A versatile solid Poly-methyl-methacrylate (PMMA) composite containing porphyrin-covalently functionalized multi-walled carbon nanotubes (MWNTs-TPP) was prepared through free radical polymerization without additional dispersion stabilizer. Using nanosecond, femtosecond pulse Z-scan and degenerate femtosecond pump-probe techniques, we studied the optical limiting effect, ultrafast saturable absorption and transient differential transmission of the composite. Results show that the solid composite exhibits weaker optical limiting effects than that of the suspension at 532 nm under nanosecond pulse, due to the absence of nonlinear scattering mechanism. The composite also shows ultrafast saturable absorption with a relaxation time about 190 fs at 800 nm under femtosecond pulse due to band-filling effect, comparably to the suspension. The versatile solid composite can be the candidate for uses in applications of ultrafast optical switching and mode-locking element or optical limiter for nanosecond pulse.
© 2013 Optical Society of America
Carbon nanotubes (CNTs) with highly delocalized π-electrons have been widely explored for their nonlinear optical (NLO) properties. CNTs are versatile NLO material, which exhibit alternative NLO properties for different requirements. For example, CNTs exhibit nonlinear scattering or reverse saturable absorption (RSA)-like behavior in the visible and near infrared region under nanosecond pulse laser [1–5], while show saturable absorption (SA) near infrared region under short pulse laser [6–8]. Nonlinear scattering properties enable CNTs used as optical limiter for laser protection, while SA enable them used as mode-locking element for the generation of ultrafast laser pulses [9,10]. However, CNTs have poor solubility in common solvents and tend to aggregate into large rope-like bundles due to their relatively high surface energy, which is a serious hurdle in the way of practical applications .
To simultaneously promote the stability and NLO properties of the CNTs suspensions, CNTs were functionalized covalently or non-covalently by organic nonlinear chromospheres, and these functionalized CNTs showed good dispersion in solvents and enhanced NLO properties due to the combination of NLO mechanism and the photoinduced electron transfer (PET) or energy transfer (ET) between chromospheres and CNTs moiety [11–13]. However, NLO materials of liquid and suspensions forms are not suitable for practical NLO devices, and preparing solid composites has attract a great interest. In most of the reports, solid CNTs composites were limited to thin films with polymer matrix [14–16], and this limits the interaction length between the propagating light and the CNTs, thus the efficient exploitation of their NLO properties were hindered. Bulk composites are of great significance due to their processable properties. Poly-methyl-methacrylate (PMMA) is one of the most commonly used polymers for preparation of solid film as matrix, bulk organic glasses based PMMA also can be prepared through methyl-methacrylate (MMA) polymerization. Martinez et al embed the CNTs into MMA and prepared CNT/PMMA bulk organic glasses, however, this method still needs the help of CNTs dispersion stabilizer . Zhang et al prepared PMMA composites containing non-covalently functionalized multi-walled carbon nanotubes (MWNTs)/copper phthalocyanine (CuPc) hybrid, however, the hybrid of MWNTs/CuPc may decompose when they are introduced into PMMA due to the low strength of the non-covalent interaction and different solubility of MWNTs and phthalocyanines in the pre-polymerization solution .
In this paper, we functionalized the MWNTs with amine functionalized porphyrin (TPP-NH2), and then prepared the solid bulk PMMA composite containing porphyrin-covalently functionalized multi-walled carbon nanotubes (MWNTs-TPP) without additional dispersion stabilizer. The optical limiting effects and ultrafast saturable absorption of the solid composite (MWNTs-TPP/PMMA) were measured in nanosecond and femtosecond regimes, respectively. MWNTs-TPP dispersed in N, N-dimethylformamide (DMF) was used as a reference. Results show that MWNTs-TPP/PMMA exhibits optical limiting properties under nanosecond laser pulses, though weaker than that of the suspension. The solid composite also exhibits ultrafast SA with a relaxation time about 190 fs under femtosecond laser pulses.
