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Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer
Optics Communications 283 (2010) 146–150
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer Xiaoqiang Zhang a, Changshun Wang a,*, Xu Pan a, Sufang Xiao b, Yi Zeng a, Tingchao He a, Xuemin Lu b a b
Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 23 September 2009 Accepted 23 September 2009
PACS: 42.65.An 42.65.Jx 42.25.Lc 42.70.Df
a b s t r a c t The nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer were investigated. The single beam Z-scan measurement showed the polymer film possessed a value of nonlinear refractive index n2 = 1.07 109 cm2/W under a picosecond 532 nm excitation. Photoinduced anisotropy in the polymer was studied through dichroism and photoinduced birefringence. A photoinduced birefringence value Dn 102 was achieved in the polymer film. The mechanism for the nonlinear optical response and the physical process of photoinduced anisotropy in the polymer were discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Azobenzene ionic liquid–crystal Nonlinear refractive index Photoinduced anisotropy
1. Introduction Great interests have been plunged into the nonlinear optical properties of polymer liquid crystals (LC) with azobenzene group in the side chain for their large nonlinear refraction and long-term optical stability. Azobenzene LC polymer has been widely used in applications of optical storage, optical-limiting and optical switching [1–6]. Among various LC materials, ionic liquids crystal is being studied due to their interesting properties such as thermal stability, non-flammability and reusability [7]. Therefore, synthesis and application of an azobenzene ionic liquid–crystalline polymer should be considered for its charming properties, especially for high nonlinear photosensitivity. On the other hand, it is well known that the azobenzene molecules can undergo trans–cis–trans isomerization under the irradiation at a wavelength lying in the absorption region [8]. Especially, for an azobenzene ionic liquid–crystalline polymer, under actinic irradiation, azobenzene groups display trans to cis transformation and induce LC alignment in the direction perpendicular to the polarization of pump light. This photoinduced orientation results in macroscopic anisotropy, thus the dichroism and birefringence
* Corresponding author. E-mail address: [email protected] (C. Wang). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.09.054
in polymer films. The dichroism represents the absorbance variations along the directions parallel and perpendicular to the pump polarization, while the birefringence represents the phase modulation effect of the photo-oriented polymer film. The photoinduced anisotropy in azobenzene materials has potential applications in many fields, particularly for holographic recording and polarization reconstruction [9]. In the paper, employing a novel ionic liquid–crystalline polymer containing azobenzene groups, the nonlinear optical properties and the characteristics of photoinduced anisotropy were investigated. The polymer was constructed by the methyl orange (MO) dye as the building unit due to its photo-isomerization and the poly (ionic liquid) (PIL) as a main chain segment. The nonlinear optical properties of the polymer were studied using Z-scan method under picosecond pulse 532 nm excitation [10], and the characteristics of photoinduced anisotropy were investigated in detail through dichroism and birefringence. 2. Experiment 2.1. Material and film preparation The sample is a supramolecular material by the ionic selfassembly of poly (ionic liquid) (PIL) and azobenzene dye, the
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Fig. 1. (a) Molecular structure of the complex PILMO and (b) Typical polarized optical micrographs of the texture of PILMO at 30 °C during cooling from their isotropic phase.
molecular structure of which is shown in Fig. 1a. The charged polymer poly(1-butyl-vinylpyridinium bromide) PIL was selected as the main chain segment, and the methyl orange (MO) dye was selected as the building unit due to its capability for photo-isomerization. For the preparation of ionic self-assembly complex, 2 mg/ ml PIL aqueous solution was added dropwise to MO aqueous solution with the same concentration, i.e. in a 1:1 molar charge ratio. The precipitated complex was filtrated and washed several times with doubly distilled water to remove residual salts and possible noncomplexed precursors, then dried in vacuum at 60 °C for 12 h. Polarized optical microscopy with hot stage was performed and displayed that the complexes powder melt at about 180 °C and the Schlieren textures appeared during cooling. The pronounced Schlieren textures indicated high orientational order of the complexes. The thickness of the resultant films was about 275 nm, measured by a Dektak profilometer. Typical polarized optical micrographs of the texture of PILMO at 30 °C during cooling from their isotropic phase are shown in Fig. 1b. The absorption spectrum of the complex films is shown in Fig. 2.
