- Email: [email protected]
Ultrafast third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer
Chemical Physics Letters 558 (2013) 100–103
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Ultrafast third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer Fuli Zhao a, Changshun Wang a,⇑, Yi Zeng a, Zuanming Jin b, Guohong Ma b a b
Department of Physics, State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, PR China Department of Physics, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 19 September 2012 In final form 19 December 2012 Available online 3 January 2013
a b s t r a c t The ultrafast third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer were investigated by optical Kerr effect (OKE) and pump–probe techniques with femtosecond pulses. The third-order nonlinear optical susceptibility of the sample solution was obtained to be 2.1 1014 esu at a concentration of 2 103 M. The temporal response of the sample solution was decomposed into two components, which could be explained by the relaxation of different singlet excited states. The polymer thin film exhibited strong ultrafast nonlinear optical response of 8.26 1010 esu and show its potential applications for further nonlinear optical devices. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Nonlinear optical materials with large third-order optical nonlinearity and fast response time are essential requirement for optical communication, optical limiter and all-optical switching applications [1–4]. Azobenzene-functionalized polymer is one of the most attractive nonlinear optical materials owing to its unusual chemical and optical properties [5,6]. There have been many reports about the nonlinear optical characteristic and optical storage based on their photoisomerization and photoinduced anisotropy [7–9]. Most of these works were done with CW or long pulse lasers as excitation source due to the slow process of trans–cis–trans isomerization cycles of azobenzene molecules. However, these results were not applicable for fast optical response and high-bandwidth optical signal processing because of their long excitation and relaxation times. Employing a ultrashort pulse laser, high power light can be output to create strong nonlinear light–matter interactions, making it possible to explore physical phenomena with unprecedented time resolution [10]. To further exploit the potential applications of azobenzene materials in ultrafast field, it is imperative to accurately determine the nonlinear parameters under the excitation of ultrashort pulse lasers and then to understand the photodynamic behaviors which are very important for designing all optical switching devices. In this Letter, we employed an azobenzene-containing ionic liquid crystalline polymer which can be easily formulated from readily available components based on ionic self-assembly, and reported its ultrafast third-order nonlinear optical properties by using optical Kerr effect (OKE) technique with femtosecond pulses.
The sample is a supramolecular prepared through self-assembly. 5 mg/mL sodium polyacrylate (PANa) aqueous solution, obtained from the neutralization of polyacrylate with sodium hydroxide, was added dropwise to NDAZO (3-(6-(4-((4-(dimethylamino)phenyl)diazenyl)phenoxy)hexyl)-1-methyl-1H-imidazol-3ium bromide) aqueous solution with the concentration of 1 mg/ mL, in a 1:1 M charge ratio. The resulting precipitated was collected by filtration and washed thoroughly with deionized water to remove residual salts and possible noncomplexed precursors and then dried in vacuum at 60 °C for 24 h. To investigate its stability and phase behaviors, TGA and DSC of the polymer were obtained. TGA curve showed that the polymer was stable under high temperature of 250 °C. DSC thermograms of the polymer displayed phase transition peak at 93 °C, indicating its crystalline nature. The polymer solution with a concentration of 2 103 M is prepared by dissolving the appropriate amount of the solute into the chloroform solvent. The molecular structure and the UV–vis absorption spectra of the polymer are shown in Figure 1. Thin films of the synthesized polymer were prepared by spin-coating chloroform solution (concentration: 30 mg/mL) onto the quartz slides (speed: 2000 rpm, time: 20 s). The thickness of the resultant films was about 300 nm, measured by a Dektak profilometer. The third-order nonlinear optical (NLO) properties of the polymer were examined via a time-resolved optical Kerr effect (OKE) and pump–probe experiments. A standard femtosecond OKE experiment was performed. The femtosecond laser pulse was generated from a mode-locked Ti:Sapphire oscillator (Mai Tai HP-1020) generating 100 fs pulses at 800 nm. A regenerative amplifier system (Spectra-Physics, Spitfire Pro) was used to amplify the pulses 106 times at a repetition rate of 1 kHz. The laser beam was divided into
⇑ Corresponding author. E-mail address: [email protected] (C. Wang). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.12.043
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103
Figure 1. Linear absorption spectrum of the polymer solution and film. Inset: the molecular structure of the polymer.
