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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 24 — Nov. 19, 2012
  • pp: 26845–26851
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Femtosecond third-order optical nonlinearity of an azobenzene-containing ionic liquid crystalline polymer

Fuli Zhao, Changshun Wang, Jinwen Zhang, and Yi Zeng  »View Author Affiliations


Optics Express, Vol. 20, Issue 24, pp. 26845-26851 (2012)
http://dx.doi.org/10.1364/OE.20.026845


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Abstract

The nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer were investigated using single beam Z-scan and optical Kerr effect (OKE) techniques. The nonlinear refractive index of electronic origin (3.1×10−19 m2/W) and the nonlinear absorption coefficient (3.63×10−13 m/W) were determined with 800 nm femtosecond laser pulses at a repetition rate of 1 KHz. The corresponding one-photon and two-photon figures of merit are determined to be 6.05 and 0.94, respectively, at irradiance of 50 GW/cm2. The response time of the observed nonlinearities is estimated to be as fast as 300 fs. These experiment results demonstrate that the polymer is a promising candidate for applications in all-optical switching modulators and nonlinear photonic devices.

© 2012 OSA

1. Introduction

In this paper, we present our experimental investigation into the nonlinear response of a newly synthesized azobenzene-containing ionic liquid crystalline polymer in the off-resonant region. Our results show that the polymer possesses a response time of 300 fs with W=6.05 and T=0.94, indicating its potential applications for optical switching. The influence of thermal effect on the sign of the nonlinear refractive index at high pulse repetition rates is also discussed.

2. Experimental details

The molecular structure of azobenzene-containing ionic liquid crystalline polymer used in the work is shown in Fig. 1(a)
Fig. 1 (a) The molecular structure of the polymer. (b)UV-vis absorption spectrum of the polymer in chloroform solution.
. The sample is a supramolecular material prepared by the ionic self-assembly. For the preparation of ionic self-assembly complex, 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-3-ium bromide) aqueous solution with the concentration of 1 mg/mL, i.e. in a 1:1 molar charge ratio. The precipitated complex was filtrated and washed several times with deionized water to remove residual salts and possible noncomplexed precursors and then dried in vacuum at 60 °C for 24 h. Differential scanning calorimetry (DSC) thermogram of the polymer displayed phase transition peak at 93°C, indicating its crystalline nature. The linear absorption spectrum of the polymer solution in chloroform with a concentration of 2 × 10−3 M is shown in Fig. 1(b). The maximum absorbance is located at 395 nm.

To determine the nonlinear optical response time of the polymer, a standard femtosecond optical Kerr-effect (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 beam was split into two parts with intensity ratio of 10:1 and the polarization of the probe beam was adjusted by 45⁰ to the pump. 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 μm. The generated OKE signal was detected by a silicon photodiode connected to a lock-in amplifier.

3. Results and discussion

The first set of Z-scan experiments was carried out using laser TSA. By assuming a spatially and temporally Gaussian profile for input laser pulses, the normalized energy transmittance TOA under open aperture (OA) condition can be expressed as [20

20. 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]

]
T(z)=1πq0+ln(1+q0exp(x2))dx,
(1)
where q0=βI0Leff,I0=I00/(1+z2/z02) is the excitation intensity at position Z, I00 is the on-axis peak irradiance at the focus, z0=πω02/λ is the Rayleigh range, ω0 is the beam waist radius at the focal point, λ is the wavelength, β is the nonlinear absorption coefficient, Leff=(1-exp(-α0L))/α0 is the effective path length in the sample, α0 is the linear absorption coefficient, and L is the length of the sample.

The Z-scan experiments were performed at input irradiance range from 50 to 100 GW/cm2, and strong reverse saturable absorption (RSA) was observed for all of the OA data. Figure 2(a)
Fig. 2 (a) Normalized open-aperture Z-scan curve excited by the amplified kilohertz laser with irradiance of 50 GW/cm2 at 800 nm. The solid curve is the theoretical fit. (b) The relation of 2PA coefficient β versus I00.
shows our OA Z-scan experimental data of the polymer solution excited with TSA at 50 GW/cm2. By applying the Eq. (1), the Z-scan experimental data can be fitted very well. The value of β is evaluated to be β=3.63×10-13m/W, and it remains constant with increasing irradiance as shown in Fig. 2(b), indicating that the mechanism of the nonlinear absorption is effective 2PA process.