2.2. Measurement of the nonlinear refractive index The Z-scan experimental set-up in our experiment was shown in Fig. 3. We used a picosecond pulse 532 nm Nd: YAG laser (Continuum Co. Ltd.) as the excitation source in Z-scan technique. The laser’s repetition frequency was 10 Hz and the pulse duration was 38 picoseconds, respectively. The spot size at the focal point for 532 nm was 26.7 lm in radius. The focal length of the lens L was 30 cm. The detector was a dual-channel joulemeter EPM2000 (Molectron Co. Ltd.). From Z-scan curve, the difference between normalized peak and valley transmittance DTP–V denoting TP–TV can be directly measured by Z-scan technique. The variation of this quantity as a function of |DU0| is given by Sheik-Bahae et al. [10]
DT P—V ¼ 0:406ð1 sÞ0:25 jDU0 j
ð1Þ
Here, |DU0| is the on-axis phase shift at the focus, s ¼ 1 expð2r20 =x20 Þ is the aperture linear transmittance with r0 denoting the aperture radius and x0 denoting beam radius at the aperture in the linear region. DU0 can be obtained by [10]
DU0 ¼ kLeff n2 I0 ¼ ð2p=kÞLeff n2 I0 :
ð2Þ
Here, I0 is the intensity of the laser beam at focus z ¼ 0; Leff ¼ ½1 expðaLÞ=a is the effective thickness of the sample, a is the linear absorption coefficient and L is the thickness of the sample. The value of DTP–V, the difference between the normalized peak and valley transmittance, could be obtained through the best theoretical fit from the results of the Z-scan curve. Using the Eq. (1) and (2) with the obtained DTP–V from the curve, the nonlinear refractive index n2 (m2/W) could be obtained. 2.3. Measurement of the photoinduced anisotropy
Fig. 2. Absorption spectra of the complex film of PILMO.
The experimental set-up for the measurement of the photoinduced dichroism in azobenzene ionic liquid–crystalline polymer is shown in Fig. 4. A beam from an argon-ion laser at 488 nm was separated with a beam-splitter in a pump and probe beams. The probe beam was polarized in the s direction. The pump was polarized and then went through a k/2 plate. The sample was
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Fig. 3. Schematic diagram of Z-scan experiment set-up: A, Attenuator; BS, Beam splitter; L, Lens; P, Aperture; S, Sample; D1, D2, Detector.
3. Results and discussion
Fig. 4. Schematic diagram of photoinduced dichroism experiment set-up: P, polarizer; A, attenuator.
placed in path of the probe beam. Under the action of pump light, a light signal of transmittance was measured with a detector, and then processed by a lock-in amplifier and recorded using a computer. The modulation frequency of the chopper was set at 1000 Hz. The sample began to be irradiated as the pump beam is turned on at time = 80 s, and then the pump is turned off at time = 250 s. The pump light was linearly polarized at 0°, 45° and 90°, respectively to the probe beam. The power of the pump keeps constant. The ratio between the power of the pump (1.25 mW) and the probe beam on the sample is about 100:1. The experimental set-up for the measurement of photoinduced birefringence in azobenzene ionic liquid–crystalline polymer was similar to one in Ref. [11]. The photoinduced birefringence was investigated with a diode laser at 650 nm as probe light, which was far from the absorption band of azobenzene ionic liquid–crystalline polymer, and an argon-ion 488 nm laser as pump light. The pump wavelength lies in the absorption band of the films. The realtime behavior of photoinduced birefringence of the polymer film can be observed. The birefringence value can be obtained by Todorov et al. [12] 2
I? ¼ I0 sin ðpDnd=kÞ:
The nonlinear refractive index n2 and nonlinear absorption b can be obtained by the normalized transmittance measured for the closed and open aperture versus sample position. Fig. 5 is the open aperture experimental data and it shows that nonlinear absorption cannot be neglected. The solid line in Fig. 5 is theoretical fit according to Ref. [10]. The nonlinear absorption index can be calculated as b = 0.72 104 cm/GW. The closed transmittance is affected by both nonlinear refraction and absorption. The determination of nonlinear refractive index is less straight-forward from the closed aperture Z-scan experimental data. It is necessary to separate the effect of nonlinear absorption by performing the open aperture Z-scan experiment. Fig. 6 shows division of closed aperture experimental data and open experimental data using a picosecond pulse laser. The solid line in Fig. 6 is theoretical fit according to Ref. [10]. The peak followed by the curve in the figure indicated that the sign of the nonlinear refraction was negative, which resulted from self-defocusing. The input energy was 0.33 lJ and peak intensity of the incident laser was calculated to be 7.77 108 W/cm2 at the focal point, DTP–V = 0.5, s = 0.4 and
ð3Þ
Here, Dn is the photoinduced birefringence, I? is the probe intensity passing through the crossed polarizers, I0 is the probe intensity passing through the parallel polarizers before pump irradiation.
