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A comparative research on the near infrared performance of three kinds of widely used graphene saturable absorbers, namely, graphene polymer composite films, neat graphene films and graphene dispersions, was performed by using Z-scan technique with 340 fs pulses at 1030 nm. The polymer films and graphene films were fabricated through solution cast method and vacuum filtration technique based on the liquid-phase exfoliated graphene dispersions, respectively. The polymer films reveal the best saturable absorption (SA) response and the lowest saturation intensity Is, in comparison with the neat films and dispersions. The graphene films show the largest SA coefficient and figure of merit due to its highest linear absorption coefficient and refractive index. By employing slow SA modeling, the excited state and ground state absorption cross sections were estimated to be ~10−17 cm2, and the ratio were 0.61, 0.57 and 0.71 for the dispersions, polymer films and neat films, respectively.
© 2015 Optical Society of America
1. Introduction
Graphene has attracted enormous attention as a promising material for photonics and optoelectronics applications, due to 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 [1–8]. Its zero band gap and linear dispersion of Dirac electrons, combined with Pauli blocking property, make graphene to be an ideal ultrafast saturable absorber over a wide spectral range from the visible to the near infrared [2]. In recent years, a number of graphene saturable absorbers with different forms have been reported, such as graphene solutions, graphene composite films, pure graphene films, etc [9–13]. However, there is lack of a comprehensive study on saturable absorption (SA) performance of graphene in different hosts under the same assessment condition. Thus, in this work, we prepared three kinds of widely used graphene saturable absorbers, i.e., graphene polymer composite films, neat graphene films, as well as graphene dispersions. For a convincing evaluation, we keep the linear transmittance, i.e., amount of effective graphene nanoflakes the same. All absorbers were tested by the same open-aperture Z-scan setup with 340 fs pulses at 1030 nm from a mode-locked fiber laser. Absorption saturation was observed in all three absorbers. With the same experimental condition, the polymer absorbers reveal the best SA response and the lowest saturation intensity Is, in comparison with the neat films and the dispersions. The graphene films show the largest SA coefficient and figure of merit (FOM) due to its highest linear absorption coefficient and refractive index. In addition, the excited state and ground state absorption cross sections were analyzed by employing a slow SA model [14]. The result is helpful for the design and application of graphene-based saturable absorbers.
2. Materials
Figure 1 shows the process to fabricate the three kinds of graphene absorbers. The left part is the preparation flow of graphene dispersions, and the right part corresponds to the preparation of the graphene-polyvinyl alcohol composite films (G/PVA films) and deposited graphene neat films based on the graphene dispersions. The graphene dispersions were produced by liquid-phase exfoliation technique [15]. The graphite powders were dispersed in an aqueous solution of the surfactant sodium cholate (SC) (CSC = 1.5 mg/ml) with an initial concentration of 5 mg/ml. The resultant dispersions were sonicated by using 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 to remove unexfoliated or aggregated powders. The top 3/4 centrifuged dispersions were gently extracted by pipetting. The obtained graphene dispersions were stable against sedimentation over a few weeks. The graphene dispersions in SC were used to fabricate G/PVA films and graphene films. The G/PVA films were manufactured by solution-cast method [16, 17]. The prepared graphene SC dispersions were added into 10 ml PVA water solutions (50 mg/ml) at ~90°C. The resulting mixtures were stirred for 24 h and ultrasonically agitated for 4 h to obtain homogeneous solutions before transferring into polymer petri-dishes (55 mm diameter). Then high quality transparent films with uniform surfaces were obtained by drying at 55°C for 3-4 days, and the thicknesses located in a range of 130~160 µm measured by a micrometer. The graphene films were obtained by vacuum filtration method [18]. The graphene SC dispersions were diluted with further sonication for 1 h and vacuum-filtrated with membrane (225 nm aperture). Then, the obtained wet membranes were pressed against cleaned glass substrates with the graphene sides touching with the substrates, followed by drying at 60 °C with a 1 kg flat stainless steel on them overnight. Graphene films on glass were obtained after the membranes were removed through acetone. For the comparison of the SA performance, the graphene dispersions in N-methylpyrrolidone (G/NMP) were also prepared for Z-scan measurement. The effective linear transmittance of all the three graphene absorbers was kept constant of ~59% by finely controlling the amount of graphene in the hosts. Estimation of the effective linear transmittance had excluded the influence of the hosts, i.e., PVA, glass substrate, NMP and quartz cuvette in the G/PVA films, graphene films and graphene dispersions, respectively. That is to say, the amount of effective graphene nanoflakes is the same in the three absorbers. Thickness of the G/PVA films, neat films and G/NMP dispersions are 160 μm, 105 nm and 1 mm, respectively.
