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Synthesis and characterization of photorefractive materials based on polymers containing photoconductors and nonlinear chromophores
Materials Letters 57 (2003) 4372 – 4377 www.elsevier.com/locate/matlet
Synthesis and characterization of photorefractive materials based on polymers containing photoconductors and nonlinear chromophores Yiwang Chen a,b,*, Qihuang Gong b, Feng Wang b, Bo Zhang b, Zhijian Chen b a
Department of Chemistry, Institute of Macromolecular Chemistry, Philipps-University of Marburg, Hans-Meerwein Str., 35039 Marburg, Germany b Department of Physics, Peking University, 100871 Beijing, PR China Received 14 March 2003; accepted 1 April 2003
Abstract We report the synthesis and characterization of a new, low-Tg polymer exhibiting the photorefractive (PR) effect. The photorefractive polymer contains azo derivative as a second-order nonlinear optical chromophore and carbazole as a chargetransporting group. The photorefractive sample was fabricated by doping 2,4,7-trinitrofluorenone (TNF) of 2 wt.% as photosensitizer, which could be poled by applied electric field at room temperature to achieve large orientational birefringence without doping plasticizer. Photorefractive experiments showed a high two-beam coupling gain coefficient of 122 cm 1 at applied electric field of 85 V/Am. In comparison to host – guest polymer composite systems, long lifetime and high stability of the photorefractive sample was proved. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymers; Thin films; Photorefractive materials; Hole transport agents; Electro-optic chromophores
1. Introduction Photorefractive (PR) polymer is a new class of photorefractive materials which has been investigated widely in the past few years [1 – 5]. Since 1990, PR studies have been extended into organic polymer materials [6 – 12]: host – guest polymer composites [6– 9] and fully functional polymers [10 –12]. Host– * Corresponding author. Department of Chemistry, Institute of Macromolecular Chemistry, Philipps-University of Marburg, HansMeerwein Str., 35039 Marburg, Germany. Tel.: +49-6421-2825782; fax: +49-6421-2825785. E-mail address: [email protected] (Y. Chen).
guest polymer composites exhibit an obvious disadvantage: low stability due to phase separation; while fully functional polymers are difficult to be synthesized due to complex synthesis routes and very low content of photosensitizer being difficult to be linked to the fully functional polymer. The driving force to pursue research on semi-fully functional photorefractive polymers, which possess both photoconductivity and electro-optic (EO) effect, comes from their high stability and easy synthesis. Based on the above knowledge, in a previous paper, a novel semi-fully functional photorefractive polymer (PHTEO2) containing carbazolyl as hole transport (HT) agents and azo derivative as EO
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00327-6
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
Fig. 1. The chemical structure of copolymers containing hole transport agents and electro-optic moieties (PHTEOn, n = 2, 6).
chromophores with flexible spacer n = 2 was designed and synthesized [13], whose structure was shown in Fig. 1. Designing this kind of PR system is based on the facts that alkoxy azo dyes/poly(N-vinyl carbazole)/2,4,7-trinitrofluorenone (TNF) composite systems are well-known photorefractive systems and exhibit high EO response and large asymmetric optical energy exchange, as verified by Peyghambarian’s
4373
group [5] and our research group [14 – 16]. Due to short flexible spacer between chromophores and backbone, the copolymer was measured to be high glass transition temperature of above 200 jC. To be able to be poled at room temperature to achieve orientational birefringence, the photorefractive sample was obtained by doping 9-ethyl carbazole as a plasticizer. In this article, we report a semi-fully functional polymer PHTEO6, as shown in Fig. 1, containing carbazolyl groups as HT agents and azo derivatives as EO chromophores with long flexible spacer n = 6. PR sample was prepared without using plasticizer and exhibited high PR properties.
2. Experimental 2.1. Monomer synthesis All reagents are commercially available and used as received except that 2,4,7-trinitrofluorenone (TNF) was purified by recrystallization before using.
Scheme 1. Synthetic routes to the monomers.
