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Abstract
NiO, a 3d transition-metal oxide with the strong electron correlation, has attracted great physical attention due to the spin-orbit splitting of 3d electrons. By taking advantage of electron transition process originated from 3d spin-orbit splitting, it may be applied to many photonics areas by linear or nonlinear optical response. To further broaden the photonics applications of NiO, we originally explore the nonlinear optical response, saturable absorption, during the electronic transition due to 3d spin-orbit splitting under a strong optical field and successfully applied in the ultrafast photonics as a mode-locker for the generation of visible laser pulses, which is the result of dynamic balancing process by the electron transition arising from ground state (3A2g) to excited state (1Eg) of spin-orbit splitting in the Ni2+ 3d configurations. With the NiO nanosheet film for saturable absorption, we experimentally realize a pulsed visible laser at a wavelength of 640.3 nm for the first time to our knowledge. These results indicate that the study of electron transition process generated by 3d spin-orbit splitting in 3d transition-metal oxides should be helpful for the development of ultrafast photonics and related devices design.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
3d transition-metal oxides have become the subject of increased interest for decades due to their unique optoelectronic properties [1,2]. Previous studies showed that 3d transition-metal oxides have a high optical sensitivity, a high electron mobility, and a large absorption coefficient at the band edge, making them promising for applications in catalysis, solar cells, sensors, and photodetectors [3–6]. Yet, these devices constructed with optoelectronic properties are mostly focused on the interband and multiphoton absorption processes [2,7]. In fact, the strong electron correlation between the 3d electrons exists in transition-metal oxides with anisotropic-shaped 3d-orbital electrons, which makes the study of the 3d transition-metal oxides an important research interest [8]. Due to the strong electron correlation and narrow band, the 3d electrons are strictly localized on the transition metal sites in the crystal field that lead to the spin-orbit splitting of 3d electrons [9,10]. Because of that, the study of spin-orbit splitting of the 3d electrons will open up an opportunity for practical applications on a new territory of functional optoelectronic devices. However, numerous studies are widely concerned on the fundamental research of the spin-orbit splitting of 3d electrons, the study of these photonics devices has been neglected under a strong optical field, particularly for nonlinear optical devices.
NiO is a typical 3d transition-metal oxide with strong electron correlation, which is strongly affected by Ni 3d electrons influenced in an octahedral crystal field because of strong d-d Coulomb interaction between 3d electrons [9,11]. Based on a great deal of fundamental research of NiO in physics, it is well known that the electron transition process arising from 3d spin-orbit splitting can be realized by the 3d8 configurations in NiO. This electron transition should, of course, be quite weak optical absorption in the visible and near-infrared light ranges since it is a d-d transition and the 3d electrons are also extremely localized [12–14]. Previously, NiO, as a wide-gap (3–4 eV) semiconductor material, was studied and successfully applied to many optoelectronic areas, such as photocatalysis, solar cells, optical limiters by two-photon absorption, and UV photodetectors [15–18]. Inspired by this, the kinetic mechanisms about the nonlinear optical response during the electron transition process originated from 3d spin-orbit splitting in NiO also play important roles for studying the light-matter interaction and revealing the ultrafast dynamics. Therefore, NiO should be become a new family of good candidates that show great potentials for nonlinear optical devices. So far, the nonlinear optical properties of the prepared diameter tuned NiO microrods have been reported by a Z-scan measurements conducted at 532 nm pulsed laser, and revealed that the NiO microrods were good optical limiters by two-photon absorption [17]. The 532 nm pulsed laser was also used to study the third-order nonlinearity of NiO nanoparticles in toluene [19]. In addition, the high-order nonlinear optical properties of NiO were reported via different electron transition processes with an increase of thickness at the wavelength of 1064 nm, including intraband transitions and three-photon interband transitions processes [20]. However, less attention has been paid to nonlinear optical response and photonics devices designed through spin-orbit splitting of 3d electrons in the visible region. Recently, MoS2 and semiconductor saturable absorber mirror (SESAM) based on well-designed GaInP quantum wells have been used in passive mode locking as the optical modulators in the visible range by using its electron interband transition processes [21–23]. Besides, NiO was employed as a mode-locker at the wavelengths of 1065.45 and 1343.12 nm by electron intraband transition arising from ground state (3A2g) to excited state (3T2g) [20]. In this paper, based on the spin-orbit splitting of 3d electrons, we have successfully designed an NiO nanosheet film which was prepared by the hydrothermal route and spin coating technique, and demonstrated the nonlinear optical response with the increase of incident pulse fluence in the visible region. By taking advantage of the saturable absorption properties of the NiO nanosheet film, a passively mode-locked laser at a wavelength of 640.3 nm has been achieved with a blue laser diode as the pump source and an NiO nanosheet film as the saturable absorber.