The synthesis of MWNTs-TPP was carried out using amine functionalized porphyrin (TPP-NH2) and MWNTs with the diameter of 20-30 nm in DMF following standard chemistry . The bulk PMMA composites were prepared through free radical polymerization (FRP) in which azobisisobutyronitrile (AIBN) was used as free radical initiator. AIBN was dissolved in MMA and MWNTs-TPP/DMF suspension was homogeneously added to the solution. The mixture was stirred in the water of 70°C and pre-polymerized until the viscosity of mixture was similar with glycerol. Then the resultant viscous mixture was poured into molds. The molds were subsequently heated in an oven at 45°C for 2 days. The solid PMMA composite of 3.3 mm-thick was obtained after removing the mold, TPP-NH2/PMMA composite of 3.3 mm-thick was also prepared in the same way. Pristine MWNTs suspension was also prepared by dispersing MWNTs into DMF after sonicating for 30 min, it was used as a reference for absorption and fluorescence spectrum of the samples.
Nanosecond pulse Z-scan and optical limiting experiments were performed with a Q-switched Nd: YAG laser (Continuum Surelite-II) with a pulse time duration of 5 ns. The pulsed laser was set at a repetition rate of 10 Hz for Z-scan  and single pulse mode for optical limiting experiments, the setup is the same as Ref. 20. Theses solutions and suspensions were filled in 5-mm thick quartz cells for nanosecond pulse experiments, while 1-mm thick quartz cell was used for femtosecond pulse experiments. Femtosecond pulse Z-scan experiments was carried out using femtosecond laser pulses from a Ti: sapphire laser amplifier (1 kHz, Spitfire, Spectra Physics) at 800 nm with the pulse duration of 130 fs. For degenerate pump-probe experiment, both pump and probe pulse were at 800 nm, the pump pulse was modulated at 383 Hz with the help of an optical chopper, the probe pulse was delayed and the transmitted probe pulse was sent to one photodiode of a balanced detector, which was connected to a lock-in amplifier referenced to the optical chopper.
3. Results and discussion
Figure 1 shows the absorption and fluorescence spectrum of the samples. As shown in Fig. 1(a), TPP-NH2/PMMA shows a Soret band near 418 nm and four Q bands between 500 nm and 700 nm, The MWNTs-TPP/PMMA composite exhibits broadband absorption with a broadening Soret band compared with TPP-NH2/PMMA, this indicates that the interaction between porphyrin and MWNTs. As a reference, pristine MWNTs in DMF shows broadband absorption from visible to near-infrared wavelength range, it should has a surface plasmon resonance band close to 270 nm as reported by Torres-Torres et al , however we did not observed such a band, because of the strong absorption of solvent DMF in ultraviolet regions and its unlikely complete compensation. As shown in Fig. 1(b), there is no fluorescence in pristine MWNTs, TPP-NH2/PMMA exhibit two fluorescence bands around 650 nm and 718 nm, respectively. MWNTs-TPP/PMMA shows quenching fluorescence compared with TPP-NH2/PMMA, indicating PET and ET from porphyrin to MWNTs moiety, similarly to the case in DMF solvent [12, 13].
Figure 2(a) shows the open-aperture Z-scan curves of MWNTs-TPP/PMMA solid composite under nanosecond pulse, TPP-NH2/PMMA, MWNTs-TPP and TPP-NH2 in DMF were used as references. We fit the curves by using the equation of , is linear absorption coefficient, is intensity and is effective nonlinear absorption coefficient.
