Nonlinear optical properties of colloidal systems containing single wall carbon nanotubes (SWNT) disperse in different surfactants are investigated. Thermal nonlinearity management Z-scan technique was performed to measure the nonlinear refractive index (n2) of colloidal system. The results presented in this letter show that the presence of SWNT enhances significantly the electronic nonlinear responses of the colloid and that the surfactants play an important role in the determination of electronic part of n2.
© 2012 OSA
The studies of properties of single wall carbon nanotubes (SWNT) have evolved from rather fundamental studies to applications, reaching from nanoelectronics  to biosensors  and nonlinear optics devices . Particularly the latter, SWNT composites have been widely investigated as a potential candidate to be used in nonlinear optical devices in many applications: optical switches, passive mode looked, optical limiting, among others.
SWNT have attracted much attention as a promising candidate for ultrafast devices due to its large third-order nonlinearity and ultrafast electronic response as a consequence of delocalized π-electrons cloud along the tube axis [4–6]. Thus, special interest has been devoted to study SWNT composites in the regime where electronic contributions to the third-order nonlinear susceptibility take place [7–10]. In this regime the experiments are typically carry out using femtosecond mode-locked laser at high repetition rate. Interesting enough, high repetition rate laser irradiation is also responsible for the thermo-optical effect in the nonlinear response due to cumulative effect, which is undesirable for ultrafast optical devices. In this sense, great care must be taken with experiments using high repetition rate laser where thermal contribution may substantially alter the nonlinear response. It is interesting to notice that, for same experiments, even for low repetition rate as low as 1 KHz the cumulative heating may not be neglected and the thermal contribution can be important. It worthwhile to mention that thermal contribution to the nonlinear effect using SWNT composites is responsible for a considerable number of applications such as optical limiting [3,11–13].
Another interesting point is to understand the role of the dispersant in SWNT composites in the nonlinear characterization of colloidal system. It is well known that some changes in the optical response are observed as a consequence of the changes in the environmental screening. For example, the influence of the nanotube and surfactant concentrations on the absorption and emission of light by individualized carbon nanotubes was studied in .
In this work, we investigate nonlinear optical properties of colloidal systems containing SWNT using a technique that allows discriminating between electronic and thermal contributions to the nonlinear refractive index (n2) [15,16]. We measured the electronic contribution only to the n2 of different colloidal systems using Z-scan measurements with high repetition rate laser. The colloidal systems consist of two types of SWNT disperse in different surfactants with different nanotube concentrations. We also investigate the influence of surfactants on the determination of n2 values for different types of SWNT.
SWNT produced by two different methods were investigated in this work. SWNT were obtained by electric arc discharge method  in the laboratories of Federal University of Minas Gerais, using Co, Ni and Fe catalyst. These nanotubes are called here as CoNiFe and presented 1.4 ± 0.2 nm diameters with a Gaussian distribution. The second kind of SWNT was purchased from South-West Nanotechnologies, Inc., named CoMoCat. They were produced by catalytic decomposition of cobalt , and have 0.9 ± 0.2 nm diameters, also with a Gaussian distribution.
These two types of nanotubes were dispersed in NaC (sodium cholate) and NaDDBS (dodecyl benzene sulfonate) surfactants following the procedure described in . We used different amounts of to obtain the dispersions in different concentrations. We dispersed 2 mg, 0.2mg or 0.02mg of SWNT in 10mL of deionized water with 1%wt of surfactant to prepare the dispersions in the three concentrations 0.2, 0.02 and 0.002 mg/mL, respectively. Raman and UV-VIS-NIR spectroscopies were employed in the structural and optical characterization of the studied SWNT. For each set of SWNT kind and dispersant, we investigated the optical response of colloids with different nanotube concentration.
