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A comparative research on saturable absorption (SA) behavior dependence on wavelength and pulse duration was performed for graphene polymer composites. Free-standing graphene-polyvinyl alcohol (PVA) composite films were fabricated by using solution cast method in combination of liquid phase exfoliation. SA responses were observed by using an open-aperture Z-scan technique for 340 fs pulses at 1030 nm and 515 nm from a mode-locked fiber laser, and 6 ns pulses at 1064 nm and 532 nm from a Q-switched Nd:YAG laser. The graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity Is, for ns pulses than that for fs pulses at the similar near infrared (NIR) wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, the excited state and ground state absorption cross sections were estimated to be ~10−17 cm2, and the ratio was ~0.6 at NIR for both fs and ns pulses.
© 2015 Optical Society of America
1. Introduction
Saturable absorption (SA) materials and devices are research hot spots in the field of laser technology as it plays a critical role on pulse generation, beam shaping, slow light, optical switching, and so on [1–6]. In order to fulfill the requirement of wide-ranging laser applications, SA materials should possess characteristics of high nonlinear response over broad wavelength and time ranges, high optical quality and mechanical stability, easy preparation procedure, low cost, etc. Since the first generation of saturable absorber, the dye saturable absorbers, was used to passively mode lock the Nd:glass laser in 1966, a large family of SA materials has been invested into this research field [7]. Semiconductor saturable absorber mirrors (SESAMs) are widely used as mode lockers, but these require complex fabrication and packaging and have relatively narrow tuning range (tens of nanometers) [2]. Single-walled carbon nanotubes (SWNTs) are also a candidate for a tunable broadband saturable absorber, however, the working wavelength is defined by choosing the SWNT diameter (i.e., band gap) [8, 9]. Recently, graphene entered into this family and has received much attention because of its excellent physical and chemical properties, such as, high carrier mobility, strict optical transparency of single layer, high thermal conductivity, high chemical stability, ultrafast carrier dynamics, etc [10–14]. In particular, the zero band gap and linear dispersion of Dirac electrons offer graphene the advantage as a saturable absorber over a wide spectral range from the visible to the near infrared (NIR) [11].
Polymer-based graphene composites, as an important material form for photonic and optoelectronic devices, can combine the optical properties of graphene and the structural properties of the host matrix. In addition to low cost and non-toxic, these polymeric systems possess advantages like gas-barrier properties, ease of integration into photonic devices, high mechanical flexibility, improved transparency and stability [15–18]. With graphene-PVA (polyvinyl alcohol) composite used as a saturable absorber, Sun et al [19, 20]. has realized a mode-locked fiber laser that works over a wide wavelength range of 1525 to 1559 nm. Fu et al [21] reported broadband ultrafast fiber lasers at 1, 1.5, and 2 μm mode-locked by a single graphene SA device. Popa et al [22] achieved a wideband Q-switched fiber laser tunable between 1552 and 1555 nm exploiting a graphene-based SA, and Rosdin et al. [23] proposed a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 mW. We recently demonstrated a linearly polarized 1180 nm passively mode-locked Raman fiber laser [24] and an all-polarization maintaining Q-switching Yb-doped fiber laser using graphene-based saturable absorber [25]. Graphene-PVA solution composite prepared by Husaini et al. [26] have been studied at 785 nm and 1064 nm in the ns and ps temporal regimes, respectively. The authors found that the output fluence readily clamped with increasing input fluence and graphene concentration, due to nonlinear absorption and scattering. However, there is still a lack of a comparative research on nonlinear absorption property dependence on wavelength and pulse duration for graphene polymer composites. In this work, we prepared free-standing graphene-PVA (G-PVA) composite films to study the intrinsic nonlinear absorption property of graphene. Absorption saturation was observed at a broad wavelength range (from the visible to the NIR) and wide time regime (from ns to fs). We show that the graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity Is, for ns pulses than that for fs pulses at the similar NIR wavelength. For fs pulses, the films show better SA response at NIR (1030 nm) than that at the visible (515 nm). By employing slow and fast SA modeling [27], the excited state and ground state absorption cross sections were estimated to be ~10−17 cm2, and the ratio was ~0.6 at NIR for both fs and ns pulses.