probe beam and pump beam by a beam splitter. The pump beam traveled through a 1-mm thickness type-I BBO nonlinear crystal to generate pulse at 400 nm. This wavelength falls in the range of relatively strong absorption of the sample allowing us to obtain information on the dynamic behavior of the resonant third-order nonlinearity. A polarizer P1 was put into the probe beam path to set its polarization to be 45° with respect to that of the pump beam. The two beams were focused by a 50 cm focal length lens and overlapped on the same spot of the sample with a spot size of 200 lm. After transmitting the sample cell, the pump beam was blocked, while the transmitted probe beam passed through an analyzer P2 with crossed polarization to P1. The generated OKE signal was detected by a silicon photodiode connected to a lock-in amplifier. In the OKE experiment, CS2 was used as the reference. The vð3Þ of CS2 is accredited as 1 1013 esu in fs domain [11]. The value of the third-order NLO susceptibility of the sample can be calculated from the OKE measurement by using the following formula [12,13]:
1=2 2 Is ns Lr Ir nr Ls aLs exp a2Ls ½1 expðaLs Þ
vð3Þ ¼ vrð3Þ
ð1Þ
where I is the intensity of the OKE signal at zero delay time, a is the linear absorption coefficient of the sample, n is the refractive index, and L is the interaction length of pump beam and probe beam over the CS2 and the sample. The subscripts s and r denote the sample and CS2, respectively. For pump–probe measurement, a half-wave plate and a polarizer were inserted into the probe beam. The polarization of both pump and probe beams was set to be parallel by rotating the polarizer. There was no second polarizer in the probe beam after the sample, and the other parts in the pump–probe experiment were the same as that of the OKE experiment. 3. Results and discussion The typical optical Kerr signal of the polymer solution is illustrated in Figure 2, consisting of two different processes: an ultrafast peak near the zero point followed by a slow decay process. Exploiting the method reported in Refs. [11,13], a Gaussian function was used to represent the ultrafast peak and a biexponential decay function It ¼ A expðt=s1 Þ þ B expðt=s2 Þ was employed
101
Figure 2. The OKE response of the polymer solution and chloroform solvent under a pump intensity of 10.6 GW/cm2. The inset is the OKE signal of CS2 measured under the same conditions.
to describe the slow decay process. The fit yields a fast component of 900 fs and a slow component of 50 ps. The OKE signal of chloroform is also shown in Figure 2, and the results suggest that the OKE signal of the sample solution does not include the influence of the solvent in the delay time of 0.8–25 ps. Based on the magnitudes of the OKE signal of the sample and the reference under identical experimental condition, the vð3Þ of the sample solution is determined as 2.1 1014 esu. The corresponding second hyperpolarizability was deduced to be 2.3 1032 esu according to the formula [14]:
c ¼ vð3Þ =NL;
ð2Þ
where N is the concentration of the molecule and L is the local field correction factor, which is defined as L ¼ ½ðn2 þ 2Þ=34 . This value is much larger than that of C60 (1.5 1036 esu), C70 (7.0 1035 esu) as well as carbon nanotube (4.3 1033 esu) reported before [15– 17]. A 300-nm thick polymer film was also prepared and tested. The absorption spectrum of this film is similar to the polymer solution as shown in Figure 1. A typical temporal behavior of the optical Kerr signal observed for the polymer thin film was shown in Figure 3. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 300 fs. From calculated results, the third-order nonlinear optical susceptibility of the thin film was estimated to be 8.26 1010 esu. Ikeda and Tsutsumi have studied the nonlinear response of azobenzene liquid–crystal film by ns time-resolve technique, and found a nematic-to-isotropic phase transition with a response of 200 ls [18]. Mysliwiec et al. have investigated the nonlinear response in a biopolymer matrix containing azo-dye molecules by using of ns and CW OKE techniques, and got the response time in the range of microseconds [19]. Dong et al. have reported pure electronic origin nonlinear response in azobenzene-doped polymer films using sub-picosecond time-resolved degenerate four-wave mixing and Z-scan techniques, and observed a fast dephasing time of 1 ps [20]. In our experiments, the sub-picosecond ultrafast response was mainly attributed to the resonant electronic nonlinearities. It is estimated that the characteristic response time of the electronic nonlinearity could be as short as 1015 s, which is much shorter than that of other physical processes, such as molecular orientation (1012 s), electrostriction (109 s), and thermal effects (103 s) [21].