The nonlinear optical properties of the colloids were investigated using the Z-scan technique managed thermally [15,16]. A mode-locked Ti-Sapphire laser, linearly polarized, delivering pulses of 200 fs at a 76 MHz repetition rate, tuned at 791 nm was employed as the excitation source. The laser beam was modulated by a chopper and focused on to the sample by convergent lens 7.5 cm focal length. The modulation frequency was 14 Hz, which provided a duty cycle equal to 0.09 and a chopper opening risetime of 24 µs. The samples consisted of 1 mm width quartz cell filled with the colloids of carbon nanotubes. The cell was mounted on a translation stage and moved around the lens focal plane (z = 0). The light transmittance was then measured by a closed-aperture photodetector as a function of the sample position. The detected signal was temporally analyzed by digital oscilloscope and then processed by a computer. Nonlinear absorption measurements were performed with the same experimental setup but using a configuration without aperture.
Owing to the laser large repetition rate, the cumulative thermo-optical effect dominates the refractive response of the medium after some time of the sample being irradiated. Hence, the measured transmittance in Z-scan technique can be expressed by Eq. (1) and are disregard at the analysis procedure. However, the temporal evolution of the Z-scan traces can be followed from the opening risetime onwards .
Ideally, the electronic contribution to the observed nonlinear refraction gives an instantaneous response. Although we could not measure the normalized transmittance at t = 0, we can reconstruct this curve extrapolating the time evolution curves of the measured normalized transmittance, at all sample positions, using Eq. (1) . Hence, the value of n2 can be obtained fitting the normalized transmittance curve at t = 0 employing the standard equation of Z-scan method 
3. Result and discussions
3.1 Samples characterization
Figure 1 shows Raman spectra of CoNiFe and CoMoCat SWNT samples obtained by exciting the samples with a 514.5 nm line of an Ar+ ion laser. As can be observed, both systems present a characteristic of Raman spectrum for single-walled carbon nanotubes. The radial breathing mode (RBM) and D and G bands are clearly visible in these spectra.
Figure 2 shows the linear absorption for the investigated colloids. For the CoMoCat samples, the increase of the nanotube concentration raises the absorption of the colloids and improves the definition of absorption bands related to the optical transitions associated with the presence of tubes with different diameters and chiralities on the samples. The colloids with NaDDBS as surfactant present higher absorption than the systems with NaC. On the other hand, although the samples containing CoNiFe SWNT also displayed an increase of absorption as the nanotube concentration was raised, they presented higher absorption levels when NaC was used as surfactant. Taking the ratio between the resonant absorption peaks area and background area (see inset to Fig. 2(a)) we observe that NaDDBS is the best surfactant for disperse the CoMoCat sample and NaC is the best in the case of CoNiFe sample.
Due to the larger diameters of the nanotubes in the CoNiFe sample, they did not present well defined absorption peaks in this spectral region. The nanotubes present in the sample have electronic transition energies very close together with each other, thus, instead of well defined absorption features we observe a broad band associated with the superposition of the optical absorption contributions from a whole set of semiconducting or metallic nanotubes in the sample. The band observed in Fig. 2(c) around 700 nm is associated with the first optical transition from metallic nanotubes.
3.2 Z-scan measurements
A typical result of the thermally managed Z-scan measurement, obtained at two different time instants, for the colloid consisting of 0.2 mg/ml CoNiFe SWNT using NaDDBS as surfactant is shown in Fig. 3 . As it can be observed, both Z-scan curves, at time instants t = 80 µs (black curve) and at t = 300 µs (red curve), the colloid presents negative nonlinear refraction responses and the difference between the peak-valley transmittance increases with time, which indicates that the cumulative thermo-optical effect is present in this sample. It is also shown in this figure the measured transmittance using the open-aperture Z-scan measurement (inset). This result demonstrates that nonlinear absorption is absent in this experimental condition for the investigated colloid, hence we can use q = 1 in Eq. (1) in the extrapolation process to reconstruct the Z-scan curve at t = 0 s. Similar results were observed for the different SWNT concentration. Similar behaviors were obtained using NaC as surfactant as well as for CoMoCat SWNT colloidal dispersions with NaDDBS and NaC.