The closed-aperture Z-scan experiments are also measured at different irradiances that range from 50 to 100 GW/cm2. In order to determine the nonlinear refraction contribution, the Z-scan experimental data in closed-aperture configuration are divided by those under the open aperture, which will remove away the influence of the nonlinear absorption on the nonlinear refraction. The valley-peak configuration as shown in Fig. 3(a)
Fig. 3 (a) The divided Z-scan experiment curve measured at 50 GW/cm2. The solid curve is the theoretical fit. (b) Intensity dependence of ΔZp-v/z0, the distance of the normalized peak and valley transmittance of the divided closed-aperture Z-scan data, and ΔTp-v, the difference between the peak and valley transmittance of the normalized divided closed-aperture Z-scan data.
, indicates the positive nonlinearity of the sample, which results from self-focusing. The experimental data are fitted by the equation [20

20. 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]

]
T(z)=14xΔϕ0(x2+9)(x2+1),
(2)
where Δϕ0=knI20Leff,k=2π/λ is the wave vector,x=z/z0. From the theoretical fit, the nonlinear refractive index is calculated to be 3.1 × 10−19 m2/W. Figure 3(b) shows the relations of ΔTp-v and ΔZp-v/z0 as a function of incident intensity. The peak and valley are separated by approximately 1.7z0, and the difference between normalized peak and valley transmittance shows linear relation with no significant deviation from the intensity dependence. All the results indicate that the nonlinear effect is a third-order response. Similar measurements were performed with the solvent (chloroform) only, and no clear Z-scan curves were recorded for the pump power used. Therefore, the relatively weak contribution of the solvent was negligible.

There are some different processes contributed at short laser pulses to the overall nonlinear refractive index. It has been proven that linearly polarized light in Z-scan measurement will induce a refractive-index associated with the photoisomerization of azo materials, and this effect could occur even if the incident wavelength is not in the absorption band of the sample. In our experiments, no birefringence signal was observed when pumping at 800 nm. Moreover, Rosales et al. showed that the nonlinear refraction effect associated with photoisomerization was intensity dependent [21

21. A. A. Rodriguez-Rosales, O. G. Morales-Saavedra, C. J. Roman-Moreno, and R. Ortega-Martinez, “Variation of nonlinear refractive index in dye-doped liquid crystals by local and nonlocal mechanisms,” Opt. Mater. 31(2), 350–360 (2008). [CrossRef]

]. As mentioned previously, the third-order optical nonlinearity obtained in this experiment was intensity independent. Thus, the effect of photoisomerization on the observed nonlinear response could be ignored. The nonlinear refraction was also not induced by laser heating effect because the sign of the nonlinear refractive index due to laser heating effect was negative, but the sign of nonlinear refractive index observed in this experiment was positive. Thus, we attribute the observed nonlinear refraction to the nonresonant electronic nonlinearity, which is due to the electronic response of molecules. A molecule underwent a transition from the ground state to the excitation after absorbing two photons. The dipole moment of the azo molecule changed during such a transition. This change would give birth to electronic nonlinearity.

To evaluate the material requirements for all-optical switching devices, we calculated the one-photon and two-photon figures of merit for the azobenzene-containing ionic liquid crystalline polymer. For λ = 800 nm, β = 3.63 × 10−13 m/W, n2 = 3.1 × 10−19 m2/W, I = 50 GW/cm2, and α = 0.32 cm−1, the figures of merit are calculated to be W=n2I/(α0λ)=6.05>1 and T=βλ/n2=0.94<1, indicating that the polymer has great potential for optical switching applications.

For practical use, ultrafast response times are also required for the nonlinear processes involved. The nonlinear optical response time of the polymer solution was determined using OKE technique. The typical optical Kerr signal of the polymer solution at the concentration of 2 × 10−3 M is illustrated in Fig. 4
Fig. 4 Time-resolved optical Kerr response of the polymer solution under a pump intensity of 60 GW/cm2. The inset is the OKE signal of CS2 measured under the same conditions. The solid curve is the theoretical fit.
. The circles are the experimental results under a pump intensity of 60 GW/cm2. We also measured the OKE signal of chloroform to separate the influence of the solvent, and no detectable OKE signal was observed, indicating that the third-order susceptibility of the solvent is negligible. The OKE signal of the solution is nearly symmetric with a valley centered at zero delay point, and the full width at the half maximum of the Kerr signal is as fast as 300 fs, indicating that the optical response of the sample is very fast.

