Open Access
Add to BibSonomy
Abstract
In this work, a thin-film MoS2/WS2/MoS2/WS2 heterostructure is employed as a saturable absorber (SA) material for efficient ultrafast pulsed laser operation, achieving a 17.54 GHz Q-switched mode-locked Nd:GGG waveguide laser fabricated by femtosecond laser direct-writing (FsLDW). The mode-locked pulse duration is measured to be as short as 31 ps. The maximum laser slope efficiency and the average output power are determined to be 29.27% and 310 mW, respectively. Such a high-performance compact pulsed laser exhibits promising applications of crystalline waveguide structures and layered heterostructures in ultrafast integrated photonics. This work represents the very first experimental demonstration of a pulsed laser based on FsLDW Nd:GGG waveguides and the very first demonstration of using a MoS2/WS2/MoS2/WS2 heterostructure as an efficient SA.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
By combining the compact geometries of waveguide structures and the multi-functional properties of various dielectric crystals, dielectric waveguides are important photonic elements in modern integrated optical systems, serving as passive and even active photonic components. One of the typical examples is solid-state waveguide lasers, which are ideal candidates as multi-functional and complex photonic chip light sources. The device optimization of waveguide lasers in terms of lasing threshold, lasing efficiency, output power, operation regime and thermal stability are therefore of significant importance for the overall performance of a photonic chip [1–6]. Particularly, mode-locked waveguide lasers with repetition rates up to GHz have recently gained increasing research interests because they possess the potential of applications in various fields, such as nonlinear microscopy, photomedicine and high-speed optical communication [4,5]. Up to date, mode-locked waveguide lasers with repetition rates up to 21.25 GHz have been demonstrated in a Ti:Sapphire waveguide [7]. Other demonstrations of GHz mode-locked waveguide lasers (including continuous-wave mode-locked and Q-switched mode-locked operations) can be referenced elsewhere [4]. A number of micro-/nano-fabrication techniques have been employed for waveguide definition in transparent dielectrics. In particular, femtosecond laser direct writing (FsLDW) has become one of the most important methods recently due to its compatibility with various dielectrics and the intrinsic characteristics of true three-dimensional (3D) micromachining of on-demand geometries via laser-induced localized material modification [8–12]. It is therefore very convenient to combine FsLDW with multi-functional laser crystals to achieve desirable waveguide fabrication and to obtain high-performance waveguide lasing [13–20]. As one of the most useful gain medium for solid state lasers operating at 1-µm wavelength, neodymium doped gadolinium gallium garnet (Nd:Gd3Ga5O12 or Nd:GGG) has attracted much attention due to its outstanding optical and thermal properties. So far, there have been several reports of continuous wave (CW) and pulsed waveguide lasers realized in this crystal [21–23]. In particular, GHz waveguide lasers based on Nd:GGG ridge waveguide fabricated by diamond blade dicing have been recently demontrated [23]. However, the very first demonstration of FsLDW Nd:GGG waveguide laser operating with GHz repetition rate is still missing.