102
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103
Figure 5. Five-level model for the nonlinear absorption behavior with the typical decay lifetimes of organic materials. S: singlet states; T: triplet states.
Figure 3. The OKE signal of the polymer thin film. The inset is the OKE signal of CS2 measured under the same conditions.
From the above-mentioned OKE experiment for the polymer solution, it was found that the photoinduced anisotropy of the polymer solution could keep up to dozens of picosecond. We suggest that it comes from the contribution of the excited state. In order to explore the origins of the decay components, we performed pump–probe experiment with parallel polarization configuration. The temporal evolution of transmittance change of the polymer solution is shown in Figure 4. An ultrafast bleaching or saturated absorption followed by slow recovery process was observed. The bleaching recovery process consist of two component, a fast process with time constant about 900 fs and a slow process of 50 ps time scale, which agrees very well with the OKE result. The transient bleaching could come from the saturation absorption or stimulated emission. Under resonant excitation, the singlet excited state would be populated, and the saturation absorption will take place if the absorption cross section of the excited state is smaller than that of ground state. As we did not observe fluorescence emission from the polymer solution, stimulated emission process could be excluded. Hence, the transient bleaching should arise from the saturation absorption effect. The recovery processes of photoinduced bleaching in our experiment reflect the relaxation of the
population in the singlet excited state. Combining with the electronic structures of azobenzene molecules, the five-energy-level model is used to analyze the dynamic process [22–24], as illustrated in Figure 5. In particular, the typical decay lifetimes of different relaxation processes for the organic materials at room temperature are also shown. Electrons in the singlet ground state S0 can be promoted to the singlet excited state S1 based on linear or nonlinear absorption. From the first singlet excited state, electrons may relax to S0 by a radiative or nonradiative transition, with a relaxation time ss1 , or be promoted to the higher-lying singlet state S2 by absorbing another photon. Another possibility for the electrons in S1 is to undergo an intersystem crossing process to the lowest triplet state T1. Some papers pointed out that, for the nonlinear optical behavior under resonant subpicosecond excitation, the one-photon absorption induced (S0 ? S1 ? S2) excited state nonlinearity is dominant [22,24]. Thus, we attribute the fast component of 900 fs to the relaxation of the high singlet excited state S2. In general, the temporal duration for the population transfer from the singlet excited states to the triplet states is ranged in ns domain for organic materials, so the contribution of the triplet states can be neglected. As there is no fluorescence emission observed from the polymer solution, the radiative process is really ineffective in the sample. Therefore, the slow component with decay time of 50 ps can be assigned to the nonradiative relaxation from singlet state S1 to the ground state S0. 4. Conclusions We measured the third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer solution using femtosecond OKE and pump–probe techniques. The magnitude of third-order nonlinear susceptibility of the polymer was calculated to be 2.1 1014 in solution. The corresponding second hyperpolarizability was deduced to be 2.3 1032 esu. The polymer thin film exhibited strong nonlinear optical response as large as 8.26 1010 esu. The large and ultrafast nonlinear optical response of the azobenzene-containing ionic liquid crystalline polymer, together with its unusual chemical features, makes it a potential material for future nonlinear optical devices applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11174203 and 11174195) and the fund of State Key Laboratory of Advanced Optical Communication Systems and Networks. References
Figure 4. The relative transient transmittance change of the polymer solution. The inset shows the response within 5 ps delay time.