Next, in order to investigate the variations of the refractive index due to the thermal effect, we performed the Z-scan experiments using the high repetition rate TiS laser with the input irradiance range from 0.2 to 1.0 GW/cm2. In this case, the electronic nonlinear mechanism is suppressed and thermal effects are considered to be a major influence on the behavior of nonlinear optical properties. There is no significant peak or valley characteristic shape for the OA Z-scan data, which indicates that the nonlinear absorption is negligible. The CA Z-scan data show that the polymer solution possesses negative nonlinear refractive index at different input irradiances, as shown in Fig. 5
Fig. 5 Normalized closed-aperture Z-scan curves excited by the high repetition rate (80 MHz) Ti:sapphire laser at various excitation irradiances, and the solid lines are the best-fit curves calculated by Z-scan theory.
. By using the theory of the third-order optical nonlinearity to fit the experimental data, we obtained the nonlinear refractive index of −9.1 × 10−18 m2/W, which was about one order of magnitude larger than CS2 [22

22. R. A. Ganeev, A. I. Ryasnyansky, M. Baba, M. Suzuki, N. Ishizawa, M. Turu, S. Sakakibara, and H. Kuroda, “Nonlinear refraction in CS2,” Appl. Phys. B 78(3-4), 433–438 (2004). [CrossRef]

], and was comparable with the values of bulk CdS material [23

23. N. Sugimoto, A. Koiwai, S. Hyodo, T. Hioki, and S. Noda, “Nonresonant third-order nonlinear optical susceptibility of CdS clusters encapsulated in zeolite A and X,” Appl. Phys. Lett. 66(8), 923–925 (1995). [CrossRef]

].

Even though the influence of nonlinear absorption at high repetition rate is insignificant, our spectral measurements give the linear absorption coefficient of this polymer solution α0=0.32cm1 at 800 nm. Therefore, the observed thermal lensing effect is mainly caused by linear absorption in this medium. The thermal lensing effect generated by a 100 fs pulse can be accumulated across neighboring pulses because their associated pulse-to-pulse separation tp-p is considerably shorter than the millisecond (ms) order thermal diffusivity time constant, which is the time taken for the generated thermal lensing effect to disappear. Other groups have also observed the nonlinearity induced by the thermal lensing effect under femtosecond pulses excitation. Tang et.al. have reported thermal lensing effect in two transparent liquids [24

24. C. W. Chen, J. L. Tang, K. H. Chung, T. H. Wei, and T. H. Huang, “Negative nonlinear refraction obtained with ultrashort laser pulses,” Opt. Express 15(11), 7006–7018 (2007). [CrossRef] [PubMed]

]. They attributed the negative lensing effect to the excitation of librations and/or vibrations, which overwhelm the simultaneously excited nonresonant skeletal motions. Ganeev et al. observed the thermal lensing effect in CS2 solution due to nonlinear absorption [19

19. R. A. Ganeev, A. I. Ryasnyansky, N. Ishizawa, M. Baba, M. Suzuki, M. Turu, S. Sakakibara, and H. Kuroda, “Two- and three-photon absorption in CS2,” Opt. Commun. 231(1-6), 431–436 (2004). [CrossRef]

]. Whatever the applications envisaged, fundamental investigation of laser heating induced nonlinearity will be valuable in better understanding the physics inherent of the nonlinear optical properties of azo materials. Moreover, thermal lens effect has been used as a spectrometric technique for the measurement of weak absorption.

4. Conclusion

We have investigated the third-order nonlinear optical properties of an azobenzene-containing ionic liquid crystalline polymer. The measurements have been conducted by using both Z-scan and OKE techniques with 800 nm, 100 fs laser pulses at a repetition rate of 1KHz. The observed electronic nonlinearities have a response time of 300 fs with the figures of merit: W=6.05 and T=0.94. Such a good property may allow this polymer to be a good candidate of all-optical switching and storage for all optical processing, which require the large third-order refraction as well as short linear and nonlinear absorptions. In contrast to the positive electronic nonlinearities, negative thermal effect was investigated at high pulse repetition rates. This effect may play an important role in applications of spectrometric technique for measuring weak absorption and thermooptical properties.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 11174203) and the fund of State Key Laboratory of Advanced Optical Communication Systems and Networks.

References and links

1.

Q. Y. Chen, L. Kuang, E. H. Sargent, and Z. Y. Wang, “Ultrafast nonresonant third-order optical nonlinearity of fullerenecontaining polyurethane films at telecommunication wavelengths,” Appl. Phys. Lett. 83(11), 2115–2117 (2003). [CrossRef]

2.

