Y. Jia and F. Chen, “Compact solid-state waveguide lasers operating in the pulsed regime: a review [Invited],” Chin. Opt. Lett. 17(1), 012302 (2019). [CrossRef]
30. J. W. Kim, S. Y. Choi, D. I. Yeom, S. aravazhi, M. Pollnau, U. Griebner, V. Petrov, and E. Rotermund, “Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes,” Opt. Lett. 38(23), 5090–5093 (2013). [CrossRef]
31. Z. Q. Li, R. Li, C. Pang, N. N. Dong, J. Wang, H. H. Yu, and F. Chen, “8.8 GHz Q-switched mode-locked waveguide lasers modulated by PtSe2 saturable absorber,” Opt. Express 27(6), 8727–8737 (2019). [CrossRef]
32. R. Li, C. Pang, Z. Q. Li, M. Yang, H. Amekura, N. N. Dong, J. Wang, F. Ren, Q. Wu, and F. Chen, “Fused silica with embedded 2D-like Ag nanoparticle monolayer: Tunable saturable absorbers by interparticle spacing manipulation,” Laser Photonics Rev. 14(2), 1900302 (2020). [CrossRef]
33. F. Lou, L. Cui, Y. B. Li, J. Hou, J. L. He, Z. T. Jia, J. Q. Liu, B. T. Zhang, K. J. Yang, Z. W. Wang, and X. T. Tao, “High-efficiency femtosecond Yb:Gd3Al0.5Ga4.5O12 mode-locked laser based on reduced graphene oxide,” Opt. Lett. 38(20), 4189–4192 (2013). [CrossRef]
34. H. F. Shi, C. Wang, Z. P. Sun, Y. L. Zhou, K. J. Jin, S. A. T. Redfern, and G. Z. Yang, “Tuning the nonlinear optical absorption of reduced graphene oxide by chemical reduction,” Opt. Express 22(16), 19375–19385 (2014). [CrossRef]
35. Y. L. Su, X. N. Huang, H. Hu, Y. Wen, X. P. Xie, J. H. Si, Y. S. Wang, and W. Zhao, “A dual-wavelength Q-switched fiber laser based on reduced graphene oxides,” Laser Phys. 29(6), 065101 (2019). [CrossRef]
36. J. M. Lv, Z. Shang, Y. Tan, J. R. V. de Aldana, and F. Chen, “Cladding-like waveguide fabricated by cooperation of ultrafast laser writing and ion irradiation: characterization and laser generation,” Opt. Express 25(16), 19603–19608 (2017). [CrossRef]
37. C. H. Kiang, M. Endo, P. M. Ajayan, G. Dresselhaus, and M. S. Dresselhaus, “Size effects in carbon nanotubes,” Phys. Rev. Lett. 81(9), 1869–1872 (1998). [CrossRef]
38. B. K. Barman and K. K. Nanda, “The dual role of Zn-acid medium for one-step rapid synthesis of M@rGO (M = Au, Pt, Pd and Ag) hybrid nanostructures at room temperature,” Chem. Commun. 49(79), 8949–8951 (2013). [CrossRef]
39. P. Kar, S. Sardar, B. Liu, M. Sreemany, P. Lemmens, S. Ghosh, and S. K. Pal, “Facile synthesis of reduced graphene oxide-gold nanohybrid for potential use in industrial wastewater treatment,” Sci. Technol. Adv. Mater. 17(1), 375–386 (2016). [CrossRef]
40. B. A. Ruzicka, S. Wang, J. W. Liu, K. P. Loh, J. Z. Wu, and H. Zhao, “Spatially resolved pump-probe study of single-layer graphene produced by chemical vapor deposition,” Opt. Mater. Express 2(6), 708–716 (2012). [CrossRef]
41. B. Anand, R. Podila, K. Lingam, S. R. Krishnan, S. Siva Sankara Sai, R. Philip, and A. M. Rao, “Optical diode action from axially asymmetric nonlinearity in an all-carbon solid-state device,” Nano Lett. 13(12), 5771–5776 (2013). [CrossRef]