[1] X. Michalet et al., Science 307 (2005) 538. [2] J. He, W. Ji, J. Mi, Y. Zheng, J.Y. Ying, Appl. Phys. Lett. 88 (2006) 181114. [3] H. Xu, S. Yin, W. Zhu, Y. Song, B. Tang, Polymer 47 (2006) 6986.
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Q. Chen, L. Kuang, E.H. Sargent, Z.Y. Wang, Appl. Phys. Lett. 83 (2003) 2115. L. Brzozowski, E.H. Sargent, J. Mater. Sci. Mater. Electron. 12 (2001) 483. A.Y.G. Fuh, H.C. Lin, T.S. Mo, C.H. Chen, Opt. Express 13 (2005) 10634. T.C. He, C.S. Wang, X. Pan, C.Z. Zhang, G.Y. Lu, Appl. Phys. B 94 (2009) 653. R.R. Rojo, S. Yamada, H. Matsuda, D. Yankelevich, Appl. Phys. Lett. 72 (1998) 1021. T.C. He, C.S. Wang, J.W. Zhang, X.Q. Zhang, X.M. Lu, Synth. Met. 160 (2010) 1896. F. Dong, E. Koudoumas, S. Couris, Y. Shen, L. Qiu, X. Fu, J. Appl. Phys. 81 (1997) 7073. D. Li, Y. Liu, H.Q. Yang, S.X. Qian, Appl. Phys. Lett. 81 (2002) 2088. M.K. Casstevens, M. Samoc, J. Pfleger, P.N. Prasad, J. Chem. Phys. 92 (1990) 2019. D.G. Kong et al., Opt. Lett. 34 (2009) 2471. S.F. Wang et al., Chem. Phys. Lett. 320 (2000) 411.
103
[15] Z.W. Wang, C.L. Liu, Z.G. Liu, H. Xiang, Z. Li, Q.H. Gong, Chem. Phys. Lett. 407 (2005) 35. [16] J.L. Li, S.F. Wang, H. Yang, Q.H. Gong, X. An, H.Y. Chen, D. Qiang, Chem. Phys. Lett. 288 (1998) 175. [17] S.S. Chu, S.F. Wang, Q.H. Gong, Chem. Phys. Lett. 523 (2012) 104. [18] T. Ikeda, O. Tsutsumi, Science 268 (1995) 1873. [19] J. Mysliwiec, M. Ziemienczuk, Andrzej Miniewicz, Opt. Mater. 33 (2011) 1382. [20] F. Dong, E. Koudoumas, S. Couris, Y.Q. Shen, L. Qiu, X.F. Fu, J. Appl. Phys. 81 (1997) 7073. [21] R.W. Boyd, Nonlinear Optics, Academic Press, New York, 2003. [22] S. Scuppa, L. Orian, D. Dini, S. Santi, M. Meneghetti, J. Phys. Chem. A 113 (2009) 9286. [23] G. Shi et al., J. Opt. Soc. Am. B 26 (2009) 754. [24] B. Gu, K. Lou, J. Chen, Y.N. Li, H.T. Wang, W. Ji, Opt. Express 18 (2010) 26843.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Ultrafast third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer Fuli Zhao a, Changshun Wang a,⇑, Yi Zeng a, Zuanming Jin b, Guohong Ma b a b
Department of Physics, State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, PR China Department of Physics, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 19 September 2012 In final form 19 December 2012 Available online 3 January 2013
a b s t r a c t The ultrafast third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer were investigated by optical Kerr effect (OKE) and pump–probe techniques with femtosecond pulses. The third-order nonlinear optical susceptibility of the sample solution was obtained to be 2.1 1014 esu at a concentration of 2 103 M. The temporal response of the sample solution was decomposed into two components, which could be explained by the relaxation of different singlet excited states. The polymer thin film exhibited strong ultrafast nonlinear optical response of 8.26 1010 esu and show its potential applications for further nonlinear optical devices. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Nonlinear optical materials with large third-order optical nonlinearity and fast response time are essential requirement for optical communication, optical limiter and all-optical switching applications [1–4]. Azobenzene-functionalized polymer is one of the most attractive nonlinear optical materials owing to its unusual chemical and optical properties [5,6]. There have been many reports about the nonlinear optical characteristic and optical storage based on their photoisomerization and photoinduced anisotropy [7–9]. Most of these works were done with CW or long pulse lasers as excitation source due to the slow process of trans–cis–trans isomerization cycles of azobenzene molecules. However, these results were not applicable for fast optical response and high-bandwidth optical signal processing because of their long excitation and relaxation times. Employing a ultrashort pulse laser, high power light can be output to create strong nonlinear light–matter interactions, making it possible to explore physical phenomena with unprecedented time resolution [10]. To further exploit the potential applications of azobenzene materials in ultrafast field, it is imperative to accurately determine the nonlinear parameters under the excitation of ultrashort pulse lasers and then to understand the photodynamic behaviors which are very important for designing all optical switching devices. In this Letter, we employed an azobenzene-containing ionic liquid crystalline polymer which can be easily formulated from readily available components based on ionic self-assembly, and reported its ultrafast third-order nonlinear optical properties by using optical Kerr effect (OKE) technique with femtosecond pulses.
The sample is a supramolecular prepared through self-assembly. 5 mg/mL sodium polyacrylate (PANa) aqueous solution, obtained from the neutralization of polyacrylate with sodium hydroxide, was added dropwise to NDAZO (3-(6-(4-((4-(dimethylamino)phenyl)diazenyl)phenoxy)hexyl)-1-methyl-1H-imidazol-3ium bromide) aqueous solution with the concentration of 1 mg/ mL, in a 1:1 M charge ratio. The resulting precipitated was collected by filtration and washed thoroughly with deionized water to remove residual salts and possible noncomplexed precursors and then dried in vacuum at 60 °C for 24 h. To investigate its stability and phase behaviors, TGA and DSC of the polymer were obtained. TGA curve showed that the polymer was stable under high temperature of 250 °C. DSC thermograms of the polymer displayed phase transition peak at 93 °C, indicating its crystalline nature. The polymer solution with a concentration of 2 103 M is prepared by dissolving the appropriate amount of the solute into the chloroform solvent. The molecular structure and the UV–vis absorption spectra of the polymer are shown in Figure 1. Thin films of the synthesized polymer were prepared by spin-coating chloroform solution (concentration: 30 mg/mL) onto the quartz slides (speed: 2000 rpm, time: 20 s). The thickness of the resultant films was about 300 nm, measured by a Dektak profilometer. The third-order nonlinear optical (NLO) properties of the polymer were examined via a time-resolved optical Kerr effect (OKE) and pump–probe experiments. A standard femtosecond OKE experiment was performed. The femtosecond laser pulse was generated from a mode-locked Ti:Sapphire oscillator (Mai Tai HP-1020) generating 100 fs pulses at 800 nm. A regenerative amplifier system (Spectra-Physics, Spitfire Pro) was used to amplify the pulses 106 times at a repetition rate of 1 kHz. The laser beam was divided into
⇑ Corresponding author. E-mail address: [email protected] (C. Wang). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.12.043
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103
Figure 1. Linear absorption spectrum of the polymer solution and film. Inset: the molecular structure of the polymer.