Fig. 5. Open aperture Z-scan experimental data of the PILMO film.
X. Zhang et al. / Optics Communications 283 (2010) 146–150
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Fig. 6. Division of closed aperture Z-scan experimental data and open experimental data of the PILMO film.
the thickness of sample was 275 nm. Then the nonlinear refractive index was calculated as n2 = 1.07 109 cm2/W. The result shows that the PILMO (poly ionic liquid methyl orange) film has a much larger value of nonlinear refractive index than many other LC materials, such as DDLC [13]. The large nonlinearity of this material indicates it has potential applications in nonlinear optical devices. It is necessary to discriminate the origin of nonlinearity rises from different effects, such as resonant electronic nonlinear effect, laser heating effect and photoinduced trans to cis transformation. Under picosecond pulse excitation, neither laser heating effect nor resonant electronic nonlinear effect are very remarkable usually and could cause such a big value of n2. It is most likely that the nonlinearity mainly comes from photoinduced trans to cis transformation. The further research is under process now. Using a CW linearly 488 nm laser as the excitation light, which is near to the peak of absorption band of polymer, the real-time behavior of photoinduced dichroism was investigated. Typical absorbance variations of polymer during the writing and the relaxation periods are presented in Fig. 7a. When the pump is on, at least two regimes are involved in the writing process [14]. The fast one is due to the pump efficiency and depends on the intensity of the pump laser, the angle between the laser polarization direction and the long molecular axis of the azobenzene groups by an ‘‘I0cos2u” factor. This selective optical pumping from the trans to cis isomers produces an ‘‘angular hole burning” (AHB) in the angular distribution of the trans isomers. From Fig. 7a, in the first few seconds, the fast AHB process takes the main part and induces the significant increase in both A\ and A45 components. Additionally, upon irradiation and during the multiple trans–cis-trans isomerization cycles, a slow ‘‘angular redistribution” (AR) of the molecules according to their ability to rotate by thermal random interactions with the host medium and to diffuse by thermal back-relaxation will occur. Consequently, during the writing period, for azobenzene ionic liquid–crystalline polymer, the AHB and AR processes occur simultaneously and producing an increase in the concentration of molecules oriented perpendicularly to the laser polarization direction. The increase of the A\ is bigger than A||. It indicates that the AR process is more intense in A\. When the pump is off, in the first few seconds, the thermal cis–trans backrelaxation takes the main factor and is the origin of the decrease of both A\ and A|| components. And then the AR process principally governs the perpendicular and parallel absorbance. But still the initial isotropic disorder of the azobenzene molecules is never recovered even after a long time delay. Through the calculation of the normalized average absorbance T 0 ¼ ðAjj þ 2A? Þ=3A0 and the normalized linear dichroism
Fig. 7. Time dependence of (a) the normalized absorbance (h is the angle between polarization directions of probe and pump), (b) the normalized average absorbance T0 and the normalized linear dichroism T2 of the PILMO film.