Since the appearance of graphene, many graphene-like two-dimensional (2D) material families, such as transition mental dichalcogenides (TMDCs), black phosphorus (BP), and topological insulators (TIs), have aroused great research enthusiasm due to their exotic optoelectronic nature as a result of quantum confinement effect [24,25]. In particular, these 2D layered materials possess excellent nonlinear optical properties and ultrafast dynamic response especially ideal for integrated ultrafast laser applications because of their easy fabrication and integration. Up to now, a variety of 2D materials have been well explored and they are playing significant roles in visible, near-infrared (NIR), and middle-infrared (MIR) Q-switched and mode-locked pulsed laser generations [7,26–36]. For example, a continuous-wave mode-locked visible laser at 639 nm modulated by monolayer graphene has been reported based on Pr3+:LuLiF4 crystal [37]. However, there are still shortcomings for their each working as high-performance SAs. For example, graphene possesses a low modulation depth even though it can be in resonance for excitation at any frequency due to its zero-bandgap. TMDCs are mostly in resonance at visible wavelengths and BP is prone to oxidation in air [4]. In view of this, 2D layered heterostructures, which are constructed by combining different 2D crystals in one vertical stack, provide a variety of possibilities and opportunities in material science. In these heterostructures, the respective advantages of different 2D layered materials are well combined and can be flexibly designed by engineering each layer separately, offering high flexibility in their main optical and electronic properties [24,38]. For example, it has been proved that the saturable absorption properties of 2D heterostructure in terms of modulation depth and nonlinear optical absorption are more preferred for ultrafast laser applications in contrast to that of pure 2D layered materials due to heterointerface reaction. For example, MoS2-WS2 heterostructure has been recently applied for generation of 154-fs mode-locked pulsed fiber lasers [39]. To the best of our knowledge, MoS2/WS2/MoS2/WS2 heterostructure, which is favorite to modulate ultrafast pulsed lasers owing to more heterointerfaces, has not yet been applied for pulsed laser operation.
In this work, we employed MoS2/WS2/MoS2/WS2 heterostructure as saturable absorber (SA) to modulate depressed cladding waveguide laser fabricated by FsLDW in Nd:GGG crystal, obtaining a fundamental repetition rate as high as 17.54 GHz under Q-switched mode-locked (QML) operation regime. The pulse duration is determined to be 31 ps and the maximum output power is 310 mW. This work represents the very first experimental demonstration of pulsed laser based on FsDWL Nd:GGG waveguides and the very first demonstration of using MoS2/WS2/MoS2/WS2 heterostructure as an efficient SA for any kinds of pulsed lasers.
2. Sample preparation
2.1 Waveguide fabrication by FsLDW
We use a Nd:GGG (1 at.% Nd3+ ions) crystal wafer with dimensions of 10 × 4 × 2 mm3 as the substrate for waveguide fabrication. Two 10 × 4 mm2 crystal surfaces and two 10 × 2 mm2 parallel crystal end facets have been well polished to optical grade prior to further processing. For the fabrication of the cladding waveguides, we employ a 1030-nm femtosecond laser (FemtoYL-25) delivering 400-fs pulses at a repetition rate of 25 kHz. The laser writing setup is schematically illustrated in Fig. 1(a). A 50× microscope objective (N.A. = 0.67) is used to focus the femtosecond laser at a depth of 160 µm beneath the top crystal surface (with a size of 10 × 4 mm2). During the fabrication, i.e., laser writing process, the precise movement of the Nd:GGG substrate is enabled by a motorized XYZ translation stage. As a result, a number of parallel scans (with a pulse energy of 0.56 µJ, a constant scan speed of 0.5 mm/s and a separation of 3 µm between two adjacent tracks) are defined in the Nd:GGG crystal, constructing cladding waveguides with hexagonal cross sections (the dimensions of the waveguide cross section are indicated in Fig. 1(b), the waveguide length is around 4 mm) due to easy fabrication in terms of time-saving and high-precision without weakening the waveguiding properties. Moreover, such a geometry does not lead to polarization dependency of the waveguiding light or additional attenuation compared to the commonly used circle cladding geometry [4]. Choosing such a combination of femtosecond laser writing parameters in this work is intended to introduce optimized refractive index change for efficient waveguiding while minimizing the localized stress effect and avoiding crystal cracking. To further investigate the localized lattice changes induced by femtosecond laser-writing process, the confocal micro-photoluminescence (µ-PL) analysis (Figs. 1(c)–1(e)) at 473 nm of the waveguide cross sections is performed. It can be observed that a reduction of µ-PL intensity (in Fig. 1(c)) appears in the cladding regions, where the laser-induced damage and crystalline distortion occur. More evidence of laser-induced localized lattice damage and stress field modification can be found in Figs. 1(d) and 1(e), where the bule shift of µ-PL spectra and bandwidth broadening from the fabricated cladding region confirm the localized lattice damage and distortion [40].