probe beam and pump beam by a beam splitter. The pump beam traveled through a 1-mm thickness type-I BBO nonlinear crystal to generate pulse at 400 nm. This wavelength falls in the range of relatively strong absorption of the sample allowing us to obtain information on the dynamic behavior of the resonant third-order nonlinearity. A polarizer P1 was put into the probe beam path to set its polarization to be 45° with respect to that of the pump beam. The two beams were focused by a 50 cm focal length lens and overlapped on the same spot of the sample with a spot size of 200 lm. After transmitting the sample cell, the pump beam was blocked, while the transmitted probe beam passed through an analyzer P2 with crossed polarization to P1. The generated OKE signal was detected by a silicon photodiode connected to a lock-in amplifier. In the OKE experiment, CS2 was used as the reference. The vð3Þ of CS2 is accredited as 1 1013 esu in fs domain [11]. The value of the third-order NLO susceptibility of the sample can be calculated from the OKE measurement by using the following formula [12,13]:
1=2 2 Is ns Lr Ir nr Ls aLs exp a2Ls ½1 expðaLs Þ
vð3Þ ¼ vrð3Þ
ð1Þ
where I is the intensity of the OKE signal at zero delay time, a is the linear absorption coefficient of the sample, n is the refractive index, and L is the interaction length of pump beam and probe beam over the CS2 and the sample. The subscripts s and r denote the sample and CS2, respectively. For pump–probe measurement, a half-wave plate and a polarizer were inserted into the probe beam. The polarization of both pump and probe beams was set to be parallel by rotating the polarizer. There was no second polarizer in the probe beam after the sample, and the other parts in the pump–probe experiment were the same as that of the OKE experiment. 3. Results and discussion The typical optical Kerr signal of the polymer solution is illustrated in Figure 2, consisting of two different processes: an ultrafast peak near the zero point followed by a slow decay process. Exploiting the method reported in Refs. [11,13], a Gaussian function was used to represent the ultrafast peak and a biexponential decay function It ¼ A expðt=s1 Þ þ B expðt=s2 Þ was employed
101
Figure 2. The OKE response of the polymer solution and chloroform solvent under a pump intensity of 10.6 GW/cm2. The inset is the OKE signal of CS2 measured under the same conditions.
to describe the slow decay process. The fit yields a fast component of 900 fs and a slow component of 50 ps. The OKE signal of chloroform is also shown in Figure 2, and the results suggest that the OKE signal of the sample solution does not include the influence of the solvent in the delay time of 0.8–25 ps. Based on the magnitudes of the OKE signal of the sample and the reference under identical experimental condition, the vð3Þ of the sample solution is determined as 2.1 1014 esu. The corresponding second hyperpolarizability was deduced to be 2.3 1032 esu according to the formula [14]:
c ¼ vð3Þ =NL;
ð2Þ
where N is the concentration of the molecule and L is the local field correction factor, which is defined as L ¼ ½ðn2 þ 2Þ=34 . This value is much larger than that of C60 (1.5 1036 esu), C70 (7.0 1035 esu) as well as carbon nanotube (4.3 1033 esu) reported before [15– 17]. A 300-nm thick polymer film was also prepared and tested. The absorption spectrum of this film is similar to the polymer solution as shown in Figure 1. A typical temporal behavior of the optical Kerr signal observed for the polymer thin film was shown in Figure 3. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 300 fs. From calculated results, the third-order nonlinear optical susceptibility of the thin film was estimated to be 8.26 1010 esu. Ikeda and Tsutsumi have studied the nonlinear response of azobenzene liquid–crystal film by ns time-resolve technique, and found a nematic-to-isotropic phase transition with a response of 200 ls [18]. Mysliwiec et al. have investigated the nonlinear response in a biopolymer matrix containing azo-dye molecules by using of ns and CW OKE techniques, and got the response time in the range of microseconds [19]. Dong et al. have reported pure electronic origin nonlinear response in azobenzene-doped polymer films using sub-picosecond time-resolved degenerate four-wave mixing and Z-scan techniques, and observed a fast dephasing time of 1 ps [20]. In our experiments, the sub-picosecond ultrafast response was mainly attributed to the resonant electronic nonlinearities. It is estimated that the characteristic response time of the electronic nonlinearity could be as short as 1015 s, which is much shorter than that of other physical processes, such as molecular orientation (1012 s), electrostriction (109 s), and thermal effects (103 s) [21].