R. S. S. Kumar, S. V. Rao, L. Giribabu, and D. N. Rao, “Femtosecond and nanosecond nonlinear optical properties of alkyl phthalocyanines studied using Z-scan technique,” Chem. Phys. Lett. 447(4-6), 274–278 (2007). [CrossRef]

3.

Z. Y. Zhao, T. Q. Jia, J. Lin, Z. G. Wang, and Z. R. Sun, “Femtosecond non-resonant optical nonlinearity of silver chloride nanocrystal doped niobic tellurite glass,” J. Phys. D Appl. Phys. 42(4), 045107 (2009). [CrossRef]

4.

T. Y. Ning, P. Gao, W. L. Wang, H. Lu, W. Y. Fu, Y. L. Zhou, D. X. Zhang, X. D. Bai, E. Wang, and G. Z. Yang, “Nonlinear optical properties of composite films consisting of multi-armed CdS nanorods and ZnO,” Opt. Mater. 31(6), 931–935 (2009). [CrossRef]

5.

Y. M. Chen, J. F. Zhang, Y. X. Wang, X. R. Zhang, K. Yang, C. Zhang, and Y. L. Song, “Third- and fifth-order nonlinearities of heterobimetallic cluster [WOS3Cu3(4-pic)6]·ClO4,” Mater. Chem. Phys. 117(1), 66–69 (2009). [CrossRef]

6.

T. He, Y. Cheng, Y. Du, and Y. Mo, “Z-scan determination of third-order nonlinear optical nonlinearity of three azobenzenes doped polymer films,” Opt. Commun. 275(1), 240–244 (2007). [CrossRef]

7.

L. Brzozowski and E. H. Sargent, “Azobenzenes for photonic network applications:Third-order nonlinear optical properties,” J. Mater. Sci. Mater. Electron. 12(9), 483–489 (2001). [CrossRef]

8.

A. Y. G. Fuh, H. C. Lin, T. S. Mo, and C. H. Chen, “Nonlinear optical property of azo-dye doped liquid crystals determined by biphotonic Z-scan technique,” Opt. Express 13(26), 10634–10641 (2005). [CrossRef] [PubMed]

9.

R. Rangel-Rojo, S. Yamada, H. Matsuda, and D. Yankelevich, “Large near-resonance third-order nonlinearity in an azobenzenefunctionalized polymer film,” Appl. Phys. Lett. 72(9), 1021–1023 (1998). [CrossRef]

10.

T. C. He, L. Zhang, Y. F. Yin, Y. G. Cheng, L. Ding, and Y. J. Mo, “Resonant electronic nonlinearity and laser heating induced nonlinearity of chlorophosphonazo I,” Phys. Lett. A 372(21), 3937–3940 (2008). [CrossRef]

11.

T. C. He, C. S. Wang, C. Z. Zhang, and G. Y. Lu, “Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses,” Physica B 406(3), 488–493 (2011). [CrossRef]

12.

B. Gu, W. Ji, P. S. Patil, and S. M. Dharmaprakash, “Ultrafast optical nonlinearities and figures of merit in acceptor-substituted 3,4,5-trimethoxy chalcone derivatives: Structure-property relationships,” J. Appl. Phys. 103(10), 103511 (2008). [CrossRef]

13.

T. C. He and C. S. Wang, “The study on the nonlinear optical response of Sudan I,” Opt. Commun. 281(15-16), 4121–4125 (2008). [CrossRef]

14.

S. F. Xiao, X. M. Lu, and Q. H. Lu, “Photosensitive liquid-crystalline supramolecules self-assembled from ionic liquid crystal and polyelectrolyte for laser-induced optical anisotropy,” Macromolecules 40, 7944–7950 (2007). [CrossRef]

15.

T. C. He, C. S. Wang, J. W. Zhang, X. Q. Zhang, and X. M. Lu, “Nonlinear absorption in an azo-containing ion liquid crystal polymer in the different excitation regimes,” Synth. Met. 160(17-18), 1896–1901 (2010). [CrossRef]

16.

X. Q. Zhang, C. S. Wang, X. Pan, S. F. Xiao, Y. Zeng, T. C. He, and X. M. Lu, “Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer,” Opt. Commun. 283(1), 146–150 (2010). [CrossRef]

17.

A. Agnesi and G. C. Reali, “Exploiting the Z-scan method for mode-locked laser design,” Opt. Lett. 18(9), 717–719 (1993). [CrossRef] [PubMed]

18.