Fig. 1. (a) Schematic illustration of an experimental setup for FsLDW operation. (b) Optical transmission micrograph of the cladding waveguide fabricated in Nd:GGG crystal. The spatial 2D distributions of µ-PL (c) intensity, (d) shift and (e) bandwidth of 866.6-nm emission line obtained from the cladding waveguide cross section.
2.2 MoS2/WS2/MoS2/WS2 heterostructure characterization
The SA material we used in this work is a multi-layer thin-film MoS2/WS2/MoS2/WS2 heterostructure (provided by 6Carbon Technology, China), which is prepared by chemical vapor deposition (CVD) on a sapphire substrate. The surface morphology and the thickness information (Fig. 2(a)) of the deposited MoS2/WS2/MoS2/WS2 thin film is investigated by atomic force microscopy (AFM) measurement operating in the tapping mode. A small section of the film has been erased by a knife and the film thickness is determined from a topological line profile across the scratch. The good homogeneity of the as-deposited thin film can be confirmed by the AFM-measured height profile in Fig. 2(a), indicating a thickness of around 38.4 nm. Since the thickness of monolayer MoS2 and WS2 are both around 1 nm, this as-deposited MoS2/WS2/MoS2/WS2 heterostructure is therefore multi-layer. Both the vibration modes of MoS2 and WS2 can be observed in the Raman spectrum (Fig. 2(b)) of the as-deposited MoS2/WS2/MoS2/WS2 heterostructure. The results observed here are in fairly good agreement with previous studies [39]. The linear optical transmittance information (studied by a UV/VIS/NIR spectrophotometer U-3500 HITACHI) of the MoS2/WS2/MoS2/WS2 heterostructure sample in Fig. 3(a) indicates an optical transmittance of 83.14% at 1064 nm, which is higher than that of MoS2/WS2/MoS2 heterostructure (around 56%) in previous report [38].
Fig. 2. (a) Height profile of the section marked in the AFM image of MoS2/WS2/MoS2/WS2 heterostructure thin film. Inset: AFM image of MoS2/WS2/MoS2/WS2 heterostructure thin film. (b) The Raman spectrum analysis of MoS2/WS2/MoS2/WS2 heterostructure thin film.
Fig. 3. (a) The linear optical transmittance of MoS2/WS2/MoS2/WS2 heterostructure. (b) The nonlinear transmission curves of MoS2/WS2/MoS2/WS2 heterostructure.
The nonlinear optical absorption, or to say the saturable absorption response, of the thin-film MoS2/WS2/MoS2/WS2 heterostructure sample is studied by a home-designed open-aperture Z-scan experimental setup, in which a 1030-nm femtosecond fiber laser (FemtoYL-10, YSL Photonics, China) with a pulse duration of 400 fs, a pulsed energy of 4.6 µJ and a repetition rate of 25 kHz is employed. The sample is placed on a PC-controlled translation stage during the Z-scan measurement and moved through the laser beam focus, which has a beam waist radius of around 52 µm at the sample position. The nonlinear optical transmittance information of the sample at different incident femtosecond laser intensities (see Fig. 3(b)) is studied to verify the effective saturable absorption properties of the heterostructure sample. The tendency of the nonlinear transmittance (equivalent to nonlinear absorption) to be saturated with the increase of the laser intensity can be clearly identified, giving a modulation depth of 23.85% and a saturation intensity of 13.8 µJ/cm2 at 1030 nm. By comparison, such a modulation depth is higher than that of pure few-layered MoS2 and WS2 or that of MoS2/WS2 heterostructure [39,41]. This is mainly because the larger quantity of the heterointerfaces, the stronger light-matter interaction in the heterostructure, and thus potentially offering a higher modulation depth [42]. Moreover, photons in the additional layers of MoS2/WS2/MoS2/WS2 heterostructure in contrast to that of, for example, MoS2/WS2 heterostructure may experience secondary absorption process, leading to a higher nonlinear optical absorption and thus a higher modulation depth [42].