T 0 ¼ ðAjj 2A? Þ=3A0 , defined as in Reference [14], the characterization of the photoinduced dichroism can be performed. The results for the polymer are shown in Fig. 7b. When the writing laser is turned on, T0 increases during the first few seconds and reaches a plateau value (0.85) which is characteristic of a photostationary state. In contrast with T0, T2 decreases and become a negative value. As mentioned above, the processes are mainly due to AHB process firstly and the AR process later. Fig. 8 shows the experimental results of photoinduced birefringence at various pump intensities. When the pump is on, a fast process and a slow process in the buildup of birefringence can be seen under irradiation. For azobenzene LC polymer, the fast process is related to the local movement of azobenzene groups in the trans–cis–trans photo-isomerization, and the slow process is attributed to the movement of azobenzene groups linked to the LC polymeric chain [15,16]. As the pump intensity increases, the saturation value of birefringence reaches a maximum. From the figure, when the pump intensity was 100 mW/cm2, the maximum of the saturation value was Dn = 5.3 102. The large value of photoinduced birefringence (in the order of 102) indicates that the material has potential value in the application of nonlinear photonic devices and optical storage. Below the intensity 100 mW/ cm2, it is clear that the transmittance increased with increase of intensity of irradiation light. Irradiation light at higher intensity produced a higher concentration of cis-azobenzene at first, and then the concentration decreased, which means a trans–cis–trans isomerization process [15]. Hence, irradiation light at higher inten-
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azobenzene groups. Using the Z-scan technique, under a picoseconds pulse 532 nm excitation, the nonlinear refractive index of the polymer film can be obtained as n2 = 1.07 109 cm2/W. Using an argon-ion 488 nm laser as pump light, we also investigated the photoinduced anisotropy through dichroism and birefringence in the polymer. A birefringence value Dn 102 was achieved in the polymer film. The mechanism responsible for the nonlinear optical properties and the process of photoinduced anisotropy in the polymer was discussed. The large value of nonlinear refractive index, remarkable and stable photoinduced molecular reorientation of the polymer film indicates that the material has potential value in the application of nonlinear photonic devices, holographic recording and polarization reconstruction. Acknowledgements
Fig. 8. Photoinduced birefringence of the PILMO film at various pump intensities: (a) 50 mW/cm2, (b) 80 mW/cm2, (c) 200 mW/cm2, and (d) 400 mW/cm2.
sity gave rise to a higher trans–cis–trans isomerization rate, therefore a higher achievement of the saturation order and a bigger value of photoinduced birefringence. Over the intensity 100 mW/ cm2, the saturation value decreased. This phenomenon can be discussed in terms of two competing processes: the photoinduced reorientation and thermal random motion. When the light intensity increased from 50 to 200 mW/cm2, the number of molecules that may take isomerization increased. When the light intensity reached 200 mW/cm2, the laser heating effect became obvious, and it disorientated the aligned azobenzene molecules [11]. When the pump is off, the birefringence does not show significant decay and even exhibits a slight increase. In fact, we found that the characteristic of long-term and reversible optical storage of the polymer can last for months. The phenomenon can be explained with the self-organization effect of the molecules [17]. The molecules keep well-ordered (or continue the reorientation) spontaneously after removing the pump irradiation, instead of relaxing back to the isotropic state. 4. Conclusion We have investigated nonlinear optical properties and photoinduced anisotropy in an ionic liquid–crystalline polymer containing
This work was supported by the National Science Foundation of China under Grant No. 10675083 and the fund of the State Key Laboratory on Fiber Optic Local Area Communication Networks and Advanced Optical Communication Systems. References [1] F. Simoni, L. Lucchetti, D.E. Lucchetta, O. Francescangeli, Opt. Express 9 (2001) 85. [2] T. Ikeda, O. Tsutsumi, Science 268 (1995) 1873. [3] L.G. Deng, H.K. Liu, Opt. Eng. 42 (2003) 2936. [4] J.M. Geary, J.W. Goodbye, J.S. Patel, J. Appl. Phys. 62 (1987) 4100. [5] M. Eich, J.H. Wendorff, B. Reck, H. Ringsdorf, Makromol. Chem. Rapid Commun. 8 (1987) 59. [6] Q. Zhang, G. Bazuin, C.J. Barrett, Chem. Mater. 20 (2008) 29. [7] D.J. Abdallah, A. Robertson, H.