2. Experiments
The graphene dispersions were produced by liquid exfoliation technique [28]. Figure 1(a) shows the process flow of the graphene dispersions being treated in sodium cholate (SC). According to previous theoretical and experimental analyses, the surface energy of SC can match very well with that of graphite (70~80 mJ/m2), resulting in effective exfoliation to graphene with single or few layers [28, 29]. The graphite powders were dispersed in an aqueous solution of the surfactant SC (CSC = 1.5 mg/ml) with a concentration of 5 mg/ml. Initial graphene dispersions were produced by sonication through a point probe (flathead sonic tip) for 60 min with a power output of 285 W, followed by centrifugation at 3000 rpm for 90 min. After centrifugation, high quality graphene dispersions were obtained by collecting the top 3/4 centrifuged samples. All dispersions were stable against sedimentation over a few weeks. The G-PVA composite films were produced by solution-cast method [30, 31], which is considered to be the most versatile and easiest method for the synthesis of free-standing polymer composite thin films [18]. Photographs of the G-PVA composite films being produced at different stages are shown in Fig. 1(b). The PVA powders (0.5 g) were dissolved into distilled water (10 ml) at ~90°C through magnetic stirring until the polymer was completely dispersed. The polymer solutions were then cooled down to room temperature. Subsequently, the prepared graphene dispersions were added into the PVA solutions, and the resulting mixtures were stirred for 24 h and ultrasonically agitated for 4 h to obtain homogeneous solutions. The mixtures were then casted into polymer petri-dishes with the diameter of 55 mm, followed by drying at 55°C for 3-4 days. High quality transparent films with uniform surface were thus obtained, and the thicknesses located in a range of 130~160 µm measured by a micrometer. The thicknesses of the films can be controlled mainly by the mass of PVA and the diameter of the petri-dishes. The influence of volume and concentration of graphene dispersions on the film thickness can be ignored. Linear transmittance of the G-PVA film was controlled by changing the amount of graphene nanoflakes in the composite film.
Fig. 1 Process flow of G-PVA composite film fabrication. (a) Photographs of graphene SC dispersions being proceed at different stages by liquid phase exfoliation technique. (b) Photographs of the G-PVA composite films being proceed at different stages by solution-cast method.
Nonlinear absorption property of G-PVA composite films was studied by using an open aperture Z-scan system [32, 33]. The optical arrangement possesses advantages of simple, sensitive, rapid, and has been widely adopted to investigate the nonlinear absorption and scattering properties, which has been used in our previous works of testing the nonlinear responses of graphene and Layered Molybdenum Dichalcogenide Semiconductors dispersions [14, 34, 35]. For a broad spectral and time domain study, the Z-scan measurements were performed by employing a fiber laser of 340 fs pulses operating at 1030 nm and its second harmonic of 515 nm with the repetition of 100 Hz, and a Q-switched Nd:YAG laser of 6 ns pulses operating at 1064 nm and its second harmonic of 532 nm with the repetition of 10 Hz. The laser beams were tightly focused through a lens with the focal length of 10 cm. In addition, the laser beam waist radius at the focus were estimated to be ~30 µm at 1030 nm, ~15.5 µm at 515 nm and ~46 µm at 1064 nm, respectively.