3. Waveguide laser characterization
3.1 Continuous-wave waveguide laser
The laser performance of the FsLDW cladding waveguide is investigated based on an end-face coupling setup, in which a tunable CW Ti:Sapphire laser (Coherent MBR-PE) is employed as the pump source. In the lasing experiment, the linearly polarized 808-nm pump light is coupled in and out of the waveguides by using a plano-convex lens (f = 25 mm) and a 20× microscope objective (NA = 0.40), respectively. The optimized coupling condition is achieved by adjusting the sample position using a 3D-translation optical stage. The pump mirror (M1 with a transmittance of 99.8% at 808 nm and a reflectivity of >99.9% at 1064 nm) and output mirror (M2 with a reflectivity of approximately 60% at 1064 nm) are butt-adhered to the waveguide end-facets, forming a compact Fabry-Pérot cavity. The whole waveguide laser characterization setup (the inserted MoS2/WS2/MoS2/WS2 heterostructure is used as an SA for Q-switched mode-locked laser generation as discussed in the next section) is schematically illustrated in Fig. 4.
Fig. 4. Schematic illustration of an experimental setup for Q-switched mode-locked waveguide laser modulated by MoS2/WS2/MoS2/WS2 heterostructure (there is no MoS2/WS2/MoS2/WS2 heterostructure inserted in the cavity in the CW operation regime).
The measured near-field laser modal profile (operating in single transversal mode) and the output power information under TE-polarized pumping (the waveguide laser performance difference along TM and TE polarizations is less than 5%) of the fabricated cladding waveguide are shown in Fig. 5(a). The lasing mode is found to be well confined in the guiding area and the maximum output power is measured to be 380 mW (with a slope efficiency of 36.3%), suggesting good preservation of lasing properties in the guiding structures. The central lasing wavelength is measured to be 1062 nm with a full wave at half maximum (FWHM) value of 0.2 nm (see Fig. 5(b), spectrometer resolution is 0.2 nm).
Fig. 5. (a) Output power as a function of launched power under the CW operation. The insert is the measured near-field modal profile of the output laser. (b) The laser emission spectrum.
To further study the waveguiding properties at lasing wavelength, the propagation losses of FsLDW Nd:GGG cladding waveguide along different polarizations are measured (see Table 1, by employing a 1064-nm solid-state laser). We believe the geometrical difference along parallel and vertical directions has an impact on the mode profile and thus the optical scattering due to the laser-induced tracks, resulting in the slight difference in the waveguide propagation losses.