-F. Hsu, R.G. Weiss, J. Am. Chem. Soc. 122 (2000) 3053. [8] C. Barret, A. Natansohn, P. Rochon, Macromolecules 27 (1994) 4781. [9] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 4139. [10] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760. [11] T.C. He, C.S. Wang, X. Pan, Phys. B 10 (2008) 2991. [12] T. Todorov, L. Nikolova, N. Tomova, Appl. Opt. 23 (1984) 4309. [13] S. Shettigar, G. Umesh, P. Poornesh, Y.L.N. Murthy, in: ICTON Mediterranean Winter Conference, 2007, pp. 302–305. [14] T. Buffeteau, F.L. Labarthet, M. Pézolet, C. Sourisseau, Macromolecules 31 (1998) 7312. [15] Y.L. Wu, Y. Demachi, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, Macromolecules 31 (1998) 349. [16] X. Meng, A. Natansohn, P. Rochon, J. Polym. Sci. B: Polym. Phys. 34 (1996) 1461. [17] D. Barada, T. Fukuda, H. Sumimura, M. Itoh, T. Yatagai, Jpn. J. Appl. Phys. 47 (2008) 1203.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer Xiaoqiang Zhang a, Changshun Wang a,*, Xu Pan a, Sufang Xiao b, Yi Zeng a, Tingchao He a, Xuemin Lu b a b
Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 15 June 2009 Received in revised form 23 September 2009 Accepted 23 September 2009
PACS: 42.65.An 42.65.Jx 42.25.Lc 42.70.Df
a b s t r a c t The nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid–crystalline polymer were investigated. The single beam Z-scan measurement showed the polymer film possessed a value of nonlinear refractive index n2 = 1.07 109 cm2/W under a picosecond 532 nm excitation. Photoinduced anisotropy in the polymer was studied through dichroism and photoinduced birefringence. A photoinduced birefringence value Dn 102 was achieved in the polymer film. The mechanism for the nonlinear optical response and the physical process of photoinduced anisotropy in the polymer were discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Azobenzene ionic liquid–crystal Nonlinear refractive index Photoinduced anisotropy
1. Introduction Great interests have been plunged into the nonlinear optical properties of polymer liquid crystals (LC) with azobenzene group in the side chain for their large nonlinear refraction and long-term optical stability. Azobenzene LC polymer has been widely used in applications of optical storage, optical-limiting and optical switching [1–6]. Among various LC materials, ionic liquids crystal is being studied due to their interesting properties such as thermal stability, non-flammability and reusability [7]. Therefore, synthesis and application of an azobenzene ionic liquid–crystalline polymer should be considered for its charming properties, especially for high nonlinear photosensitivity. On the other hand, it is well known that the azobenzene molecules can undergo trans–cis–trans isomerization under the irradiation at a wavelength lying in the absorption region [8]. Especially, for an azobenzene ionic liquid–crystalline polymer, under actinic irradiation, azobenzene groups display trans to cis transformation and induce LC alignment in the direction perpendicular to the polarization of pump light. This photoinduced orientation results in macroscopic anisotropy, thus the dichroism and birefringence
* Corresponding author. E-mail address: [email protected] (C. Wang). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.09.054
in polymer films. The dichroism represents the absorbance variations along the directions parallel and perpendicular to the pump polarization, while the birefringence represents the phase modulation effect of the photo-oriented polymer film. The photoinduced anisotropy in azobenzene materials has potential applications in many fields, particularly for holographic recording and polarization reconstruction [9]. In the paper, employing a novel ionic liquid–crystalline polymer containing azobenzene groups, the nonlinear optical properties and the characteristics of photoinduced anisotropy were investigated. The polymer was constructed by the methyl orange (MO) dye as the building unit due to its photo-isomerization and the poly (ionic liquid) (PIL) as a main chain segment. The nonlinear optical properties of the polymer were studied using Z-scan method under picosecond pulse 532 nm excitation [10], and the characteristics of photoinduced anisotropy were investigated in detail through dichroism and birefringence. 2. Experiment 2.1. Material and film preparation The sample is a supramolecular material by the ionic selfassembly of poly (ionic liquid) (PIL) and azobenzene dye, the
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147
Fig. 1. (a) Molecular structure of the complex PILMO and (b) Typical polarized optical micrographs of the texture of PILMO at 30 °C during cooling from their isotropic phase.