3. Results and discussion
Absorption spectrum of a G-PVA composite film and a reference PVA film are plotted in Fig. 2(a). The absorption peak in the UV region is ascribed to the π-π* transitions of the C = C bonds in the aromatic ring of grapheme [24]. The photographs of a pure PVA film and a G-PVA composite film are shown in the insets, from which we can confirm the good uniformity of the composite films. Raman characterization of the G-PVA film was carried out by using a Renishaw Invia Raman spectrometer excited at 488 nm. As shown in Fig. 2(b), the graphene polymer films exhibit similar Raman fingerprints to graphene except the band around~1440 cm−1 which is originated from PVA. The G peak at ~1580 cm−1 corresponds to the first-order scattering of the E2g phonon at the Brillouin [19]. The disorder-related D peak (~1350 cm−1) mainly originates from a large quantity of edges due to the small size of graphene nanoflakes [36]. The characteristics of the 2D band (~2700 cm−1) vary along with the number of layers in graphene 〉akes, from a single peak (monolayer) to a band with four component peaks (bilayer) to a broadened band (<five layers), and then finally to a graphite-type band with two component peaks (>five layers) [28, 36]. We note that the 2D band, although broader than that in pristine graphene, still can be fitted by a single Lorentzian line which implies that the graphene flakes are less than five layers [14]. Figure 2(c) shows the bright-field TEM image of a graphene nano〉ake dispersed in SC. According to our previous work, the number fraction of monolayer graphene (number of monolayers/total number of flakes) is close to 30% and the size of the graphene nanoflakes is ~0.5-2 μm [14]. The AFM image in Fig. 2(d) displays the surface morphology of the G-PVA composite film and the surface roughness is about 4 nm.
Fig. 2 (a) Absorption spectra of a G-PVA composite film and a pure PVA film. Insets: photographs of the G-PVA film and pure PVA. (b) Raman spectrum of the G-PVA film. (c) Bright-field TEM image of a graphene nanoflake. (d) AFM image of the G-PVA composite film.
The nonlinear study was focused on a couple of films with a high concentration and a relative low concentration of graphene, respectively. As given in Table 1, with a very close value of thickness, the two films show similar linear transmittance at NIR for either fs or ns pulses. Figures 3(a), 3(c) and 3(e) show the typical Z-scan results for the high concentration G-PVA film. The normalized transmittance increases as the film moves toward the beam focus (z = 0), indicating a clear SA response. The SA becomes much more pronounced as the incident energy increasing. The low concentration film has similar Z-scan traces. In order to have a better understanding on the SA responses between the high concentration and low concentration films, we selected two Z-scan curves with same I0 (on-focus intensity) and converted to the format of the normalized transmission as a function of incident intensity [see Figs. 3(b), 3(d) and 3(f)]. As we can see, the SA response rises readily with the input intensity and graphene concentration increasing. It should be pointed out that no NLO behavior was observed for ns pulses at 532 nm before the films were damaged at the excitation pulse energy ~25 µJ.
Table 1. Linear and NLO parameters of the G-PVA films.
Fig. 3 (a), (c) and (e): Z-scan curves at different incident pulse energies of the film with lower T0. The solid lines are the fitting results. (b), (d) and (f): Normalized transmission as a function of input laser intensity.
According to the NLO theory, the propagation equation in G-PVA composite films can be expressed as: , where I is the excitation intensity and is the propagation distance in the sample. The total absorption consists of the linear absorption coefficient α0 and the nonlinear absorption coefficient αNL, . In our experiment, we did not observe any clear NLO response from the pure PVA films. As a result, the contribution of αNL only comes from the nonlinear absorption of graphene in the composite films. By fitting the Z-scan data using the two equations above, we can obtain the αNL of the composite films in a series of incident intensities (left column in Fig. 4). The average values are given in Table 1. It is clearly that αNL decreases gradually in our experimental range. At same on-focus intensity (I0), the film with lower T0 possesses higher αNL than the film with higher T0. This is consistent with the SA response rising with increasing graphene concentration. The imaginary part of the third order NLO susceptibility,, is directly related to αNL, , where c is the speed of light, λ is the wavelength of the incident light, and n is the refractive index. In order to eliminate the discrepancy caused by the linear absorption α0, we define figure of merit (FOM) for the third order optical nonlinearity as . FOM for the lower T0 film can be calculated as ~(9.3 ± 4.4) × 10−15 esu cm at 1030 nm for 340 fs pulses, ~(2.2 ± 1.7) × 10−15 esu cm at 515 nm for 340 fs pulses, and ~(2.2 ± 0.5) × 10−13 esu cm at 1064 nm for 6 ns pulses, respectively. The FOM values are comparable with some other SA nanomaterials, such as graphene in DMF ~5.0 × 10−15 esu cm at 790 nm for 80 fs pulses [37], reduced graphene oxide ~0.36 × 10−15 esu cm at 395 nm for 80 fs pulses [38], SWNTs ~4.8 × 10−13 esu cm at 780 nm for 200 fs pulses [39], and AgInSe2 nanorods~8 × 10−15 esu cm at 780 nm for 200 fs pulses [40].