Table 1. FsLDW Nd:GGG cladding waveguide propagation losses
3.2 Q-switched mode-locked waveguide laser
The saturable absorption properties of MoS2/WS2/MoS2/WS2 heterostructure and the lasing performance operating at the pulsed regime (Q-switched mode-locked regime in this experiment) of FsLDW Nd:GGG cladding are studied based on the laser experimental setup illustrated in Fig. 4. The output laser spectrum under the Q-switched mode-locked operation is identical to that in the CW regime (Fig. 5(b)). The output power and the near-field laser modal profile (operating in single transversal mode) information is summarized in Fig. 6(a), exhibiting identical mode profile but higher lasing threshold and lower average output power in contrast to that in the CW regime as a result of the additional absorption/scattering loss introduced by SA. The nature of Q-switched mode-locked pulsed laser operation is verified by the Q-switched envelope (see Fig. 6(b), the pulse energy of Q-switched envelope is determined to be 185.63 µJ) and the mode-locked pulse trains (see Fig. 6(c), the pulse duration is determined to be as short as 31 ps), giving a fundamental repetition rate of 17.54 GHz (see Fig. 6(d), the signal-to-noise ratio (SNR) is up to 47 dB). Such a high-repetition-rate pulsed laser has never been reported in Nd:GGG waveguide fabricated by FsLDW, and the short pulse duration is comparable to that demonstrated in ion-irradiated Nd:GGG waveguides [23]. According to the formula frep = c/2nl (where c is the light speed, n is the effective refractive index of the waveguide which is very close to that of Nd:GGG crystal, and l is the length of the waveguide cavity), the theoretical repetition rate of the waveguide cavity used in this work can be calculated. The refractive index of Nd:GGG waveguide at the 1064 nm is close to n ≈ 1.95 according to the Sellmeier formula reported in Ref. [43], plus the waveguide cavity length l ≈ 4.25 mm (consists of the waveguide length of 4 mm and the SA thickness of 0.25 mm), making the calculated fundamental repetition rate to be around 18 GHz, which is in fairly good agreement to the experimental results in this work. The slightly lower repetition rate measured in this work is most probably caused by the small air gap due to the imperfect fit between the cavity mirrors and the waveguide end-facets, resulting in a slight longer cavity length in the experiments. Concerning the laser stability, the Q-switched and the mode-locked pulse train waveforms have been testified for around 1-hour time scale, and only <10% difference in pulse intensity can be identified, suggesting good stability of the pulsed waveguide laser achieved in this work.
Fig. 6. (a) Output power as a function of launched power under the Q-switched mode-locked operation. The insert is measured near-field modal profile of the output laser. (b) Q-switched envelope, the inset is successive Q-switched envelopes. (c) Mode-locked pulse trains. (d) RF spectrum.
In order to achieve the continuous-wave mode-locked (CWML) waveguide lasers in the future work, efforts should be made on significantly reducing the level of waveguide loss and to effectively controlling the material dispersion in the laser system. Generally, the waveguide loss could be reduced by systemically researching on fabrication parameters. While, to control cavity diversion while maintaining the compactness of such cavity, a thin air-filled gap between cavity mirror and waveguide facet can be employed [4,7]. Moreover, in order to get a shorter pulse duration, SA with larger modulation depth can be used according to the formula τp ≈ 3.52TR/ΔR, where the pulse duration (τp) is highly corelated to the cavity round-trip time (TR) and SA modulation depth (ΔR) [4].
4. Summary
In this work, the saturable absorption properties of MoS2/WS2/MoS2/WS2 heterostructure at 1-µm wavelength have been studied and its application in ultrahigh-repetition-rate laser generation has been demonstrated, achieving Q-switched mode-locked laser with a repetition rate up to 17.54 GHz based on a monolithic waveguide platform. It can be found that, the MoS2/WS2/MoS2/WS2 heterostructure exhibits superior nonlinear optical responses compared to pure materials. The results achieved in this work indicate the great potential of 2D heterostructure and compact waveguide lasers for application in future ultrafast integrated photonics.
Funding
Taishan Scholar Foundation of Shandong Province; National Natural Science Foundation of China (12074223, 61775120).
Acknowledgments
Y. Jia acknowledges the support from “Taishan Scholars Youth Expert Program” of Shandong Province and “Qilu Young Scholar Program” of Shandong University, China. F. Chen thanks the support from “Taishan Scholars Climbing Program” of Shandong Province. The authors gratefully acknowledge Mr. Q. Lu from Shandong University, Prof. H. Yu from Shandong University, Ms. L. Sun from Shandong Normal University for their kind help on crystal processing, optical characterization, and μ-PL analysis.
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 maybe obtained from the authors upon reasonable request.