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Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
All synthesized products were detailedly characterized. 6-(9-Carbazoyl)hexyl acrylate and 6-[4-nitrophenylazo-2,5-dimethylphenyloxy]hexyl acrylate were synthesized according to previous procedures [13]. In synthesis of medium product 6-[4-nitrophenylazo2,5-dimethylphenyloxy]hexanol, 6-bromo-1-hexanol was used with replacing 2-bromoethanol. The synthetic routes to the monomers are shown in Scheme 1. 2.2. Polymer synthesis The polymerizations were carried out in purified toluene solution under argon atmosphere at 60 jC in the presence of 1 wt.% AIBN for 48 h. The resulting polymer solution was cooled and poured into methanol to precipitate the polymer. The precipitated polymer was filtered, redissolved and reprecipitated, filtered, and finally dried under reduced pressure. 2.3. Photorefractive sample preparation According to the method reported in our previous paper [16], samples for photorefractive measurements were prepared by dissolving the polymer in chloroform and by adding TNF up to 2 wt.% concentration. After passage of this solution through a 0.2-Am PTFE membrane filter, the solvent was allowed to evaporate for 3 h at 80 jC. The resulting homogeneous mixture was then molten between two ITO-coated glass slides, and the thickness of the samples was controlled to be around 100 Am with the help of Teflon spacers. 2.4. Photorefractive characterization The photorefractive properties of the polymer were studied by the two-beam coupling (TBC) and the four-wave mixing (FWM) techniques using a similar experimental setup described in Fig. 2. Because of the polymer’s low Tg, no poling was performed before photorefractive measurements which were carried out at room temperature. Holographic gratings are written by two laser beams from a He –Ne laser operating at 633 nm. Two coherent p-polarized laser beams with an equal intensity of 2 mW/cm2 overlap on the sample to create an intensity grating with a spacing K of around 3 Am (depending on the material’s refractive index). The sample is tilted at an angle /ext = 60j (in air) relative to the bisector of the
Fig. 2. The typical tilted geometry for the two-beam coupling experiment.
incident beams, and the angle between incident beams 2hext = 20j (in air). The energy transfer between the p-polarized writing beams is observed by monitoring the intensity of each of the writing beams when an external electric field is applied. The same energy transfer is also observed by the increase (or decrease) of the beam intensity while the other writing beam is open and blocked.
3. Results and discussion 3.1. Synthesis and molecular structure of the polymer The composition of the polymer was determined with 1H NMR. The result of the polymer consisting of azo derivative and carbazole with molar ratio 0.6– 0.4 was consistent with the monomer feed while the feed molar ratio of monomer 6-[4-nitrophenylazo-2,5dimethylphenyloxy]hexyl acrylate and 6-(9-carbazoyl)hexyl acrylate was 0.65– 0.35. The NMR spectrum clearly indicates that all the monomer species are indeed incorporated into the polymer chain. Good optical quality film can be cast from the polymer solution. The UV/Vis spectrum of the polymer showed the absorption of the EO chromophore at 390 nm and that of transporting compound at 342 nm in chloroform which are similar to those of corresponding monomers. The FT-IR spectrum consisted of typical peaks of the two functional groups, and the relative intensities of these peaks were also identical with the results of UV/Vis data, which are related to quantities of the two functional groups. High yields with 94% and reasonable molecular weights with M n ¼ 8000 and M n ¼ 11 000 were obtained. This corresponds to the analysis of GPC
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
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beams is small enough, the two-beam coupling gain coefficient, C, can be calculated by the following approximate formula: cos/in br C¼ ln ; bþ1r d
Fig. 3. A typical energy transfer behavior in a two-beam coupling experiment: a dc external electric field of 76 V/Am was applied in advance while beam 1 was open all the time, beam 2 was turned on at t = 2.8 s.
curves that shows a monomodal distribution, and there is no evidence for the presence of homo-polymer fractions anymore. The Tg of the polymer is about 52 jC, and it is thermally stable up to 300 jC. In preparation of the photorefractive sample, external sensitizer (TNF) was added into the polymer with 2 wt.% concentration to create the charges necessary for the photorefractive effect, by selective excitation of the carbazole –TNF charge-transfer complex at 633 nm. The sensitizer was not incorporated in the polymer structure because crystallization of this unit is not likely due to its small concentration.