3. Results and discussion
The bright-field TEM image of a graphene flake is shown in Fig. 2(a).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 [5]. An atomic force microscopy (AFM) was carried out to determine the surface morphology of the graphene film. The AFM image given in Fig. 2(b) demonstrates the surface roughness is about 40 nm, while the roughness of a G/PVA film is about 4 nm in our previous work [19]. Though the surface of the graphene film is not such smooth as the G/PVA films, it is acceptable and good enough for the optical experiments. Figure 2(c) shows the absorption spectra of the G/NMP dispersions, G/PVA films and graphene films. The absorption peak in the UV region of the G/PVA films is due to the π-π* transitions of the C = C bonds in the aromatic ring of graphene [9]. The peak was not observed in absorption spectra of the G/NMP dispersions and graphene films, which may be attributed to the strong background noise of NMP in the liquid dispersions and glass substrates of the graphene films. It should be pointed out that the absorption spectra in Fig. 2(c) come from the whole samples, i.e., the hosts result in the difference of the three spectra, meanwhile the effective linear transmittance remains the same.
Fig. 2 (a) Bright-field TEM image of a graphene nanoflake in NMP. (b) AFM image of the graphene film. (c) Absorption spectra and (d) Raman spectra of the G/NMP dispersion, G/PVA film and graphene film.
Raman spectra, depicted in Fig. 2(d), were measured by using a Renishaw Invia Raman spectrometer excited at 488 nm. The graphene Raman feature peaks—G peak (~1580 cm−1) corresponding to the first-order scattering of the E2g phonon at the Brillouin, D peak (~1350 cm−1) mainly resulting from intrinsic defects and 2D peak (~2700 cm−1) whose characteristics vary along with the number of layers in graphene flakes—are all observed to be similar in the three absorbers, indicating the structure of graphene is consistent with each other among the absorbers [10, 15, 20]. The full width at half-maximum (FWHM) of the 2D band for G/PVA film, graphene film, and G/NMP dispersion are ~66.2 cm−1, ~67.8 cm−1, and ~66.8 cm−1, respectively, which is similar as the result of five-layer graphene ~66.1 ± 1.4 cm−1 in the previous report [21]. In addition, the band around ~1440 cm−1 in G/PVA films is originated from PVA.
Ultrafast SA performance of the three absorbers were studied by using an open aperture Z-scan system under 340 fs pulses operating at 1030 nm with the repetition of 100 Hz [22, 23]. The laser beam was tightly focused through a lens with the focal length of 10 cm, and the beam waist radius at the focus was estimated to be ~29 µm. In our experiment, we did not observe any clear NLO response from the NMP solvent, pure PVA films and glass substrates. Figures 3(a), 3(b) and 3(c) show the typical Z-scan results of the three absorbers at different incident energies. The normalized transmittance increases as the sample moves toward the beam focus (z = 0), indicating a clear SA response. The SA becomes much more pronounced as the incident energy increasing for all the three absorbers. For comparison, the maximum transmittance variations at different excitation pulse energies are summarized in Fig. 3(d). Obviously, the G/PVA film has the largest variation in the three absorbers at the same pulse energy, that is to say, the G/PVA film exhibits stronger SA response than the others. In contrast, the neat film is inferior. Under the excitation of 160 nJ/pulse, the SA response follows G/PVA film > G/NMP dispersion > graphene film, which is clearly shown in Fig. 3(e).
Fig. 3 (a), (b) and (c): Z-scan curves at different incident pulse energies of the G/NMP dispersion, G/PVA film and graphene film. The solid lines are the fitting results. (d) The maximum transmittance variation in Z-scan traces at different incident pulse energies for the three absorbers. (e) The Z-scan curves of the three absorbers at the excitation pulse energy ~160 nJ. (f) Is as function of input energy for the three absorbers.