References
1. E. A. J. Marcatili and R. A. Schmeltzer, “Hollow metallic and dielectric waveguides for long distance optical transmission and lasers,” Bell Syst. Tech. J. 43(4), 1783–1809 (1964). [CrossRef]
2. C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). [CrossRef]
3. R. Wolf, Y. Jia, S. Bonaus, C. Werner, S. Herr, I. Breunig, K. Buse, and H. Zappe, “Quasi-phase-matched nonlinear optical frequency conversion in on-chip whispering galleries,” Optica 5(7), 872–875 (2018). [CrossRef]
4. 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]
5. C. Grivas, “Optically pumped planar waveguide lasers: Part II: Gain media, laser systems, and applications,” Prog. Quantum Electron. 45-46, 3–160 (2016). [CrossRef]
6. Y. Jia, L. Wang, and F. Chen, “Ion-cut lithium niobate on insulator technology: Recent advances and perspectives,” Appl. Phys. Rev. 8(1), 011307 (2021). [CrossRef]
7. C. Grivas, R. Ismaeel, C. Corbari, C.-C. Huang, D. W. Hewak, P. Lagoudakis, and G. Brambilla, “Generation of multi-gigahertz trains of phase-coherent femtosecond laser pulses in Ti:Sapphire waveguides,” Laser Photonics Rev. 12(11), 1800167 (2018). [CrossRef]
8. F. Chen and J. R. V. de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). [CrossRef]
9. D. Choudhury, J. R. Macdonald, and A. K. Kar, “Ultrafast laser inscription: perspectives on future integrated applications,” Laser Photonics Rev. 8(6), 827–846 (2014). [CrossRef]
10. Y. Jia, S. Wang, and F. Chen, “Femtosecond laser direct writing of flexibly configured waveguide geometries in optical crystals: fabrication and application,” Opto-Electron. Adv. 3(10), 190042 (2020). [CrossRef]
11. M. Beresna, M. Gecevičius, and P. G. Kazansky, “Ultrafast laser direct writing and nanostructuring in transparent materials,” Adv. Opt. Photonics 6(3), 293–339 (2014). [CrossRef]
12. M. Ams, P. Dekker, S. Gross, and M. J. Withford, “Fabricating waveguide Bragg gratings (WBGs) in bulk materials using ultrashort laser pulses,” Nanophotonics 6(5), 743–763 (2017). [CrossRef]
13. T. Calmano, A. G. Paschke, S. Müller, C. Kränkel, and G. Huber, “Curved Yb:YAG waveguide lasers, fabricated by femtosecond laser inscription,” Opt. Express 21(21), 25501–25508 (2013). [CrossRef]
14. X. Sun, S. Sun, C. Romero, J. R. V. de Aldana, F. Liu, Y. Jia, and F. Chen, “Femtosecond laser direct writing of depressed cladding waveguides in Nd:YAG with “ear-like” structures: fabrication and laser generation,” Opt. Express 29(3), 4296–4307 (2021). [CrossRef]
15. Y. Jia, R. He, J. R. V. de Aldana, H. Liu, and F. Chen, “Femtosecond laser direct writing of few-mode depressed-cladding waveguide lasers,” Opt. Express 27(21), 30941–30951 (2019). [CrossRef]
16. V. A. Amorim, J. M. Maia, D. Viveiros, and P. V. S. Marques, “Loss mechanisms of optical waveguides inscribed in fused silica by femtosecond laser direct writing,” J. Lightwave Technol. 37(10), 2240–2245 (2019). [CrossRef]
17. N. Pavel, G. Salamu, F. Voicu, F. Jipa, and M. Zamfirescu, “Diode-laser pumping into the emitting level for efficient lasing of depressed cladding waveguides realized in Nd:YVO4 by the direct femtosecond-laser writing technique,” Opt. Express 22(19), 23057–23065 (2014). [CrossRef]
18. Y. Ren, G. Brown, R. Mary, G. Demetriou, D. Popa, F. Torrisi, A. C. Ferrari, F. Chen, and A. K. Kar, “7.8-GHz graphene-based 2 µm monolithic waveguide laser,” IEEE J. Sel. Top. Quantum Electron. 21(1), 1602106 (2015).