ð1Þ
where d is the thickness of the sample, /in the tilting angle inside the sample, b = I2/I1, c = I12/I21, I1 (I2) and I12 (I21) are the intensity of beam 1 (beam 2) after the sample in the absence and in the presence of beam 2 (beam 1), respectively. The two-beam coupling gain was measured as a function of the electric field, and the dependence is shown in Fig. 4. At the applied electric field of 85 V/Am, the two-beam coupling gain coefficient was measured to be 122 cm 1. According to the standard band-transport model [18], the applied electric field dependence of the TBC coefficient should be described as: E0 E0G C ¼ K qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sinðtan1 ðE0G =Eq ÞÞ; 2 G 1 þ ðE0 =Eq Þ
ð2Þ
where E0 is the applied electric field, E0G the component of E0 along the direction of the grating wavevector, and Eq the trap limited saturation space-charge field. We used Eq. (2) to fit two-beam coupling process, and the theoretical fitting curve is also presented in Fig. 4.
3.2. Photorefractive characterization The photorefractive performances of the material were investigated by two-beam coupling as well as four-wave mixing. The latter technique provides information about the amplitudes of the photorefractive space-charge field and of the resulting refractive index modulation [17]. Real proof of the photorefractive effect, however, can only be obtained by asymmetric two-beam coupling, which outrules other possible grating forming mechanisms as photochromic or thermal gratings. A typical energy transfer in two-beam coupling experiments at a bias field of 76 V/Am applied on the polymer film is evidenced by the increase in the intensity of beam 1 and decrease in that of beam 2 (Fig. 3). When the angle between two
Fig. 4. The two-beam coupling coefficient of the photorefractive sample as a function of applied electric field (circle: measured data; line: theoretical fitting).
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Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
The presence of the photorefractive effect can be demonstrated by writing gratings in the material through a holographic optical technique, four-wave mixing. Diffraction efficiency, g, is defined as the intensity ratio of the diffracted beam to the incoming read beam. Limited by the reflection and absorption losses, a maximum of about 34% was obtained at the highest applied electric field of 85 V/Am. Photorefractivity is a combined effect of the electro-optic response and photoconductivity simultaneously present in the material. The holographic grating formation dynamics is directly related to the photoconductivity (or charge mobility) of the material. Through a correct design of optimized chromophore, the polymer presented in this paper possesses chargetransporting and electro-optic properties arising from the carbazole group and the second-order nonlinear chromophore, respectively, both covalently attached to the polymer backbone. When doped with a photocharge generation sensitizer, high photorefractivity has been observed. Problems associated with phase separation are clearly avoided because of the covalent bonding and of the low concentration of the photocharge generation sensitizer. Compared with guest – host system in a previous paper [5], relatively low coupling coefficient and electro-optic response were obtained. This may be a result of the low charge mobilities in polymers where the charge-transporting agent and second-order chromophore are covalently bound to the polymer backbone, as has been observed in other fully functional photorefractive polymers [10 – 12]. The molecular motions of the charge-transporting agents in fully functional polymers are restricted to very short-range vibrations and rotations at temperatures below the glass transition temperature. Interrupting conduction pathways of charge transport occur in fully functionalized polymer systems, and result in low charge mobilities. In systems in which the charge-transport molecules are present as guests, aggregation of these molecules undoubtedly occurs, which ensures contact between the transport molecules as long as the concentration is above a percolation threshold. However, compared with a bifunctional polymer with a short spacer between functional groups and polymer backbone reported previously [13], a relatively high photorefractivity was measured in this system. This attributes to more free rotation of second-order chromophore
due to flexible spacer, and high concentration of chromophore in this system due to without adding plasticizer.
4. In conclusion A new low-Tg photorefractive polymer has been successfully synthesized and characterized which contains a hole-transporting group and a nonlinear optical chromophore. The experimental results demonstrated that the major contribution to the photorefractive effect comes from the space-charge field due to the lightinduced charge separation with a subsequent effective electric-field-induced refractive index change. The photorefractivity was successfully elevated by reasonable design of nonlinear chromophore and increasing free rotation of molecules. No evidence manifests that phase separation and degradation of the sample occur when the sample was stored in a low-humidity environment for more than 1 year. The disadvantages of host – guest systems such as phase separation and the disadvantages of fully functionalized polymers such as complicated synthesis are overcome by using bifunctional polymers as matrix.