Fig. 4 αNL and Is as functions of I0 for the two G-PVA films for fs pulses at 1030 nm ((a) and (b)), fs pulses at 515 nm ((c) and (d)), and ns pulses at 1064 nm ((e) and (f)).
To obtain the saturation intensity Is, we can describe with the form of [41]. The right column in Fig. 4 gives the Is results as a function of I0. It is clearly that Is increases gradually with I0 increasing for the two films, and the higher T0 film exhibits larger Is value than the lower one at same I0, implying the lower T0 film exhibits much stronger SA response than the higher one. All linear and NLO parameters of the two composite films are summarized in Table 1. From αNL, Imχ(3), FOM and Is, we can find that the graphene films have better NLO property, i.e., larger SA coefficient and FOM, and lower Is, for ns pulses than that for fs pulses at the NIR wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm.
In order to fully illustrate the SA response, we give the normalized transmission of the film with low T0 as a function of the input laser intensity for the four laser sources (Fig. 5). It is obvious that the film exhibits SA response at all the wavelengths except for the ns pulses at 532 nm. The largest normalized transmission variation for fs pulses at 1030 nm is ~13.4% when the excitation energy up to 440 nJ/pulse, corresponding to an estimated on-focus intensity of:33.4 GW/cm2. For fs pulses at 515 nm, the largest variation achieves~7.3% at the excitation energy 200 nJ/pulse (~77 GW/cm2). And for ns pulses at 1064 nm, the value is ~1.7% at the excitation energy 55.4 µJ/pulse (~0.1 GW/cm2). The films were damaged at the focal point when the energy was 600 nJ/pulse, 240 nJ/pulse, 70 µJ/pulse and 25 µJ/pulse, for laser pulses at 1030 nm, 515 nm, 1064 nm and 532 nm, respectively.
Fig. 5 Nonlinear transmission of the G-PVA film with low T0 as a function of the input laser intensity for fs pulses at 1030 nm and 515 nm, and ns pulses at 1064 nm and 532 nm. The solid lines are fitting results based on the fast and slow saturable absorber models. Inset is a schematic diagram of graphene level and a three-level system used for the modeling.
For the purpose of further studying the SA properties of graphene over a broad spectral and time domain, we regard graphene as a three-level system (inset of Fig. 5) according to its carrier relaxation dynamics. The SA results in Fig. 5 were modeled for two limiting cases: slow saturable absorber, analyzed by the modified Frantz-Nodvik equation, and fast saturable absorber, analyzed by the steady-state solution of Hercher's rate equations [27, 42]. The term “slow absorber” is used for the case where the pulse duration is much shorter compared to the excited state decay time τ. An analytical solution for the transmission T of a real saturable absorber is given in a closed form as
where , , σgs and σes are the ground state and the excited state absorption cross sections, L is the sample thickness, T0 is the transmission in the low pulse intensity limit, Tmax is the maximal transmission achieved at very high pulse intensities, N is the absorbers density and TFN is the transmission of an “ideal” saturable absorber (),where E(0) is the input beam fluence in units of photons per unit area. This expression is known as the Frantz-Nodvik equation [43]. The case “fast saturable absorber” occurs when the relaxation of excited atoms is very fast, namely, the excited-state decay time τ is short compared to the pulse duration. The expression is given in a closed form aswhere , , I(L) is the photon 〉ux density (photons per unit time per unit area) at distance L, and. This expression is known as Hercher’s equation [44].The excited state lifetime of graphene is around a few picoseconds [45, 46], which is quite longer than the 340 fs pulse duration and shorter than the 6 ns pulse duration in our experiment. Therefore, the results of 340 fs pulses at 1030 nm and 515 nm were modeled by the slow saturable absorber expression and the results of 6 ns pulses at 1064 nm were modeled by the fast saturable absorber expression, which are shown in Fig. 5 (solid lines). The parameters used for the fitting are given in Table 2. Particularly, the estimated ground state absorption cross sections are σgs = 4.9 × 10−17 cm2 at 1030 nm for 340 fs pulses, σgs = 3.1 × 10−17 cm2 at 515 nm for 340 fs pulses, σgs = 9.9 × 10−17 cm2 at 1064 nm for 6 ns pulses. All of these values are in the same order of magnitude. The values of the excited state absorption cross section, i.e., σes = 2.9 × 10−17 cm2 at 1030 nm, σes = 2.1 × 10−17 cm2 at 515 nm, σes = 6.1 × 10−17 cm2 at 1064 nm, are smaller than the corresponding ground state absorption cross sections, resulting in the saturable absorption phenomena. The σes/σgs are estimated to be 0.59 at 1030 nm, 0.68 at 515 nm and 0.62 at 1064 nm. The estimated excited-state lifetime τ is ~1.3 ps at 1064 nm, which is consistent with the assumption of a fast absorber that τ should be far shorter than the pulse duration of 6 ns.