2. Experimental section
The as-grown NiO nanosheets as well as in high yield were successfully prepared via a facile hydrothermal route, as we previously reported [24]. In order to achieve the uniform dispersion of NiO nanosheet film, the film was prepared by mixing 5 mg of NiO nanosheets and 20 mL of sodium carboxymethylcellulose solution with concentration of 1 wt%, followed by ultrasonication for 2 hours to be used as a spin coating precursor. Subsequently, the precursor was spin coated on the Au/Si substrate at a rate of 800 rpm. Ultimately, the film sample was dried in a vacuum oven at 30 °C for 12 h to form the solid film.
3. Results and discussion
A scanning electron microscopy (SEM) was performed to examine the morphology of the NiO nanomaterial, as shown in Fig. 1(a). The NiO nanosheets exhibit a size ranging from 150 to 200 nm, and a thickness of about 15 nm. The X-ray diffraction pattern (XRD) of the as-prepared NiO nanosheets is shown in Fig. 1(b), the diffraction peaks are observed at 37.2◦, 43.3◦, 62.9◦, 74.7◦, and 78.6◦, which can respectively be assigned to the (111), (200), (220), (311), and (222) crystal faces of cubic NiO peaks (JCPDS No. 47–1049) [24]. No signal from other crystalline phases can be detected, indicating the high purity of the obtained NiO nanosheets. The atomic force microscopy (AFM) was used to determine the thickness of nanosheet film on the substrate, which cannot be observed in SEM characterization. From Fig. 1(c) and Fig. 1(d), the thickness of a typical nanosheet film is measured to be ≈280 nm.
Fig. 1 (a) SEM image, (b) XRD pattern, (c) AFM image, and (d) corresponding height profile of NiO nanosheets.
In order to investigate the linear optical properties of the prepared NiO nanosheets, the absorption spectrum was studied by UV–vis spectroscopy with the spectrum range from 300 nm to 800 nm and is shown in Fig. 2(a). The interband absorption edge of the NiO nanosheets occurs at about 360 nm (3.44 eV), which is caused by a transition from the 2p states of oxygen to the 3d states of Ni [25]. At the same time, the absorption band recorded in the range 600–800 nm (as shown in the inset of Fig. 2(a)) belongs to the electron transition process of 3A2g(F) → 1Eg(D) in terms of the spin-orbit splitting of the Ni2+ 3d configurations in an octahedral crystal field, as marked by the arrow in the inset of Fig. 2(b) [15,26]. According to our previous reported, sodium carboxymethylcellulose was only used as the dispersing agent in the preparation of NiO nanosheet film. In addition, the sodium carboxymethylcellulose had no absorption peak when the wavelength was longer than 600 nm [27]. Therefore, we can draw the conclusion that the visible photons can transfer resulted from spin-orbit splitting of the Ni 3d electronic levels. The electron transition process in the visible region cannot only cause the linear absorptions but also lead to the nonlinear absorptions. To explore the relationship about the fluence-dependent nonlinear optical response under a strong optical field, NiO nanosheet film was characterized at the 720 nm wavelength by a femtosecond Ti:sapphire amplifier with a pulse duration of 100 fs and a repetition rate of 80 MHz, as shown in Fig. 2(b). The nonlinear optical response of NiO nanosheet saturable absorber was studied by reflection mode [28]. The experimental setup for nonlinear optical response measurement is shown in Fig. 2(c), the laser pulse fluence at the sample was varied by adjusting the optical attenuator, and the laser was divided into two beams by a beam splitter, in which one beam was used as the reference, and another beam was focused onto sample with a focus lens (f = 25 cm). The sample under test was tilted with a small enough angle. In theory, the kinetic process of saturable absorption can be described by the fluence-dependent reflectivity equation as [29]