References [1] S. Ducharine, J.C. Scott, R.J. Twieg, W.E. Moerner, Phys. Rev. Lett. 66 (1991) 1846. [2] M.C.J.M. Donckers, S. Silence, A. Walshc, F. Hache, D.M. Bruland, W.E. Moerner, R.J. Twieg, Opt. Lett. 8 (1993) 1044. [3] Y. Zhang, Y. Cui, P.N. Prasad, Phys. Rev., B 46 (1992) 9900. [4] M. Liphardt, A. Goonesekera, B.E. Joves, S. Ducharme, J.M. Tadacs, L. Zhang, Science 371 (1994) 497. [5] K. Meerholz, B.L. Volodin, B. Kippelen, N. Peyghambarian, Nature 371 (1994) 497. [6] K. Sutter, J. Hullinger, P. Gunter, Solid State Commun. 74 (1986) 867. [7] C.A. Walsh, W.E. Moerner, J. Opt. Soc. Am., B 9 (1992) 1642. [8] S.M. Silence, C.A. Walsh, J.C. Scott, W.E. Moerner, Appl. Phys. Lett. 61 (1992) 2967. [9] Y.P. Cui, Y. Zhang, P.N. Prasad, Appl. Phys. Lett. 61 (1992) 2132. [10] L.P. Yu, W.K. Chan, Z.N. Bao, S. Cao, J. Chem. Soc. Chem. Commun., (1992) 1735. [11] L.P. Yu, W.K. Chan, Z.N. Bao, S. Cao, Macromolecules 26 (1993) 2216. [12] Y.M. Chen, Z.H. Peng, W.K. Chan, L.P. Yu, Appl. Phys. Lett. 64 (1994) 1195.
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377 [13] Y.W. Chen, Y.K. He, F. Wang, H.Y. Chen, Q.H. Gong, Polymer 42 (2001) 1101. [14] F. Wang, Z.J. Chen, Q.H. Gong, Y.W. Chen, H.Y. Chen, Chin. Phys. Lett. 15 (1998) 351. [15] F. Wang, Z.J. Chen, Q.H. Gong, Y.W. Chen, H.Y. Chen, Solid State Commun. 106 (1998) 299.
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[16] Z.J. Chen, F. Wang, Q.H. Gong, Y.W. Chen, H.Y. Chen, J. Phys. D 31 (1998) 2245. [17] H. Kogelnik, Bell Syst. Tech. J. 48 (1969) 2909. [18] P. Gu¨nter, J.-P. Huignard, Photorefractive Materials and Their Applications, vols. 1 and II, Springer-Verlag, Berlin, 1988 – 1989.
Synthesis and characterization of photorefractive materials based on polymers containing photoconductors and nonlinear chromophores Yiwang Chen a,b,*, Qihuang Gong b, Feng Wang b, Bo Zhang b, Zhijian Chen b a
Department of Chemistry, Institute of Macromolecular Chemistry, Philipps-University of Marburg, Hans-Meerwein Str., 35039 Marburg, Germany b Department of Physics, Peking University, 100871 Beijing, PR China Received 14 March 2003; accepted 1 April 2003
Abstract We report the synthesis and characterization of a new, low-Tg polymer exhibiting the photorefractive (PR) effect. The photorefractive polymer contains azo derivative as a second-order nonlinear optical chromophore and carbazole as a chargetransporting group. The photorefractive sample was fabricated by doping 2,4,7-trinitrofluorenone (TNF) of 2 wt.% as photosensitizer, which could be poled by applied electric field at room temperature to achieve large orientational birefringence without doping plasticizer. Photorefractive experiments showed a high two-beam coupling gain coefficient of 122 cm 1 at applied electric field of 85 V/Am. In comparison to host – guest polymer composite systems, long lifetime and high stability of the photorefractive sample was proved. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymers; Thin films; Photorefractive materials; Hole transport agents; Electro-optic chromophores
1. Introduction Photorefractive (PR) polymer is a new class of photorefractive materials which has been investigated widely in the past few years [1 – 5]. Since 1990, PR studies have been extended into organic polymer materials [6 – 12]: host – guest polymer composites [6– 9] and fully functional polymers [10 –12]. Host– * Corresponding author. Department of Chemistry, Institute of Macromolecular Chemistry, Philipps-University of Marburg, HansMeerwein Str., 35039 Marburg, Germany. Tel.: +49-6421-2825782; fax: +49-6421-2825785. E-mail address: [email protected] (Y. Chen).