Table 2. The physical parameters used for the fitting based on a fast saturable absorber and a slow saturable absorber models.
4. Conclusion
In conclusion, we have investigated the SA behavior of G-PVA composite films over a broadband range from the visible to the NIR and from ns to fs. At the same excitation condition, the lower transmittance film exhibited stronger SA response than the higher one at all the wavelengths. The graphene films exhibit better SA, i.e., larger SA coefficient and FOM, and lower Is, for ns pulses than that for fs pulses at the NIR wavelength. For fs pulses, the films show larger SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, σes and σgs were estimated to be ~10−17 cm2, and the ratio was ~0.6 at NIR for both fs and ns pulses.
Acknowledgments
This work is supported by NSFC (No. 61178007, 61308087, 61308034, and 51072207), the External Cooperation Program of BIC, CAS (No. 181231KYSB20130007), STCSM (No. 11nm0502400, 12ZR1451800, 10XD1404600, and Shanghai Pujiang Program 12PJ1409400), China Postdoctoral Science Foundation (2014T70435 and 2012M520049). W.J.B. gratefully acknowledges Science Foundation Ireland (SFI, 12/IA/1306). J.W. thanks the National 10000-Talent Program and CAS 100-Talent Program for financial support.
References and links
1. U. Keller, K. J. Weingarten, F. X. Kartner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Honninger, N. Matuschek, and J. A. Au, “Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 2(3), 435–453 (1996). [CrossRef]
2. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]
3. N. Xiang, M. D. Guina, A. M. Vainionpää, J. Lyytikäinen, S. Suomalainen, M. J. Saarinen, O. Okhotnikov, T. Sajavaara, and J. Keinonen, “Broadband semiconductor saturable absorber mirrors in the 1.55 µm wavelength range for pulse generation in fiber lasers,” IEEE J. Quantum Electron. 38(4), 369–374 (2002). [CrossRef]
4. I. D. Jung, F. X. Fartner, N. Matuschek, D. H. Sutter, F. Morier-Genoud, Z. Shi, V. Scheuer, M. Tilsch, T. Tschudi, and U. Keller, “Semiconductor saturable absorber mirrors supporting sub-10-fs pulses,” Appl. Phys. B 65(2), 137–150 (1997). [CrossRef]
5. E. B. Aleksandrov and V. S. Zapasskii, “Chasing ‘slow light’,” Phys. Usp. 49(10), 1067–1075 (2006). [CrossRef]
6. J. Wang, C. Sun, and J.-K. Wang, “Nonlinear pulse-shaping phenomena of semiconductor saturable absorber mirror,” Appl. Phys. Lett. 89(23), 231106 (2006). [CrossRef]
7. A. J. D. Maria, D. A. Stetser, and H. Heynau, “Self mode-locking of lasers with saturable absorbers,” Appl. Phys. Lett. 8(7), 174–176 (1966). [CrossRef]
8. V. Scardaci, Z. Sun, F. Wang, A. G. Rozhin, T. Hasan, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, “Carbon nanotube polycarbonate composites for ultrafast lasers,” Adv. Mater. 20(21), 4040–4043 (2008). [CrossRef]
9. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, “Wideband-tuneable, nanotube mode-locked, fibre laser,” Nat. Nanotechnol. 3(12), 738–742 (2008). [CrossRef] [PubMed]
10. 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] [PubMed]
11. A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nat. Mater. 6(3), 183–191 (2007). [CrossRef] [PubMed]
12. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, and A. K. Geim, “Giant intrinsic carrier mobilities in graphene and its bilayer,” Phys. Rev. Lett. 100(1), 016602 (2008). [CrossRef] [PubMed]
13. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, and B. H. Hong, “Large-scale pattern growth of graphene films for stretchable transparent electrodes,” Nature 457(7230), 706–710 (2009). [CrossRef] [PubMed]
14. J. Wang, Y. Hernandez, M. Lotya, J. N. Coleman, and W. J. Blau, “Broadband nonlinear optical response of graphene dispersions,” Adv. Mater. 21(23), 2430–2435 (2009). [CrossRef]
15. S. Kamel, “Nanotechnology and its applications in lignocellulosic composites, a mini review,” Express Polym. Lett. 1(9), 546–575 (2007). [CrossRef]
16. M. Tanahashi, “Development of fabrication methods of filler/polymer nanocomposites: With focus on simple melt-compounding-based approach without surface modification of nanofillers,” Materials 3(3), 1593–1619 (2010). [CrossRef]
17. P. H. C. Camargo, K. G. Satyanarayana, and F. Wypych, “Nanocomposites: synthesis, structure, properties and new application opportunities,” Mater. Res. 12(1), 1–39 (2009). [CrossRef]
18. Y. Hou, J. Tang, H. Zhang, C. Qian, Y. Feng, and J. Liu, “Functionalized few-walled carbon nanotubes for mechanical reinforcement of polymeric composites,” ACS Nano 3(5), 1057–1062 (2009). [CrossRef] [PubMed]
19. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]
20. Z. Sun, D. Popa, T. Hasan, F. Torrisi, F. Wang, E. J. R. Kelleher, J. C. Travers, V. Nicolosi, and A. C. Ferrari, “A stable, wideband tunable, near transform-limited, graphene-mode-locked, ultrafast laser,” Nano Res. 3(9), 653–660 (2010). [CrossRef]
21. B. Fu, Y. Hua, and X. Xiao, Member, IEEEH. Zhu, Z. Sun, and C. Yang, “Broadband graphene saturable absorber for pulsed fiber lasers at 1, 1.5, and 2 μm,” IEEE J. Sel. Top. Quantum Electron. 20(5), 1100705 (2014).
22. 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]
23. R. Z. R. R. Rosdin, F. Ahmad, N. M. Ali, S. W. Harun, and H. Arof, “Q-switched Er-doped fiber laser with low pumping threshold using graphene saturable absorber,” Chin. Opt. Lett. 12(9), 091404 (2014). [CrossRef]
24. L. Zhang, G. Wang, J. Hu, J. Wang, J. Fan, J. Wang, and Y. Feng, “Linearly polarized 1180-nm Raman fiber laser mode locked by graphene,” IEEE Photonics J. 4(5), 1809–1815 (2012). [CrossRef]
25. L. Zhang, J. Fan, J. Wang, J. Hu, M. Lotya, G. Wang, R. Li, L. Zhang, W. J. Blau, J. N. Coleman, J. Wang, and Y. Feng, “Graphene incorporated Q-switching of a polarization-maintaining Yb-doped fiber laser,” Laser Phys. Lett. 9(12), 888–892 (2012).