molecular structure of which is shown in Fig. 1a. The charged polymer poly(1-butyl-vinylpyridinium bromide) PIL was selected as the main chain segment, and the methyl orange (MO) dye was selected as the building unit due to its capability for photo-isomerization. For the preparation of ionic self-assembly complex, 2 mg/ ml PIL aqueous solution was added dropwise to MO aqueous solution with the same concentration, i.e. in a 1:1 molar charge ratio. The precipitated complex was filtrated and washed several times with doubly distilled water to remove residual salts and possible noncomplexed precursors, then dried in vacuum at 60 °C for 12 h. Polarized optical microscopy with hot stage was performed and displayed that the complexes powder melt at about 180 °C and the Schlieren textures appeared during cooling. The pronounced Schlieren textures indicated high orientational order of the complexes. The thickness of the resultant films was about 275 nm, measured by a Dektak profilometer. Typical polarized optical micrographs of the texture of PILMO at 30 °C during cooling from their isotropic phase are shown in Fig. 1b. The absorption spectrum of the complex films is shown in Fig. 2.
2.2. Measurement of the nonlinear refractive index The Z-scan experimental set-up in our experiment was shown in Fig. 3. We used a picosecond pulse 532 nm Nd: YAG laser (Continuum Co. Ltd.) as the excitation source in Z-scan technique. The laser’s repetition frequency was 10 Hz and the pulse duration was 38 picoseconds, respectively. The spot size at the focal point for 532 nm was 26.7 lm in radius. The focal length of the lens L was 30 cm. The detector was a dual-channel joulemeter EPM2000 (Molectron Co. Ltd.). From Z-scan curve, the difference between normalized peak and valley transmittance DTP–V denoting TP–TV can be directly measured by Z-scan technique. The variation of this quantity as a function of |DU0| is given by Sheik-Bahae et al. [10]
DT P—V ¼ 0:406ð1 sÞ0:25 jDU0 j
ð1Þ
Here, |DU0| is the on-axis phase shift at the focus, s ¼ 1 expð2r20 =x20 Þ is the aperture linear transmittance with r0 denoting the aperture radius and x0 denoting beam radius at the aperture in the linear region. DU0 can be obtained by [10]
DU0 ¼ kLeff n2 I0 ¼ ð2p=kÞLeff n2 I0 :
ð2Þ
Here, I0 is the intensity of the laser beam at focus z ¼ 0; Leff ¼ ½1 expðaLÞ=a is the effective thickness of the sample, a is the linear absorption coefficient and L is the thickness of the sample. The value of DTP–V, the difference between the normalized peak and valley transmittance, could be obtained through the best theoretical fit from the results of the Z-scan curve. Using the Eq. (1) and (2) with the obtained DTP–V from the curve, the nonlinear refractive index n2 (m2/W) could be obtained. 2.3. Measurement of the photoinduced anisotropy
Fig. 2. Absorption spectra of the complex film of PILMO.
The experimental set-up for the measurement of the photoinduced dichroism in azobenzene ionic liquid–crystalline polymer is shown in Fig. 4. A beam from an argon-ion laser at 488 nm was separated with a beam-splitter in a pump and probe beams. The probe beam was polarized in the s direction. The pump was polarized and then went through a k/2 plate. The sample was
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Fig. 3. Schematic diagram of Z-scan experiment set-up: A, Attenuator; BS, Beam splitter; L, Lens; P, Aperture; S, Sample; D1, D2, Detector.