The normalized transmittance at focus is 0.89, 0.87, 0.86 and 0.66; is 10.5 cm/GW, 8 cm/GW, 17 cm/GW and 33 cm/GW for TPP-NH2/PMMA, TPP-NH2 in DMF, MWNTs-TPP/PMMA and MWNTs-TPP in DMF, respectively. Compared with TPP-NH2 in DMF, TPP-NH2/PMMA shows the higher normalized transmittance at focus but larger value due to the higher concentration of TPP-NH2 and larger linear absorption in the solid composite. MWNTs-TPP/PMMA shows the higher normalized transmittance at focus and much smaller effective nonlinear absorption coefficient due to the absence of nonlinear scattering compared with MWNTs-TPP in DMF. Figure 2(b) gives the optical limiting properties of the samples. From Fig. 2(b) we also can see that the solid composite shows weaker optical limiting properties than the suspension form, for example, at the input fluence of 18.3 J/cm2, the normalized transmittance are 0.28 J/cm2 and 0.20 J/cm2 for MWNTs-TPP/PMMA and MWNTs-TPP in DMF, respectively. The optical limiting thresholds (defined as the input fluences at which the transmittance falls to 50% of the normalized linear transmittance) are 10.5 J/cm2 and 3.1 J/cm2, respectively.
To measure the intrinsic NLO performance of MWNTs-TPP and evaluate the performance of the bulk solid composite in ultrafast regime, an ultrafast femtosecond pulse laser was used. Figure 3(a) shows the open-aperture Z-scan curves of MWNTs-TPP/PMMA under femtosecond pulse, TPP-NH2/PMMA, MWNTs-TPP and TPP-NH2 in DMF were used as references.The normalized transmittance of MWNTs-TPP/PMMA increase as the sample is brought closer to the focus, which is contrary to optical limiting effect, indicating the SA properties, Effective nonlinear absorption coefficient is −3.6 cm/TW, −9.0 cm/TW for MWNTs-TPP/PMMA solid and MWNTs-TPP in DMF, respectively. No NLO responses were observed in TPP-NH2/PMMA and TPP-NH2 in DMF, which is due to the zero linear absorption, small two-photon absorption cross-section at 800 nm and low concentration of TPP-NH2 in our experiments.
Figure 3(b) shows the normalized absorbance of MWNTs-TPP in DMF and PMMA composites as functions of input intensity at focus, the normalized absorbance decreased as the input intensity increase, at the intensity of 206 GW/cm2, the normalized absorbance decreased to 93% and 88% of linear absorption, corresponding the modulation depth of 7% and 12% for MWNTs-TPP in DMF and PMMA composites, respectively. Since the NLO responses of TPP-NH2 can be neglected, the SA properties in MWNTs-TPP/PMMA and MWNTs-TPP in DMF can be attributed to the band-filling effect from MWNTs moiety. That is to say, the electrons on the valence band in MWNTs transited to conduction band under the laser pulse, the high intensity of the pulse deplete many electrons on the valence band to the conduction band and lead to absorbance decrease . Usually, when many electrons transit to the conduction band, free carrier absorption will take place [8, 21]. For MWNTs-TPP/PMMA and MWNTs-TPP in DMF, we performed the open-aperture Z-scan experiment with the femtosecond pulse under a range of input intensities from 110 GW/cm2 to 247 GW/cm2, the normalized transmittance at the focus for the samples increase as input intensity increase, this means the SA is dominant compared with free carrier absorption in our experiments. The experiments were repeated for several times and the repeatable results indicate that there is no optical damage in the samples. For femtosecond pulses with a repetition rate of 1 kHz, the thermal effect in samples can be neglected .
Since is dependent on the linear absorption coefficient, so can be used for the comparison between samples with different linear absorption . The values of and of the samples were summarized in Table 1. In nanosecond regime, of TPP-NH2/PMMA is comparable to that of TPP-NH2 in DMF (20.2 vs 25.0), while of MWNTs-TPP/PMMA is much smaller than that of MWNTs-TPP in DMF (7.0 vs 103), indicating the weaken NLO properties due to the absence of nonlinear scattering mechanism in MWNTs-TPP/PMMA. In femtosecond regime, of MWNTs-TPP/PMMA is comparable to that of MWNTs-TPP in DMF (2.0 vs 1.3), this indicates they share the same SA mechanism from band-filling effect .