because both recovery time components of NiO (30–70 ps and 200–500 ps) are longer than the pulse duration (100 fs) [30], where R is the reflectivity of the sample film, ∆Rns and ∆R are the non-saturable loss and modulation depth, respectively, F is the incident pulse fluence, and Fsat is the saturation fluence. By fitting the measured data with Eq. (1), the values of ∆Rns, ∆R, and Fsat are determined to be ≈22.77%, 24.59%, and 4.41 nJ cm−2, respectively. From the dynamics of the NiO nanosheets with the increase of incident pulse fluence, it has been concluded that the saturable absorption is a balancing process from electronic transition due to 3d spin-orbit splitting under a strong optical field. Considering the saturable absorption generated by the electron transition process from ground state (3A2g) to excited state (1Eg) of 3d spin-orbit splitting in the visible region, the modulation depth of an NiO nanosheet saturable absorber is very high (up to 24.59%) compared to well-established saturable absorbers like MoS2 (about 1%−10%) [21], carbon nanotube (about 1%) [31], and well-established NiO saturable absorber at the wavelength of 1065.45 nm (1.48%) [20]. In addition, the saturation fluence is lower (4.41 nJ cm−2) than well-established MoS2 in the visible region (about 75–232 nJ cm−2) [21] and NiO as reported in our previous paper (about 25.6 nJ cm−2) [20]. Associated with the higher modulation depth and lower saturation fluence, NiO nanosheet film is reliable in the field of nanophotonics, e.g., optical switching, beam shaping, etc [32,33]. Meanwhile, NiO nanosheet saturable absorber has the advantages in the easy preparation and low cost compared with MoS2 [21] and SESAM [22,23] that were employed as mode-lockers in the visible region.Fig. 2 (a) UV–vis absorption spectrum (inset: a magnified image with the spectrum range from 600 nm to 800 nm), (b) reflectivity as a function of incident pulse fluence (inset: schematic energy level diagram of Ni2+ 3d8 configurations), and (c) experimental setup for nonlinear optical response measurement.
For ultrafast photonics applications, the mode-locking performance is related to the modulation depth and saturation fluence of the semiconductor saturable absorbers. To explore the feasibility of the obtained mode-locked laser, a quantitative confirmation of the theory is proposed in our previous paper [21]. Based on the relationship between laser dynamics and saturable absorption, the product of the saturation fluence and modulation depth (Fsat × ∆R) can be calculated as 1.6 to 2 nJ cm−2 for the visible laser at the wavelengths from green (about 520 nm) to deep red (about 720 nm). This result suggests that if we want to obtain a stable mode-locked visible laser, the nonlinear absorption parameters (Fsat × ∆R) of the NiO nanosheet optical modulator should be less than 2 nJ cm−2 orders of magnitude. For NiO nanosheet optical modulator in the letter, the value of Fsat × ∆R is determined to be ≈1.08 nJ cm−2, which is far less than the selection criteria (2 nJ cm−2). Therefore, the NiO nanosheets should be the most appropriate optical modulator in the visible mode-locked laser.
4. Ultrafast visible laser application
In the pulsed visible laser experiment, a standard X-folded cavity with four mirrors and an NiO nanosheet saturable absorber was used as shown in Fig. 3. A 3.5 W blue laser diode with a central wavelength of 444 nm was employed as the pump source. The pump beam was focused with two lens (F1 = 25 mm and F2 = 100 mm) and the laser beam radius on the gain material was determined to be about 65 μm. The gain medium was a 1.01 at. % a-cut Pr3+:LiGdF4 crystal with dimensions of 3 × 3 × 5 mm3. The 3 × 3 mm2 faces were polished and antireflection (AR) coated at the wavelengths of 440–450 nm and 500–750 nm. To protect the laser crystal from thermal fracture, the crystal was wrapped inside indium foil and mounted on a water-cooled copper holder with constant temperature control at 7 °C throughout the whole experiment. The cavity mirrors M1, M2, and M3 have the radii of curvature 100 mm with AR coated at 440–450 nm and high-reflection coated at 500–780 nm. The output coupler mirror M4 is a flat mirror with the transmittance of 1.8% at 639 nm. The reflection-type NiO nanosheet saturable absorber used in the experiment was put in the following of M3 with the waist radii of the laser beam of 30 μm. The physical length of the resonator was about 1.36 m. The output power was measured by a laser power meter (Newport, 1916-R), the mode-locked pulse trains were monitored with the mixed signal oscilloscope (Tektronix, MSO 72504DX) and an InGaAs photodetector (New focus, 1414 mode), and the laser spectrum was displayed with the spectrometer (YOKOGAWA, AQ6315) with a resolution of 0.05 nm.