guest polymer composites exhibit an obvious disadvantage: low stability due to phase separation; while fully functional polymers are difficult to be synthesized due to complex synthesis routes and very low content of photosensitizer being difficult to be linked to the fully functional polymer. The driving force to pursue research on semi-fully functional photorefractive polymers, which possess both photoconductivity and electro-optic (EO) effect, comes from their high stability and easy synthesis. Based on the above knowledge, in a previous paper, a novel semi-fully functional photorefractive polymer (PHTEO2) containing carbazolyl as hole transport (HT) agents and azo derivative as EO
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00327-6
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
Fig. 1. The chemical structure of copolymers containing hole transport agents and electro-optic moieties (PHTEOn, n = 2, 6).
chromophores with flexible spacer n = 2 was designed and synthesized [13], whose structure was shown in Fig. 1. Designing this kind of PR system is based on the facts that alkoxy azo dyes/poly(N-vinyl carbazole)/2,4,7-trinitrofluorenone (TNF) composite systems are well-known photorefractive systems and exhibit high EO response and large asymmetric optical energy exchange, as verified by Peyghambarian’s
4373
group [5] and our research group [14 – 16]. Due to short flexible spacer between chromophores and backbone, the copolymer was measured to be high glass transition temperature of above 200 jC. To be able to be poled at room temperature to achieve orientational birefringence, the photorefractive sample was obtained by doping 9-ethyl carbazole as a plasticizer. In this article, we report a semi-fully functional polymer PHTEO6, as shown in Fig. 1, containing carbazolyl groups as HT agents and azo derivatives as EO chromophores with long flexible spacer n = 6. PR sample was prepared without using plasticizer and exhibited high PR properties.
2. Experimental 2.1. Monomer synthesis All reagents are commercially available and used as received except that 2,4,7-trinitrofluorenone (TNF) was purified by recrystallization before using.
Scheme 1. Synthetic routes to the monomers.
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Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
All synthesized products were detailedly characterized. 6-(9-Carbazoyl)hexyl acrylate and 6-[4-nitrophenylazo-2,5-dimethylphenyloxy]hexyl acrylate were synthesized according to previous procedures [13]. In synthesis of medium product 6-[4-nitrophenylazo2,5-dimethylphenyloxy]hexanol, 6-bromo-1-hexanol was used with replacing 2-bromoethanol. The synthetic routes to the monomers are shown in Scheme 1. 2.2. Polymer synthesis The polymerizations were carried out in purified toluene solution under argon atmosphere at 60 jC in the presence of 1 wt.% AIBN for 48 h. The resulting polymer solution was cooled and poured into methanol to precipitate the polymer. The precipitated polymer was filtered, redissolved and reprecipitated, filtered, and finally dried under reduced pressure. 2.3. Photorefractive sample preparation According to the method reported in our previous paper [16], samples for photorefractive measurements were prepared by dissolving the polymer in chloroform and by adding TNF up to 2 wt.% concentration. After passage of this solution through a 0.2-Am PTFE membrane filter, the solvent was allowed to evaporate for 3 h at 80 jC. The resulting homogeneous mixture was then molten between two ITO-coated glass slides, and the thickness of the samples was controlled to be around 100 Am with the help of Teflon spacers. 2.4. Photorefractive characterization The photorefractive properties of the polymer were studied by the two-beam coupling (TBC) and the four-wave mixing (FWM) techniques using a similar experimental setup described in Fig. 2. Because of the polymer’s low Tg, no poling was performed before photorefractive measurements which were carried out at room temperature. Holographic gratings are written by two laser beams from a He –Ne laser operating at 633 nm. Two coherent p-polarized laser beams with an equal intensity of 2 mW/cm2 overlap on the sample to create an intensity grating with a spacing K of around 3 Am (depending on the material’s refractive index). The sample is tilted at an angle /ext = 60j (in air) relative to the bisector of the
Fig. 2. The typical tilted geometry for the two-beam coupling experiment.
incident beams, and the angle between incident beams 2hext = 20j (in air). The energy transfer between the p-polarized writing beams is observed by monitoring the intensity of each of the writing beams when an external electric field is applied. The same energy transfer is also observed by the increase (or decrease) of the beam intensity while the other writing beam is open and blocked.