19. E. Kifle, P. Loiko, C. Romero, J. R. V. de Aldana, V. Zakharov, Y. Gurova, A. Veniaminov, V. Petrov, U. Griebner, R. Thouroude, M. Laroche, P. Camy, M. Aguiló, F. Díaz, and X. Mateos, “Tm3+ and Ho3+ colasing in in-band pumped waveguides fabricated by femtosecond laser writing,” Opt. Express 46(1), 122–125 (2021). [CrossRef]
20. S. Wang, C. Pang, Z. Li, R. Li, N. Dong, Q. Lu, F. Ren, J. Wang, and F. Chen, “8.6 GHz Q-switched mode-locked waveguide lasing based on LiNbO3 crystal embedded Cu nanoparticles,” Opt. Mater. Express 9(9), 3808–3817 (2019). [CrossRef]
21. C. Zhang, N. Dong, J. Yang, F. Chen, J. R. V. de Aldana, and Q. Lu, “Channel waveguide lasers in Nd:GGG crystals fabricated by femtosecond laser inscription,” Opt. Express 19(13), 12503–12508 (2011). [CrossRef]
22. Y. Jia, N. Dong, F. Chen, J. R. V. de Aldana, S. Akhmadaliev, and S. Zhou, “Ridge waveguide lasers in Nd:GGG crystals produced by swift carbon ion irradiation and femtosecond laser ablation,” Opt. Express 20(9), 9763–9768 (2012). [CrossRef]
23. X. Dong, Z. Li, C. Pang, N. Dong, H. Jiang, J. Wang, and F. Chen, “Q-switched mode-locked Nd:GGG waveguide laser with tin disulfide as saturable absorber,” Opt. Mater. 100, 109702 (2020). [CrossRef]
24. B. Zhang, J. Liu, C. Wang, K. Yang, C. Lee, H. Zhang, and J. He, “Recent progress in 2D material-based saturable absorbers for all solid-state pulsed bulk lasers,” Laser Photonics Rev. 14(2), 1900240 (2020). [CrossRef]
25. Y. Ge, Z. Zhu, Y. Xu, Y. Chen, S. Chen, Z. Liang, Y. Song, Y. Zou, H. Zeng, S. Xu, H. Zhang, and D. Fan, “Broadband nonlinear photoresponse of 2D TiS2 for ultrashort pulse generation and all-optical thresholding devices,” Adv. Opt. Mater. 6(4), 1701166 (2018). [CrossRef]
26. Y. Wang, J. Li, K. Mo, Y. Wang, F. Liu, and Y. Liu, “14.5 GHz passive harmonic mode-locking in a dispersion compensated Tm-doped fiber laser,” Sci. Rep. 7(1), 7779 (2017). [CrossRef]
27. Z. Li, N. Dong, C. Cheng, L. Xu, M. Chen, J. Wang, and F. Chen, “Enhanced nonlinear optical response of graphene by silver-based nanoparticle modification for pulsed lasing,” Opt. Mater. Express 8(5), 1368–1377 (2018). [CrossRef]
28. S. Y. Choi, T. Calmano, F. Rotermund, and C. Kränkel, “2-GHz carbon nanotube mode-locked Yb:YAG channel waveguide laser,” Opt. Express 26(5), 5140–5145 (2018). [CrossRef]
29. A. G. Okhrimchuk and P. A. Obraztsov, “11-GHz waveguide Nd:YAG laser CW mode-locked with single-layer graphene,” Sci. Rep. 5(1), 11172 (2015). [CrossRef]
30. D. P. Shepherd, A. Choudhary, A. A. Lagatsky, P. Kannan, S. J. Beecher, R. W. Eason, J. I. Mackenzie, X. Feng, W. Sibbett, and C. T. A. Brown, “Ultrafast high-repetition-rate waveguide lasers,” IEEE J. Sel. Top. Quantum Electron. 43(23), 5753–5756 (2018).