26. S. Husaini, J. E. Slagle, J. M. Murray, S. Guha, L. P. Gonzalez, and R. G. Bedford, “Broadband saturable absorption and optical limiting in graphene-polymer composites,” Appl. Phys. Lett. 102(19), 191112 (2013). [CrossRef]
27. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]
28. Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun’Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari, and J. N. Coleman, “High-yield production of graphene by liquid-phase exfoliation of graphite,” Nat. Nanotechnol. 3(9), 563–568 (2008). [CrossRef] [PubMed]
29. S. D. Bergin, V. Nicolosi, P. V. Streich, S. Giordani, Z. Y. Sun, A. H. Windle, P. Ryan, N. P. Niraj, Z. T. Wang, L. Carpenter, W. J. Blau, J. Boland, J. P. Hamilton, and J. N. Coleman, “Towards solutions of single-walled carbon nanotubes in common solvents,” Adv. Mater. 20(10), 1876–1881 (2008). [CrossRef]
30. H. Zidan, “Structural properties of CrF3- and MnCl2-filled poly (vinyl alcohol) films,” J. Appl. Polym. Sci. 88(5), 1115–1120 (2003). [CrossRef]
31. Z. Guo, D. Zhang, S. Wei, Z. Wang, A. B. Karki, Y. Li, P. Bernazzani, D. P. Young, J. A. Gomes, D. L. Cocke, and T. C. Ho, “Effects of iron oxide nanoparticles on polyvinyl alcohol: interfacial layer and bulk nanocomposites thin film,” J. Nanopart. Res. 12(7), 2415–2426 (2010). [CrossRef]
32. M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]
33. H. Yan and J. S. Wei, “False nonlinear effect in z-scan measurement based on semiconductor laser devices: theory and experiments,” Photon. Res. 2(2), 51–58 (2014). [CrossRef]
34. 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]
35. K. Wang, Y. Feng, C. Chang, J. Zhan, C. Wang, Q. Zhao, J. N. Coleman, L. Zhang, W. J. Blau, and J. Wang, “Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors,” Nanoscale 6(18), 10530–10535 (2014). [CrossRef] [PubMed]
36. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene layers,” Phys. Rev. Lett. 97(18), 187401 (2006). [CrossRef] [PubMed]
37. S. Kumar, M. Anija, N. Kamaraju, K. S. Vasu, K. S. Subrahmanyam, A. K. Sood, and C. N. R. Rao, “Femtosecond carrier dynamics and saturable absorption in graphene suspensions,” Appl. Phys. Lett. 95(19), 191911 (2009). [CrossRef]
38. S. Kumar, N. Kamaraju, K. S. Vasu, A. Nag, A. K. Sood, and C. N. R. Rao, “Graphene analogue BCN: Femtosecond nonlinear optical susceptibility and hot carrier dynamics,” Chem. Phys. Lett. 499(1–3), 152–157 (2010). [CrossRef]
39. S. Tatsuura, M. Furuki, Y. Sato, I. Iwasa, M. Tian, and H. Mitsu, “Semiconductor carbon nanotubes as ultrafast switching materials for optical telecommunications,” Adv. Mater. 15(6), 534–537 (2003). [CrossRef]
40. H. I. Elim, W. Ji, M. T. Ng, and J. J. Vittal, “AgInSe2 nanorods: A semiconducting material for saturable absorber,” Appl. Phys. Lett. 90(3), 033106 (2007). [CrossRef]
41. G. Xing, H. Guo, X. Zhang, T. C. Sum, and C. H. A. Huan, “The physics of ultrafast saturable absorption in graphene,” Opt. Express 18(5), 4564–4573 (2010). [CrossRef] [PubMed]
42. M. Pokrass, Z. Burshtein, R. Gvishi, and M. Nathan, “Saturable absorption of multi-walled carbon nanotubes/hybrid-glass composites,” Opt. Mater. Express 2(6), 825–838 (2012). [CrossRef]
43. L. M. Frantz and J. S. Nodvik, “Theory of pulse propagation in a laser amplifier,” J. Appl. Phys. 34(8), 2346–2349 (1963). [CrossRef]
44. M. Hercher, “An analysis of saturable absorbers,” Appl. Opt. 6(5), 947–954 (1967). [CrossRef] [PubMed]
45. W. J. Cao, H. Y. Wang, A. P. Luo, Z. C. Luo, and W. C. Xu, “Graphene-based, 50 nm wide-band tunable passively Q-switched fiber laser,” Laser Phys. Lett. 9(1), 54–58 (2012). [CrossRef]
46. J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]