Fig. 3 Experimental configuration of the pulsed visible laser based on NiO nanosheet saturable absorber.
The stable mode-locked laser at a wavelength of 639 nm was obtained with the NiO nanosheet saturable absorber. The details of the 639 nm mode-locked laser are shown in Fig. 4. Figure 4(a) shows the measured average output power as a function of the absorbed pump power. With a careful adjustment to the cavity elements, the laser absorbed pump power has a threshold of 507 mW. When the absorbed pump power is beyond 667 mW, the laser exhibits stable continuous wave mode-locked (CWML) operation. The highest average output power is 6 mW with an absorbed pump power of 797 mW. In the experiment, the mode-locked pulse trains are monitored for nearly an hour, and it remains stable. Figure 4(b) and inset show the pulse trains on 20 ns and 2 ms per division scale, it can be seen that the pulse trains are fully modulated with good pulse stability. Single pulse is also recorded and shown in Fig. 4(c) with a full width half maximum (FWHM) of 30.2 ps. At the same pump power, the spectrum of the passively mode-locked laser is shown in Fig. 4(d) with an FWHM of 0.1 nm at a central wavelength of 640.3 nm. The time-bandwidth product of the mode locked pulse is calculated to be 2.21, which indicates large chirp inside the pulse and their duration can be further narrowed [34]. The pulse repetition rate of NiO nanosheets is 109.8 MHz, which matches well with the cavity round trip time. The radio frequency (RF) signal is shown in the inset of Fig. 4(d) and the signal-to-noise ratio reaches 52 dB above the background, which is a sign of pulse stability. To verify whether the plused visible laser is purely contributed by the NiO saturable absorber, the NiO-based substrate is purposely replaced with only a substrate. In this case, no plused visible laser is observed, despite that the pump power is increased to the maximum available power. As we know, this study is the first to experimentally demonstrate pulsed visible laser using an NiO nanosheet saturable absorber.
Fig. 4 (a) Average output power versus absorbed pump power, (b) pulse train recorded in 20 ns/div (inset: 2 ms/div), (c) a single pulse trace, and (d) output spectrum (inset: an RF spectrum with a frequency of 109.8 MHz).
5. Conclusions
In summary, NiO nanosheets should be excellent media for realizing electron transition process caused by spin-orbit splitting of 3d electrons in the Ni2+ 3d configurations. Based on the investigation of spin-orbit splitting of 3d electrons, we comprehensively demonstrate the fluence-dependent nonlinear optical response of NiO nanosheets in the visible region, accompanying with the saturable absorption, which is attributed to a dynamic balancing process by electron transition process arising from 3d spin-orbit splitting in NiO nanosheets. Such saturable absorption properties of NiO nanosheets have potential application in modern photonics devices. Then, the use of NiO nanosheets as an optical modulator is demonstrated with the construction of laser diode-pumped visible mode-locked lasers with a center wavelength of 640.3 nm. Our results indicate that NiO nanosheets are a potential optical switcher for ultrafast photonics, which will broaden the photonics applications of 3d transition-metal oxides. In addition, the electron transition process originated from 3d spin-orbit splitting advocates a great promise of the 3d transition-metal oxides for nonlinear optical devices.
Funding
National Natural Science Foundation of China (NSFC) (51422205, 51372139, 51632004); National Key Research and Development Program of China (2016YFB0701002); Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201415); Taishan Scholar Foundation of Shandong Province, China.