3. Results and discussion
Fig. 4. Schematic diagram of photoinduced dichroism experiment set-up: P, polarizer; A, attenuator.
placed in path of the probe beam. Under the action of pump light, a light signal of transmittance was measured with a detector, and then processed by a lock-in amplifier and recorded using a computer. The modulation frequency of the chopper was set at 1000 Hz. The sample began to be irradiated as the pump beam is turned on at time = 80 s, and then the pump is turned off at time = 250 s. The pump light was linearly polarized at 0°, 45° and 90°, respectively to the probe beam. The power of the pump keeps constant. The ratio between the power of the pump (1.25 mW) and the probe beam on the sample is about 100:1. The experimental set-up for the measurement of photoinduced birefringence in azobenzene ionic liquid–crystalline polymer was similar to one in Ref. [11]. The photoinduced birefringence was investigated with a diode laser at 650 nm as probe light, which was far from the absorption band of azobenzene ionic liquid–crystalline polymer, and an argon-ion 488 nm laser as pump light. The pump wavelength lies in the absorption band of the films. The realtime behavior of photoinduced birefringence of the polymer film can be observed. The birefringence value can be obtained by Todorov et al. [12] 2
I? ¼ I0 sin ðpDnd=kÞ:
The nonlinear refractive index n2 and nonlinear absorption b can be obtained by the normalized transmittance measured for the closed and open aperture versus sample position. Fig. 5 is the open aperture experimental data and it shows that nonlinear absorption cannot be neglected. The solid line in Fig. 5 is theoretical fit according to Ref. [10]. The nonlinear absorption index can be calculated as b = 0.72 104 cm/GW. The closed transmittance is affected by both nonlinear refraction and absorption. The determination of nonlinear refractive index is less straight-forward from the closed aperture Z-scan experimental data. It is necessary to separate the effect of nonlinear absorption by performing the open aperture Z-scan experiment. Fig. 6 shows division of closed aperture experimental data and open experimental data using a picosecond pulse laser. The solid line in Fig. 6 is theoretical fit according to Ref. [10]. The peak followed by the curve in the figure indicated that the sign of the nonlinear refraction was negative, which resulted from self-defocusing. The input energy was 0.33 lJ and peak intensity of the incident laser was calculated to be 7.77 108 W/cm2 at the focal point, DTP–V = 0.5, s = 0.4 and
ð3Þ
Here, Dn is the photoinduced birefringence, I? is the probe intensity passing through the crossed polarizers, I0 is the probe intensity passing through the parallel polarizers before pump irradiation.
Fig. 5. Open aperture Z-scan experimental data of the PILMO film.
X. Zhang et al. / Optics Communications 283 (2010) 146–150
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Fig. 6. Division of closed aperture Z-scan experimental data and open experimental data of the PILMO film.
the thickness of sample was 275 nm. Then the nonlinear refractive index was calculated as n2 = 1.07 109 cm2/W. The result shows that the PILMO (poly ionic liquid methyl orange) film has a much larger value of nonlinear refractive index than many other LC materials, such as DDLC [13]. The large nonlinearity of this material indicates it has potential applications in nonlinear optical devices. It is necessary to discriminate the origin of nonlinearity rises from different effects, such as resonant electronic nonlinear effect, laser heating effect and photoinduced trans to cis transformation. Under picosecond pulse excitation, neither laser heating effect nor resonant electronic nonlinear effect are very remarkable usually and could cause such a big value of n2. It is most likely that the nonlinearity mainly comes from photoinduced trans to cis transformation. The further research is under process now. Using a CW linearly 488 nm laser as the excitation light, which is near to the peak of absorption band of polymer, the real-time behavior of photoinduced dichroism was investigated. Typical absorbance variations of polymer during the writing and the relaxation periods are presented in Fig. 7a. When the pump is on, at least two regimes are involved in the writing process [14]. The fast one is due to the pump efficiency and depends on the intensity of the pump laser, the angle between the laser polarization direction and the long molecular axis of the azobenzene groups by an ‘‘I0cos2u” factor. This selective optical pumping from the trans to cis isomers produces an ‘‘angular hole burning” (AHB) in the angular distribution of the trans isomers. From Fig. 7a, in the first few seconds, the fast AHB process takes the main part and induces the significant increase in both A\ and A45 components. Additionally, upon irradiation and during the multiple trans–cis-trans isomerization cycles, a slow ‘‘angular redistribution” (AR) of the molecules according to their ability to rotate by thermal random interactions with the host medium and to diffuse by thermal back-relaxation will occur. Consequently, during the writing period, for azobenzene ionic liquid–crystalline polymer, the AHB and AR processes occur simultaneously and producing an increase in the concentration of molecules oriented perpendicularly to the laser polarization direction. The increase of the A\ is bigger than A||. It indicates that the AR process is more intense in A\. When the pump is off, in the first few seconds, the thermal cis–trans backrelaxation takes the main factor and is the origin of the decrease of both A\ and A|| components. And then the AR process principally governs the perpendicular and parallel absorbance. But still the initial isotropic disorder of the azobenzene molecules is never recovered even after a long time delay. Through the calculation of the normalized average absorbance T 0 ¼ ðAjj þ 2A? Þ=3A0 and the normalized linear dichroism
Fig. 7. Time dependence of (a) the normalized absorbance (h is the angle between polarization directions of probe and pump), (b) the normalized average absorbance T0 and the normalized linear dichroism T2 of the PILMO film.