3. Results and discussion 3.1. Synthesis and molecular structure of the polymer The composition of the polymer was determined with 1H NMR. The result of the polymer consisting of azo derivative and carbazole with molar ratio 0.6– 0.4 was consistent with the monomer feed while the feed molar ratio of monomer 6-[4-nitrophenylazo-2,5dimethylphenyloxy]hexyl acrylate and 6-(9-carbazoyl)hexyl acrylate was 0.65– 0.35. The NMR spectrum clearly indicates that all the monomer species are indeed incorporated into the polymer chain. Good optical quality film can be cast from the polymer solution. The UV/Vis spectrum of the polymer showed the absorption of the EO chromophore at 390 nm and that of transporting compound at 342 nm in chloroform which are similar to those of corresponding monomers. The FT-IR spectrum consisted of typical peaks of the two functional groups, and the relative intensities of these peaks were also identical with the results of UV/Vis data, which are related to quantities of the two functional groups. High yields with 94% and reasonable molecular weights with M n ¼ 8000 and M n ¼ 11 000 were obtained. This corresponds to the analysis of GPC
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
4375
beams is small enough, the two-beam coupling gain coefficient, C, can be calculated by the following approximate formula: cos/in br C¼ ln ; bþ1r d
Fig. 3. A typical energy transfer behavior in a two-beam coupling experiment: a dc external electric field of 76 V/Am was applied in advance while beam 1 was open all the time, beam 2 was turned on at t = 2.8 s.
curves that shows a monomodal distribution, and there is no evidence for the presence of homo-polymer fractions anymore. The Tg of the polymer is about 52 jC, and it is thermally stable up to 300 jC. In preparation of the photorefractive sample, external sensitizer (TNF) was added into the polymer with 2 wt.% concentration to create the charges necessary for the photorefractive effect, by selective excitation of the carbazole –TNF charge-transfer complex at 633 nm. The sensitizer was not incorporated in the polymer structure because crystallization of this unit is not likely due to its small concentration.
ð1Þ
where d is the thickness of the sample, /in the tilting angle inside the sample, b = I2/I1, c = I12/I21, I1 (I2) and I12 (I21) are the intensity of beam 1 (beam 2) after the sample in the absence and in the presence of beam 2 (beam 1), respectively. The two-beam coupling gain was measured as a function of the electric field, and the dependence is shown in Fig. 4. At the applied electric field of 85 V/Am, the two-beam coupling gain coefficient was measured to be 122 cm 1. According to the standard band-transport model [18], the applied electric field dependence of the TBC coefficient should be described as: E0 E0G C ¼ K qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sinðtan1 ðE0G =Eq ÞÞ; 2 G 1 þ ðE0 =Eq Þ
ð2Þ
where E0 is the applied electric field, E0G the component of E0 along the direction of the grating wavevector, and Eq the trap limited saturation space-charge field. We used Eq. (2) to fit two-beam coupling process, and the theoretical fitting curve is also presented in Fig. 4.
3.2. Photorefractive characterization The photorefractive performances of the material were investigated by two-beam coupling as well as four-wave mixing. The latter technique provides information about the amplitudes of the photorefractive space-charge field and of the resulting refractive index modulation [17]. Real proof of the photorefractive effect, however, can only be obtained by asymmetric two-beam coupling, which outrules other possible grating forming mechanisms as photochromic or thermal gratings. A typical energy transfer in two-beam coupling experiments at a bias field of 76 V/Am applied on the polymer film is evidenced by the increase in the intensity of beam 1 and decrease in that of beam 2 (Fig. 3). When the angle between two
Fig. 4. The two-beam coupling coefficient of the photorefractive sample as a function of applied electric field (circle: measured data; line: theoretical fitting).
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Y. Chen et al. / Materials Letters 57 (2003) 4372–4377
The presence of the photorefractive effect can be demonstrated by writing gratings in the material through a holographic optical technique, four-wave mixing. Diffraction efficiency, g, is defined as the intensity ratio of the diffracted beam to the incoming read beam. Limited by the reflection and absorption losses, a maximum of about 34% was obtained at the highest applied electric field of 85 V/Am. Photorefractivity is a combined effect of the electro-optic response and photoconductivity simultaneously present in the material. The holographic grating formation dynamics is directly related to the photoconductivity (or charge mobility) of the material. Through a correct design of optimized chromophore, the polymer presented in this paper possesses chargetransporting and electro-optic properties arising from the carbazole group and the second-order nonlinear chromophore, respectively, both covalently attached to the polymer backbone. When doped with a photocharge generation sensitizer, high photorefractivity has been observed. Problems associated with phase separation are clearly avoided because of the covalent bonding and of the low concentration of the photocharge generation sensitizer. Compared with guest – host system in a previous paper [5], relatively low coupling coefficient and electro-optic response were obtained. This may be a result of the low charge mobilities in polymers where the charge-transporting agent and second-order chromophore are covalently bound to the polymer backbone, as has been observed in other fully functional photorefractive polymers [10 – 12]. The molecular motions of the charge-transporting agents in fully functional polymers are restricted to very short-range vibrations and rotations at temperatures below the glass transition temperature. Interrupting conduction pathways of charge transport occur in fully functionalized polymer systems, and result in low charge mobilities. In systems in which the charge-transport molecules are present as guests, aggregation of these molecules undoubtedly occurs, which ensures contact between the transport molecules as long as the concentration is above a percolation threshold. However, compared with a bifunctional polymer with a short spacer between functional groups and polymer backbone reported previously [13], a relatively high photorefractivity was measured in this system. This attributes to more free rotation of second-order chromophore
due to flexible spacer, and high concentration of chromophore in this system due to without adding plasticizer.