References and links
1. M. A. Chougule, S. G. Pawar, P. R. Godse, R. D. Sakhare, S. Sen, and V. B. Patil, “Sol–gel derived Co3O4 thin films: effect of annealing on structural, morphological and optoelectronic properties,” J. Mater. Sci. Mater. Electron. 23(3), 772–778 (2012). [CrossRef]
2. B. Gu, J. He, W. Ji, and H. T. Wang, “Three-photon absorption saturation in ZnO and ZnS crystals,” J. Appl. Phys. 103(7), 073105 (2008). [CrossRef]
3. X. C. Dong, H. Xu, X. W. Wang, Y. X. Huang, M. B. Chan-Park, H. Zhang, L. H. Wang, W. Huang, and P. Chen, “3D graphene-cobalt oxide electrode for high-performance supercapacitor and enzymeless glucose detection,” ACS Nano 6(4), 3206–3213 (2012). [CrossRef] [PubMed]
4. V. I. Sokolov, A. V. Druzhinin, G. A. Kim, N. B. Gruzdev, A. Y. Yermakov, M. A. Uimin, I. V. Byzov, N. N. Shchegoleva, V. B. Vykhodets, and T. E. Kurennykh, “Fundamental absorption edge of NiO nanocrystals,” Phys. B 430, 1–5 (2013). [CrossRef]
5. L. T. Hoa, H. N. Tien, and S. H. Hur, “A highly sensitive UV sensor composed of 2D NiO nanosheets and 1D ZnO nanorods fabricated by a hydrothermal process,” Sens. Actuators A Phys. 207, 20–24 (2014). [CrossRef]
6. H. L. Xue, X. Z. Kong, Z. R. Liu, C. X. Liu, J. R. Zhou, W. Y. Chen, S. P. Ruan, and Q. Xu, “TiO2 based metal-semiconductor-metal ultraviolet photodetectors,” Appl. Phys. Lett. 90(20), 201118 (2007). [CrossRef]
7. G. Chen, S. Ji, Y. Sang, S. Chang, Y. Wang, P. Hao, J. Claverie, H. Liu, and G. Yu, “Synthesis of scaly Sn3O4/TiO2 nanobelt heterostructures for enhanced UV-visible light photocatalytic activity,” Nanoscale 7(7), 3117–3125 (2015). [CrossRef] [PubMed]
8. Y. Tokura and N. Nagaosa, “Orbital physics in transition-metal oxides,” Science 288(5465), 462–468 (2000). [CrossRef] [PubMed]
9. W. Nolting, L. Haunert, and G. Borstel, “Temperature-dependent electronic structure and magnetic behavior of mott insulators,” Phys. Rev. B Condens. Matter 46(8), 4426–4445 (1992). [CrossRef] [PubMed]
10. R. J. Powell and W. E. Spicer, “Optical properties of NiO and CoO,” Phys. Rev. B 2(6), 2182–2193 (1970). [CrossRef]
11. O. Bengone, M. Alouani, P. Blochl, and J. Hugel, “Implementation of the projector augmented-wave LDA+U method: application to the electronic structure of NiO,” Phys. Rev. B 62(24), 16392–16401 (2000). [CrossRef]
12. D. Adler and B. Feinleib, “Electrical and optical properties of narrow-band materials,” Phys. Rev. B 2(8), 3112–3134 (1970). [CrossRef]
13. J. E. Hirsch, “Spin hall effect,” Phys. Rev. Lett. 83(9), 1834–1837 (1999). [CrossRef]
14. S. Murakami, N. Nagaosa, and S. C. Zhang, “Dissipationless quantum spin current at room temperature,” Science 301(5638), 1348–1351 (2003). [CrossRef] [PubMed]
15. Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, and Y. Liu, “Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity,” ACS Appl. Mater. Interfaces 2(10), 2915–2923 (2010). [CrossRef] [PubMed]
16. S. Mori, S. Fukuda, S. Sumikura, Y. Takeda, Y. Tamaki, E. Suzuki, and T. Abe, “Charge-transfer processes in dye-sensitized NiO solar cells,” J. Phys. Chem. C 112(41), 16134–16139 (2008). [CrossRef]
17. B. Karthikeyan, T. Pandiyarajan, S. Hariharana, and M. S. Ollakkan, “Wet chemical synthesis of diameter tuned NiO microrods: microstructural, optical and optical power limiting applications,” CrystEngComm 18(4), 601–607 (2016). [CrossRef]