T 0 ¼ ðAjj 2A? Þ=3A0 , defined as in Reference [14], the characterization of the photoinduced dichroism can be performed. The results for the polymer are shown in Fig. 7b. When the writing laser is turned on, T0 increases during the first few seconds and reaches a plateau value (0.85) which is characteristic of a photostationary state. In contrast with T0, T2 decreases and become a negative value. As mentioned above, the processes are mainly due to AHB process firstly and the AR process later. Fig. 8 shows the experimental results of photoinduced birefringence at various pump intensities. When the pump is on, a fast process and a slow process in the buildup of birefringence can be seen under irradiation. For azobenzene LC polymer, the fast process is related to the local movement of azobenzene groups in the trans–cis–trans photo-isomerization, and the slow process is attributed to the movement of azobenzene groups linked to the LC polymeric chain [15,16]. As the pump intensity increases, the saturation value of birefringence reaches a maximum. From the figure, when the pump intensity was 100 mW/cm2, the maximum of the saturation value was Dn = 5.3 102. The large value of photoinduced birefringence (in the order of 102) indicates that the material has potential value in the application of nonlinear photonic devices and optical storage. Below the intensity 100 mW/ cm2, it is clear that the transmittance increased with increase of intensity of irradiation light. Irradiation light at higher intensity produced a higher concentration of cis-azobenzene at first, and then the concentration decreased, which means a trans–cis–trans isomerization process [15]. Hence, irradiation light at higher inten-
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azobenzene groups. Using the Z-scan technique, under a picoseconds pulse 532 nm excitation, the nonlinear refractive index of the polymer film can be obtained as n2 = 1.07 109 cm2/W. Using an argon-ion 488 nm laser as pump light, we also investigated the photoinduced anisotropy through dichroism and birefringence in the polymer. A birefringence value Dn 102 was achieved in the polymer film. The mechanism responsible for the nonlinear optical properties and the process of photoinduced anisotropy in the polymer was discussed. The large value of nonlinear refractive index, remarkable and stable photoinduced molecular reorientation of the polymer film indicates that the material has potential value in the application of nonlinear photonic devices, holographic recording and polarization reconstruction. Acknowledgements
Fig. 8. Photoinduced birefringence of the PILMO film at various pump intensities: (a) 50 mW/cm2, (b) 80 mW/cm2, (c) 200 mW/cm2, and (d) 400 mW/cm2.
sity gave rise to a higher trans–cis–trans isomerization rate, therefore a higher achievement of the saturation order and a bigger value of photoinduced birefringence. Over the intensity 100 mW/ cm2, the saturation value decreased. This phenomenon can be discussed in terms of two competing processes: the photoinduced reorientation and thermal random motion. When the light intensity increased from 50 to 200 mW/cm2, the number of molecules that may take isomerization increased. When the light intensity reached 200 mW/cm2, the laser heating effect became obvious, and it disorientated the aligned azobenzene molecules [11]. When the pump is off, the birefringence does not show significant decay and even exhibits a slight increase. In fact, we found that the characteristic of long-term and reversible optical storage of the polymer can last for months. The phenomenon can be explained with the self-organization effect of the molecules [17]. The molecules keep well-ordered (or continue the reorientation) spontaneously after removing the pump irradiation, instead of relaxing back to the isotropic state. 4. Conclusion We have investigated nonlinear optical properties and photoinduced anisotropy in an ionic liquid–crystalline polymer containing
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