Linear and nonlinear optical parameters of porphyrin, MWNTs and their derivates reported in other works were given in Table 2. From Table 2, we can see that of a water-soluble porphyrin TMPyP/water solution and TMPyP/ Gelatin solid film are much different due to the different , but the values of are comparable, while the value of of PU/MWNTs film is much smaller than PcH2-MWNTs /toluene in nanosecond pulse regime. This phenomenon is similar to the case of TPP-NH2 and MWNTs-TPP under nanosecond pulse regime. The values of of MWNTs film and MWNTs-ZnO film in femtosecond pulse regime are much larger than that of MWNTs-TPP/PMMA, this can be attributed to the high concentration of MWNTs and MWNTs-ZnO in the film on substrates.
An important physical property for ultrafast devices is the excited carriers’ relaxation time, to evaluate the relaxation time of MWNTs-TPP/PMMA as mode-locker or optics switching, the transient differential transmission as a function of delay time for the composites were measured using femtosecond pump-probe techniques. Figure 4 shows the transient differential transmission as a function of delay time for MWNTs-TPP/PMMA, MWNTs-TPP dispersed in DMF was used as a reference. The pump-induced changes in transmissions are dominated by SA in the samples, resulting in a positive differential transmission, and are the sample transmissions with and without excitation, respectively. The data were fitted by using single exponentially decaying function, convoluted with the cross correlation of the pump and probe pulses. is the relaxation time. Since the differential transmission curves are close to Gaussian shape, and also close to the 130 fs pulse shape, the fitting error could be high. The fitting values of of MWNTs-TPP/PMMA solid composite and MWNTs-TPP/DMF suspension are 190 ± 40 fs, 230 ± 45 fs, respectively. It means that the fitting errors are about 20%. The different environment of MWNTs-TPP seemly has less effect on the ultrafast SA and this time is comparable to the fast component of relaxation time of CNTs [8, 16]. This subpicosecond delay time can be interpreted as the relaxation time of electrons from conduction band back to the valence band in MWNTs.
Under nanosecond pulse at 532 nm, TPP-NH2 shows strong RSA arising from excited state absorption like other porphyrin derivates [12,13], while MWNTs-TPP in DMF exhibits strong optical limiting effects in the nanosecond regime, which mainly arises from strong nonlinear light scatterings due to the creation of new scattering centers consisting of ionized carbon microplasmas and solvent microbubbles [12,13]. What’s more, RSA from TPP-NH2 moiety, PET and ET from TPP-NH2 to MWNTs moiety also contribute to optical limiting effect in MWNTs-TPP/DMF suspension. Since the rigid PMMA solid state environment precludes the formation of microbubbles and microplasmas, so there is no nonlinear scattering in MWNTs-TPP/PMMA solid , only RSA from TPP-NH2 moiety, combined with PET and ET from TPP-NH2 to MWNTs moiety contribute to optical limiting effect, however it is difficult to determine which mechanism is dominant in MWNTs-TPP/PMMA solid. Under femtosecond pulse at 800 nm, the NLO response from TPP-NH2 can be neglected, the SA properties arises mainly from band-filling effect of MWNTs moiety, so MWNTs-TPP/PMMA solid and MWNTs-TPP/DMF suspension share the same NLO mechanism from band-filling effect of MWNTs moiety. Compared with MWNTs-TPP in DMF, MWNTs-TPP/PMMA has good optical quality and stability, the fast relaxation time is also kept.
In summary, optical limiting properties, ultrafast SA and transient differential transmission of MWNTs-TPP/PMMA composite has been studied by using Z-scan and pump-probe techniques. Although MWNTs-TPP/PMMA composite shows weak optical limiting properties at 532 nm under nanosecond pulse, it exhibits ultrafast SA and fast relaxation times at 800 nm under femtosecond pulse, and this makes it a potential candidate for uses in applications of ultrafast optical switching and mode-locking element or optical limiter for nanosecond pulse.
This work was supported by the Youth Foundation of Taiyuan University of Technology (No. 2012L085), the Scientific Research Starting Foundation from Taiyuan University of Technology, and the Natural Science Foundation of China (Grant 11174159).
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