From the complete time evolution of the Z-scan measurements, the electronic contribution to nonlinear refraction of the used samples was obtained applying the extrapolation method. Therefore, the reconstructed Z-scan curves were obtained in t = 0 s. Figures 4(a) and 4(c) show the reconstructed Z-scan curves using CoMoCat SWNT disperse in aqueous solution with NaDDBS and NaC, respectively. Figures 4(b) and 4(d) show the same reconstruction using only the surfactants NaDDBS and NaC in aqueous solution, respectively. The n2 values obtained in Figs. 4(a) and 4(c) at 0.2 mg/ml concentration, using 60mW laser power, were -8.34 x 10−14 cm2/W and -5.91 x 10−14 cm2/W respectively. On the other hand, + 1,02 x 10−16 cm2/W at 600 mW and -3,46 x 10−15 cm2/W at 500 mW were the n2 values obtained from Figs. 4(b) and 4(d), respectively. The same procedure was performed for the colloidal systems using CoNiFe SWNT, not shown here. The obtained results are summarized in Table 1 .
It can be observed that the presence of SWNT increases significantly the modulus of the peak-valley transmittance variation in both surfactants. This result shows that electronic contributions for the nonlinear refractive index of the colloid are enhanced owing to the presence of the SWCNT. This enhancement is related to the real part of the nonlinear third-order optical susceptibility of the individual SWNT and the local field effects associated with the interaction between the nanotubes and the dielectric dispersants.
Using Eq. (2) we could measure the electronic contribution to the nonlinear refractive index of all investigated colloids. We observed that the different combinations of SWNT and surfactants produce colloids with distinct values of the effective nonlinear refractive index. For both kind of surfactants, the CoMoCat colloids presented larger values of n2 in modulus than the CoNiFe samples. For instance, in NaDDBS at 0.2 mg/ml concentration, the value of n2 in modulus was up to five times larger for the CoMoCat SWNT. This result suggests that SWNT of smaller diameter (CoMoCat) present larger negative values of the real part of the third-order optical susceptibility. We can understand this behavior when we compare the obtained results for the optical nonlinearity and the linear absorption of the colloids. As can be observed in Fig. 2, the CoMoCat colloids present more intense peaks in the near infrared and visible region, associated with electronic excitations of the nanotubes, while these transitions are less intense or absent in the CoNiFe response. As the laser frequency is smaller than the frequencies of these excitations and the detuning between them are small, the third-order nonlinear optical response of these media are dominated by the contributions of the non resonant one-photon transitions, which have negative values.
Additionally, we could also observe that changing the surfactant also modifies the nonlinear refractive response of the colloids. While the CoMoCat colloid has more negative refractive index with NaDDBS as surfactant, the largest value of the n2 in modulus is achieved for the CoNiFe colloid when NaC is employed as surfactant. This is an expected result because, since this nonlinearity is related to the one-photon non resonant transitions, the combinations of SWNT and surfactant that give the largest absorbance should also present the higher nonlinearity, which is indeed observed when we compare the linear absorption spectra and the nonlinear refraction results.
Owing to the production method of the nanotubes used in this work, our samples consisted in mixture of SWNTs of different chiralities and diameters, dispersed randomly within the colloidal systems. Hence, it was not possible to obtain the individual contribution of a single (n,m) nanotube to the nonlinear optical response observed in these systems. Further investigations are currently been made in order to elucidate this question, employing samples of SWNT colloids that present only nanotubes of a single chirality.
In summary, we investigated nonlinear optical properties of colloidal systems containing CoNiFe and CoMoCat SWNT disperse in NaC (sodium cholate) and NaDDBS (dodecyl benzene sulfonate) surfactants using a technique that allows discrimination between electronic and thermal contributions to the nonlinear refractive index (n2). It was observed that the presence of the SWNT enhances the electronic nonlinear response of the colloid, up to two orders of magnitude in modulus. We also investigated the influence of different surfactants in the electronic part of n2 of colloidal systems for different types of SWNT. We observed that changing the surfactant also modifies the nonlinear refractive response of the colloids. Our results suggest that surfactants may play an important role in the development of photonic applications involving SWNT.
The authors thank the financial support from CAPES Pró-equipamentos/PROCAD/PROCAD-NF, CNPq/MCT, Pronex/FAPEAL, PADCT, Instituto Nacional de Ciência e Tecnologia Fotônica para Telecomunicações - FOTONICOM, and FINEP. The authors also thank the Nanomaterials Laboratory at UFMG for supplying CoNiFe SWNT samples.
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