According to the NLO theory, the propagation equation in the graphene saturable absorbers can be expressed as: dI/dz’ = -α(I)I, where I is the excitation intensity and z’ is the propagation distance in the sample. To obtain the saturation intensity Is, we can describe the total absorption α(I) with the form of α(I) = α0/(1 + I/Is) [24]. Figure 3(f) gives Is deduced by numerically fitting the Z-scan curves as a function of input energy. It is clearly that Is increases gradually with input energy increasing for the three absorbers, and the G/PVA film exhibits the lowest Is at same input energy, implying it exhibits much stronger SA response than the others, consistent with the results of Fig. 3(d) and (e). Is for the G/PVA film, G/NMP dispersion and graphene film were calculated to be ~(59.9 ± 21.6) GW/cm2, ~(67.1 ± 21.6) GW/cm2 and ~(85.1 ± 26.3) GW/cm2, respectively. Nonlinear absorption coefficient αNL can also be deduced with the nonlinear propagation equation α(I) = α0 + αNLI, where α0 is the linear absorption coefficient [25]. The imaginary part of the third order NLO susceptibility, Imχ(3), is directly related to αNL, Imχ(3) = (10−7cλn2/96π2)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 FOM for the third order optical nonlinearity as FOM = |Imχ(3)/α0|. The obtained αNL, Imχ(3) and FOM values as a function of input energy are shown in Fig. 4.It is obviously that the three parameters decrease gradually with input energy increasing in our experimental range for all absorbers. What we need to point out is that the SA responses are not directly consistent with the values of αNL, Imχ(3) and FOM because of the different linear absorption coefficients and refractive indices in the three absorbers. All linear and NLO parameters are summarized in Table 1.
Fig. 4 (a)αNL, (b)Imχ(3) and (c)FOM as functions of input energy for the G/NMP dispersion, G/PVA film and graphene film for 340 fs pulses at 1030 nm.
Table 1. Linear and NLO parameters of the G/NMP dispersion, G/PVA film and graphene film.
The interband relaxation lifetime of graphene is around a few picoseconds [26, 27], which is quite longer than the 340 fs pulse duration. Therefore the SA results in this work can be analyzed by slow saturable absorber model used for the case where the excited state decay time τ is much longer than the pulse duration [14, 28]. Details of the model has been described in the previous works [15]. In Fig. 5, we show the fitting results with the slow saturable absorber model, and the parameters are given in Table 2..Referring to the same linear transmittance, the ground state absorption cross section of the three absorbers are estimated to be the same σgs = 7.8 × 10−17 cm2. The variation of the number density N is mainly due to the same amount of effective graphene nanoflakes but the different pathlengths of the three absorbers. In consequence, the excited state absorption cross sections are deduced to be 4.8 × 10−17 cm2, 4.4 × 10−17 cm2 and 5.5 × 10−17 cm2 for the G/NMP dispersion, G/PVA film and graphene film, and the corresponding σes/σgs are 0.61, 0.57 and 0.71, respectively. In Table 2, σes are smaller than σgs, resulting in the SA phenomena in all three absorbers. The G/PVA absorber possesses the lowest σes/σgs, implying much stronger SA response than the others, which is in good agreement with the above results.
Fig. 5 Nonlinear transmission of the three graphene absorbers as a function of the input laser intensity at the excitation pulse energy of 160 nJ. The solid lines are the fitting results based on the slow saturable absorber model. Inset is a schematic diagram of graphene level and a three-level system used for the modeling.
Table 2. The physical parameters used for the fitting based on the slow saturable absorber model.
4. Conclusion
In conclusion, we prepared three kinds of widely used graphene saturable absorbers, i.e., G/PVA films, graphene films and dispersions. In the three graphene absorbers with the same amount of effective graphene nanoflakes, the G/PVA film exhibits the strongest SA response and lowest saturation intensity Is, while the graphene film shows the largest SA coefficient and FOM. By employing the slow SA modeling, σes and σgs were estimated to be ~10−17 cm2, and the ratio were 0.61, 0.57 and 0.71 for the G/NMP dispersion, G/PVA film and graphene film, respectively. The results are expected to be helpful for the design and application of graphene-based saturable absorbers.
Acknowledgments
This work is supported in part by NSFC (No. 61178007, 61308087), the External Cooperation Program of BIC, CAS (No. 181231KYSB20130007), STCSM (No. 12ZR1451800), China Postdoctoral Science Foundation (2014T70435 and 2012M520049). J.W. thanks the National 10000-Talent Program and CAS 100-Talent Program for financial support.
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