102
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103
Figure 5. Five-level model for the nonlinear absorption behavior with the typical decay lifetimes of organic materials. S: singlet states; T: triplet states.
Figure 3. The OKE signal of the polymer thin film. The inset is the OKE signal of CS2 measured under the same conditions.
From the above-mentioned OKE experiment for the polymer solution, it was found that the photoinduced anisotropy of the polymer solution could keep up to dozens of picosecond. We suggest that it comes from the contribution of the excited state. In order to explore the origins of the decay components, we performed pump–probe experiment with parallel polarization configuration. The temporal evolution of transmittance change of the polymer solution is shown in Figure 4. An ultrafast bleaching or saturated absorption followed by slow recovery process was observed. The bleaching recovery process consist of two component, a fast process with time constant about 900 fs and a slow process of 50 ps time scale, which agrees very well with the OKE result. The transient bleaching could come from the saturation absorption or stimulated emission. Under resonant excitation, the singlet excited state would be populated, and the saturation absorption will take place if the absorption cross section of the excited state is smaller than that of ground state. As we did not observe fluorescence emission from the polymer solution, stimulated emission process could be excluded. Hence, the transient bleaching should arise from the saturation absorption effect. The recovery processes of photoinduced bleaching in our experiment reflect the relaxation of the
population in the singlet excited state. Combining with the electronic structures of azobenzene molecules, the five-energy-level model is used to analyze the dynamic process [22–24], as illustrated in Figure 5. In particular, the typical decay lifetimes of different relaxation processes for the organic materials at room temperature are also shown. Electrons in the singlet ground state S0 can be promoted to the singlet excited state S1 based on linear or nonlinear absorption. From the first singlet excited state, electrons may relax to S0 by a radiative or nonradiative transition, with a relaxation time ss1 , or be promoted to the higher-lying singlet state S2 by absorbing another photon. Another possibility for the electrons in S1 is to undergo an intersystem crossing process to the lowest triplet state T1. Some papers pointed out that, for the nonlinear optical behavior under resonant subpicosecond excitation, the one-photon absorption induced (S0 ? S1 ? S2) excited state nonlinearity is dominant [22,24]. Thus, we attribute the fast component of 900 fs to the relaxation of the high singlet excited state S2. In general, the temporal duration for the population transfer from the singlet excited states to the triplet states is ranged in ns domain for organic materials, so the contribution of the triplet states can be neglected. As there is no fluorescence emission observed from the polymer solution, the radiative process is really ineffective in the sample. Therefore, the slow component with decay time of 50 ps can be assigned to the nonradiative relaxation from singlet state S1 to the ground state S0. 4. Conclusions We measured the third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer solution using femtosecond OKE and pump–probe techniques. The magnitude of third-order nonlinear susceptibility of the polymer was calculated to be 2.1 1014 in solution. The corresponding second hyperpolarizability was deduced to be 2.3 1032 esu. The polymer thin film exhibited strong nonlinear optical response as large as 8.26 1010 esu. The large and ultrafast nonlinear optical response of the azobenzene-containing ionic liquid crystalline polymer, together with its unusual chemical features, makes it a potential material for future nonlinear optical devices applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11174203 and 11174195) and the fund of State Key Laboratory of Advanced Optical Communication Systems and Networks. References
Figure 4. The relative transient transmittance change of the polymer solution. The inset shows the response within 5 ps delay time.
[1] X. Michalet et al., Science 307 (2005) 538. [2] J. He, W. Ji, J. Mi, Y. Zheng, J.Y. Ying, Appl. Phys. Lett. 88 (2006) 181114. [3] H. Xu, S. Yin, W. Zhu, Y. Song, B. Tang, Polymer 47 (2006) 6986.
F. Zhao et al. / Chemical Physics Letters 558 (2013) 100–103 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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