31. C. Cheng, H. Liu, Z. Shang, W. Nie, Y. Tan, B. R. Rabes, J. R. V. de Aldana, D. Jaque, and F. Chen, “Femtosecond laser written waveguides with MoS2 as saturable absorber for passively Q-switched lasing,” Opt. Mater. Express 6(2), 367–373 (2016). [CrossRef]
32. F. Thorburn, A. Lancaster, S. McDaniel, G. Cook, and A. K. Kar, “5.9 GHz graphene-based Q-switched modelocked mid-infrared monolithic waveguide laser,” Opt. Express 25(21), 26166–26174 (2017). [CrossRef]
33. X. Jiang, S. Gross, M. J. Withford, H. Zhang, D. Yeom, F. Rotermund, and A. Fuerbach, “Low-dimensional nanomaterial saturable absorbers for ultrashort-pulsed waveguide lasers,” Opt. Mater. Express 8(10), 3055–3071 (2018). [CrossRef]
34. Z. Li, C. Cheng, N. Dong, C. Romero, Q. Lu, J. Wang, J. R. V. de Aldana, Y. Tan, and F. Chen, “Q-switching of waveguide lasers based on graphene/WS2 van der Waals heterostructure,” Photonics Res. 5(5), 406–410 (2017). [CrossRef]
35. T. Huang, H. Liang, K. Su, and Y. Chen, “Exploring the emergence of the self-Q-switching in diode-pumped Yb:KGW monolithic lasers,” IEEE J. Sel. Top. Quantum Electron. 24(5), 1–6 (2018). [CrossRef]
36. E. Kifle, P. Loiko, C. Romero, J. R. V. de Aldana, M. Aguiló, F. Díaz, P. Camy, U. Griebner, V. Petrov, and X. Mateos, “Watt-level ultrafast laser inscribed thulium waveguide lasers,” Prog. Quantum Electron. 72, 100266 (2020). [CrossRef]
37. Y. Zhang, H. Yu, R. Zhang, G. Zhao, H. Zhang, Y. Chen, L. Mei, M. Tonelli, and J. Wang, “Broadband atomic-layer MoS2 optical modulators for ultrafast pulse generations in the visible range,” Opt. Lett. 42(3), 547–550 (2017). [CrossRef]
38. H. Chen, J. Yin, J. Yang, X. Zhang, M. Liu, Z. Jiang, J. Wang, Z. Sun, T. Guo, W. Liu, and P. Yan, “Transition-metal dichalcogenides heterostructure saturable absorbers for ultrafast photonics,” Opt. Lett. 42(21), 4279–4282 (2017). [CrossRef]
39. W. Liu, M. Liu, B. Liu, R. G. Quhe, M. Lei, S. Fang, H. Teng, and Z. Wei, “Nonlinear optical properties of MoS2-WS2 heterostructure in fiber lasers,” Opt. Express 27(5), 6689–6699 (2019). [CrossRef]
40. A. Rodenas, G. A. Torchia, G. Lifante, E. Cantelar, J. Lamela, F. Jaque, L. Roso, and D. Jaque, “Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations,” Appl. Phys. B 95(1), 85–96 (2009). [CrossRef]
41. E. Kifle, P. Loiko, J. R. V. de Aldana, C. Romero, A. Ródenas, V. Zakharov, A. Veniaminov, H. Yu, H. Zhang, Y. 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]
42. L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y.-J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, A. N. Grigorenko, A. K. Geim, C. Casiraghi, A. H. Castro Neto, and K. S. Novoselov, “Strong light-matter interactions in heterostructures of atomically thin films,” Science 340(6138), 1311–1314 (2013). [CrossRef]
43. D. L. Wood and K. Nassau, “Optical properties of gadolinium gallium garnet,” Appl. Opt. 29(25), 3704–3707 (1990). [CrossRef]