18. N. Park, K. Sun, Z. L. Sun, Y. Jing, and D. L. Wang, “High efficiency NiO/ZnO heterojunction UV photodiode by sol–gel processing,” J. Mater. Chem. C Mater. Opt. Electron. Devices 1(44), 7333–7338 (2013). [CrossRef]
19. L. A. Gómez, C. B. de Araújo, L. M. Rossi, S. H. Masunaga, and R. F. Jardim, “Third-order nonlinearity of nickel oxide nanoparticles in toluene,” Opt. Lett. 32(11), 1435–1437 (2007). [CrossRef] [PubMed]
20. B. Sun, Y. X. Zhang, R. Zhang, H. H. Yu, G. W. Zhou, H. J. Zhang, and J. Y. Wang, “High-order nonlinear optical properties generated by different electron transition processes of NiO nanosheets and applications to ultrafast lasers,” Adv. Opt. Mater. 5(8), 1600937 (2017). [CrossRef]
21. 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] [PubMed]
22. R. Bek, H. Kahle, T. Schwarzbäck, M. Jetter, and P. Michler, “Mode-locked red-emitting semiconductor disk laser with sub-250 fs pulses,” Appl. Phys. Lett. 103(24), 242101 (2013). [CrossRef]
23. M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014). [CrossRef] [PubMed]
24. B. Sun, G. W. Zhou, T. T. Gao, H. J. Zhang, and H. H. Yu, “NiO nanosheet/TiO2 nanorod-constructed p–n heterostructures for improved photocatalytic activity,” Appl. Surf. Sci. 364, 322–331 (2016). [CrossRef]
25. K. Terakura, A. R. Williams, T. Oguchi, and J. Kiibler, “Transition-metal monoxides: band or mott insulators,” Phys. Rev. Lett. 52(20), 1830–1833 (1984). [CrossRef]
26. W. Low, “Paramagnetic and optical spectra of divalent nickel in cubic crystalline fields,” Phys. Rev. 109(2), 247–255 (1958). [CrossRef]
27. S. X. Wang, Y. X. Zhang, R. Zhang, H. H. Yu, H. J. Zhang, and Q. H. Xiong, “High-order nonlinearity of surface plasmon resonance in Au nanoparticles: paradoxical combination of saturable and reverse-saturable absorption,” Adv. Opt. Mater. 3(10), 1342–1348 (2015). [CrossRef]
28. Z. Qin, G. Xie, C. Zhao, S. Wen, P. Yuan, and L. Qian, “Mid-infrared mode-locked pulse generation with multilayer black phosphorus as saturable absorber,” Opt. Lett. 41(1), 56–59 (2016). [CrossRef] [PubMed]
29. O. Okhotnikov, A. Grudinin, and M. Pessa, “Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications,” New J. Phys. 6, 177–198 (2004). [CrossRef]
30. V. V. Volkov, Z. L. Wang, and B. S. Zou, “Carrier recombination in clusters of NiO,” Chem. Phys. Lett. 337(1–3), 117–124 (2001). [CrossRef]
31. J. H. Yim, W. B. Cho, S. Lee, Y. H. Ahn, K. Kim, H. Lim, G. Steinmeyer, V. Petrov, U. Griebner, and F. Rotermund, “Fabrication and characterization of ultrafast carbon nanotube saturable absorbers for solid-state laser mode locking near 1 μm,” Appl. Phys. Lett. 93(16), 161106 (2008). [CrossRef]
32. K. Wang, J. Wang, J. Fan, M. Lotya, A. O’Neill, D. Fox, Y. Feng, X. Zhang, B. Jiang, Q. Zhao, H. Zhang, J. N. Coleman, L. Zhang, and W. J. Blau, “Ultrafast saturable absorption of two-dimensional MoS2 nanosheets,” ACS Nano 7(10), 9260–9267 (2013). [CrossRef] [PubMed]
33. X. Zhang, S. Zhang, C. Chang, Y. Feng, Y. Li, N. Dong, K. Wang, L. Zhang, W. J. Blau, and J. Wang, “Facile fabrication of wafer-scale MoS2 neat films with enhanced third-order nonlinear optical performance,” Nanoscale 7(7), 2978–2986 (2015). [CrossRef] [PubMed]
34. Y. G. Wang, H. R. Chen, W. F. Hsieh, and Y. H. Tsang, “Watt-level high power passively mode-locked Nd:LuVO4 laser with carbon nanotube saturable absorber at 1.34 μm,” Opt. Commun. 285(24), 5372–5374 (2012). [CrossRef]