4. In conclusion A new low-Tg photorefractive polymer has been successfully synthesized and characterized which contains a hole-transporting group and a nonlinear optical chromophore. The experimental results demonstrated that the major contribution to the photorefractive effect comes from the space-charge field due to the lightinduced charge separation with a subsequent effective electric-field-induced refractive index change. The photorefractivity was successfully elevated by reasonable design of nonlinear chromophore and increasing free rotation of molecules. No evidence manifests that phase separation and degradation of the sample occur when the sample was stored in a low-humidity environment for more than 1 year. The disadvantages of host – guest systems such as phase separation and the disadvantages of fully functionalized polymers such as complicated synthesis are overcome by using bifunctional polymers as matrix.
References [1] S. Ducharine, J.C. Scott, R.J. Twieg, W.E. Moerner, Phys. Rev. Lett. 66 (1991) 1846. [2] M.C.J.M. Donckers, S. Silence, A. Walshc, F. Hache, D.M. Bruland, W.E. Moerner, R.J. Twieg, Opt. Lett. 8 (1993) 1044. [3] Y. Zhang, Y. Cui, P.N. Prasad, Phys. Rev., B 46 (1992) 9900. [4] M. Liphardt, A. Goonesekera, B.E. Joves, S. Ducharme, J.M. Tadacs, L. Zhang, Science 371 (1994) 497. [5] K. Meerholz, B.L. Volodin, B. Kippelen, N. Peyghambarian, Nature 371 (1994) 497. [6] K. Sutter, J. Hullinger, P. Gunter, Solid State Commun. 74 (1986) 867. [7] C.A. Walsh, W.E. Moerner, J. Opt. Soc. Am., B 9 (1992) 1642. [8] S.M. Silence, C.A. Walsh, J.C. Scott, W.E. Moerner, Appl. Phys. Lett. 61 (1992) 2967. [9] Y.P. Cui, Y. Zhang, P.N. Prasad, Appl. Phys. Lett. 61 (1992) 2132. [10] L.P. Yu, W.K. Chan, Z.N. Bao, S. Cao, J. Chem. Soc. Chem. Commun., (1992) 1735. [11] L.P. Yu, W.K. Chan, Z.N. Bao, S. Cao, Macromolecules 26 (1993) 2216. [12] Y.M. Chen, Z.H. Peng, W.K. Chan, L.P. Yu, Appl. Phys. Lett. 64 (1994) 1195.
Y. Chen et al. / Materials Letters 57 (2003) 4372–4377 [13] Y.W. Chen, Y.K. He, F. Wang, H.Y. Chen, Q.H. Gong, Polymer 42 (2001) 1101. [14] F. Wang, Z.J. Chen, Q.H. Gong, Y.W. Chen, H.Y. Chen, Chin. Phys. Lett. 15 (1998) 351. [15] F. Wang, Z.J. Chen, Q.H. Gong, Y.W. Chen, H.Y. Chen, Solid State Commun. 106 (1998) 299.
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[16] Z.J. Chen, F. Wang, Q.H. Gong, Y.W. Chen, H.Y. Chen, J. Phys. D 31 (1998) 2245. [17] H. Kogelnik, Bell Syst. Tech. J. 48 (1969) 2909. [18] P. Gu¨nter, J.-P. Huignard, Photorefractive Materials and Their Applications, vols. 1 and II, Springer-Verlag, Berlin, 1988 – 1989.