D. G. Kong, Q. Chang, H. A. Ye, Y. C. Gao, Y. X. Wang, X. R. Zhang, K. Yang, W. Z. Wu, and Y. L. Song, “The fifth-order nonlinearity of CS2,” J. Phys. At. Mol. Opt. Phys. 42(6), 065401 (2009). [CrossRef]

19.

R. A. Ganeev, A. I. Ryasnyansky, N. Ishizawa, M. Baba, M. Suzuki, M. Turu, S. Sakakibara, and H. Kuroda, “Two- and three-photon absorption in CS2,” Opt. Commun. 231(1-6), 431–436 (2004). [CrossRef]

20.

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]

21.

A. A. Rodriguez-Rosales, O. G. Morales-Saavedra, C. J. Roman-Moreno, and R. Ortega-Martinez, “Variation of nonlinear refractive index in dye-doped liquid crystals by local and nonlocal mechanisms,” Opt. Mater. 31(2), 350–360 (2008). [CrossRef]

22.

R. A. Ganeev, A. I. Ryasnyansky, M. Baba, M. Suzuki, N. Ishizawa, M. Turu, S. Sakakibara, and H. Kuroda, “Nonlinear refraction in CS2,” Appl. Phys. B 78(3-4), 433–438 (2004). [CrossRef]

23.

N. Sugimoto, A. Koiwai, S. Hyodo, T. Hioki, and S. Noda, “Nonresonant third-order nonlinear optical susceptibility of CdS clusters encapsulated in zeolite A and X,” Appl. Phys. Lett. 66(8), 923–925 (1995). [CrossRef]

24.

C. W. Chen, J. L. Tang, K. H. Chung, T. H. Wei, and T. H. Huang, “Negative nonlinear refraction obtained with ultrashort laser pulses,” Opt. Express 15(11), 7006–7018 (2007). [CrossRef] [PubMed]

OCIS Codes
(190.0190) Nonlinear optics : Nonlinear optics
(190.4400) Nonlinear optics : Nonlinear optics, materials

ToC Category:
Nonlinear Optics

History
Original Manuscript: September 25, 2012
Revised Manuscript: October 29, 2012
Manuscript Accepted: October 31, 2012
Published: November 14, 2012

Citation
Fuli Zhao, Changshun Wang, Jinwen Zhang, and Yi Zeng, "Femtosecond third-order optical nonlinearity of an azobenzene-containing ionic liquid crystalline polymer," Opt. Express 20, 26845-26851 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-24-26845


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References

  1. Q. Y. Chen, L. Kuang, E. H. Sargent, and Z. Y. Wang, “Ultrafast nonresonant third-order optical nonlinearity of fullerenecontaining polyurethane films at telecommunication wavelengths,” Appl. Phys. Lett.83(11), 2115–2117 (2003). [CrossRef]
  2. R. S. S. Kumar, S. V. Rao, L. Giribabu, and D. N. Rao, “Femtosecond and nanosecond nonlinear optical properties of alkyl phthalocyanines studied using Z-scan technique,” Chem. Phys. Lett.447(4-6), 274–278 (2007). [CrossRef]
  3. Z. Y. Zhao, T. Q. Jia, J. Lin, Z. G. Wang, and Z. R. Sun, “Femtosecond non-resonant optical nonlinearity of silver chloride nanocrystal doped niobic tellurite glass,” J. Phys. D Appl. Phys.42(4), 045107 (2009). [CrossRef]
  4. T. Y. Ning, P. Gao, W. L. Wang, H. Lu, W. Y. Fu, Y. L. Zhou, D. X. Zhang, X. D. Bai, E. Wang, and G. Z. Yang, “Nonlinear optical properties of composite films consisting of multi-armed CdS nanorods and ZnO,” Opt. Mater.31(6), 931–935 (2009). [CrossRef]
  5. Y. M. Chen, J. F. Zhang, Y. X. Wang, X. R. Zhang, K. Yang, C. Zhang, and Y. L. Song, “Third- and fifth-order nonlinearities of heterobimetallic cluster [WOS3Cu3(4-pic)6]·ClO4,” Mater. Chem. Phys.117(1), 66–69 (2009). [CrossRef]
  6. T. He, Y. Cheng, Y. Du, and Y. Mo, “Z-scan determination of third-order nonlinear optical nonlinearity of three azobenzenes doped polymer films,” Opt. Commun.275(1), 240–244 (2007). [CrossRef]
  7. L. Brzozowski and E. H. Sargent, “Azobenzenes for photonic network applications:Third-order nonlinear optical properties,” J. Mater. Sci. Mater. Electron.12(9), 483–489 (2001). [CrossRef]
  8. A. Y. G. Fuh, H. C. Lin, T. S. Mo, and C. H. Chen, “Nonlinear optical property of azo-dye doped liquid crystals determined by biphotonic Z-scan technique,” Opt. Express13(26), 10634–10641 (2005). [CrossRef] [PubMed]
  9. R. Rangel-Rojo, S. Yamada, H. Matsuda, and D. Yankelevich, “Large near-resonance third-order nonlinearity in an azobenzenefunctionalized polymer film,” Appl. Phys. Lett.72(9), 1021–1023 (1998). [CrossRef]
  10. T. C. He, L. Zhang, Y. F. Yin, Y. G. Cheng, L. Ding, and Y. J. Mo, “Resonant electronic nonlinearity and laser heating induced nonlinearity of chlorophosphonazo I,” Phys. Lett. A372(21), 3937–3940 (2008). [CrossRef]
  11. T. C. He, C. S. Wang, C. Z. Zhang, and G. Y. Lu, “Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses,” Physica B406(3), 488–493 (2011). [CrossRef]
  12. B. Gu, W. Ji, P. S. Patil, and S. M. Dharmaprakash, “Ultrafast optical nonlinearities and figures of merit in acceptor-substituted 3,4,5-trimethoxy chalcone derivatives: Structure-property relationships,” J. Appl. Phys.103(10), 103511 (2008). [CrossRef]
  13. T. C. He and C. S. Wang, “The study on the nonlinear optical response of Sudan I,” Opt. Commun.281(15-16), 4121–4125 (2008). [CrossRef]
  14. S. F. Xiao, X. M. Lu, and Q. H. Lu, “Photosensitive liquid-crystalline supramolecules self-assembled from ionic liquid crystal and polyelectrolyte for laser-induced optical anisotropy,” Macromolecules40, 7944–7950 (2007). [CrossRef]
  15. T. C. He, C. S. Wang, J. W. Zhang, X. Q. Zhang, and X. M. Lu, “Nonlinear absorption in an azo-containing ion liquid crystal polymer in the different excitation regimes,” Synth. Met.160(17-18), 1896–1901 (2010). [CrossRef]
  16. X. Q. Zhang, C. S. Wang, X. Pan, S. F. Xiao, Y. Zeng, T. C. He, and X. M. Lu, “Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer,” Opt. Commun.283(1), 146–150 (2010). [CrossRef]
  17. A. Agnesi and G. C. Reali, “Exploiting the Z-scan method for mode-locked laser design,” Opt. Lett.18(9), 717–719 (1993). [CrossRef] [PubMed]
  18. D. G. Kong, Q. Chang, H. A. Ye, Y. C. Gao, Y. X. Wang, X. R. Zhang, K. Yang, W. Z. Wu, and Y. L. Song, “The fifth-order nonlinearity of CS2,” J. Phys. At. Mol. Opt. Phys.42(6), 065401 (2009). [CrossRef]
  19. R. A. Ganeev, A. I. Ryasnyansky, N. Ishizawa, M. Baba, M. Suzuki, M. Turu, S. Sakakibara, and H. Kuroda, “Two- and three-photon absorption in CS2,” Opt. Commun.231(1-6), 431–436 (2004). [CrossRef]
  20. 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]
  21. A. A. Rodriguez-Rosales, O. G. Morales-Saavedra, C. J. Roman-Moreno, and R. Ortega-Martinez, “Variation of nonlinear refractive index in dye-doped liquid crystals by local and nonlocal mechanisms,” Opt. Mater.31(2), 350–360 (2008). [CrossRef]
  22. R. A. Ganeev, A. I. Ryasnyansky, M. Baba, M. Suzuki, N. Ishizawa, M. Turu, S. Sakakibara, and H. Kuroda, “Nonlinear refraction in CS2,” Appl. Phys. B78(3-4), 433–438 (2004). [CrossRef]
  23. N. Sugimoto, A. Koiwai, S. Hyodo, T. Hioki, and S. Noda, “Nonresonant third-order nonlinear optical susceptibility of CdS clusters encapsulated in zeolite A and X,” Appl. Phys. Lett.66(8), 923–925 (1995). [CrossRef]
  24. C. W. Chen, J. L. Tang, K. H. Chung, T. H. Wei, and T. H. Huang, “Negative nonlinear refraction obtained with ultrashort laser pulses,” Opt. Express15(11), 7006–7018 (2007). [CrossRef] [PubMed]

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