APPLIED PHYSICS LETTERS 87, 261111 共2005兲
Photorefractive properties of an unsensitized polymer composite based on a dicyanostyrene derivative as nonlinear optical chromophore José A. Quintana and Pedro G. Boj Instituto Universitario de Materials de Alicante (IUMA), Departamento Interuniversitario de Óptica, Universidad de Alicante, 03080-Alicante, Spain
José M. Villalvilla Instituto Universitario de Materiales de Alicante (IUMA), Departamento de Física Aplicada, Universidad de Alicante, 03080-Alicante, Spain
Javier Ortíz, Fernando Fernández-Lázaro, and Ángela Sastre-Santos División de Química Orgánica, Instituto de Bioingeniería, Universidad Miguel Hernández, Elche 03202 (Alicante), Spain
María A. Díaz-Garcíaa兲 Instituto Universitario de Materiales de Alicante (IUMA), Departamento de Física Aplicada and Unidad Asociada CSIC-UA, Universidad de Alicante, 03080-Alicante, Spain
共Received 1 July 2005; accepted 1 November 2005; published online 23 December 2005兲 We report on the photorefractive 共PR兲 properties at the 633 nm laser wavelength of a polymer composite based on the polymer poly共n-vinyl carbazole兲 共PVK兲, doped with the dicyanostyrene derivative 4-piperidinobenzylidene-malonitrile 共PDCST兲 as nonlinear optical chromophore and the liquid plasticizer butyl benzyl phthalate 共BBP兲, without the presence of sensitizer. The PR-effect is observed only when samples are previously subjected to an electric field 共i.e., 20 V / m for 10 min兲. Photoconductivity and birefringence of the composite become significant when the electric field treatment is performed at temperatures higher than room temperature 共24 ° C兲. Gain coefficient and PR speed, determined from two-beam coupling experiments, are compared to those obtained with the PVK/ PDSCT/ BBP/ C60 standard sensitized composite. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2158032兴 During the last decades much effort has been devoted to the investigation of photorefractive 共PR兲 materials, mainly motivated by the wide range of potential applications for which they can be used 共e.g., optical processing, phase conjugation, optical storage, and many others兲.1–3 One of the most investigated PR organic materials are the polymer composites that are based on a photoconducting polymer doped with a nonlinear optical 共NLO兲 chromophore to provide the properties needed for photorefractivity, i.e., photoinduced charge generation, transport, trapping, and electro-optical activity.3 In order to increase the charge generation efficiency, a sensitizer is generally added. In addition, after the discovery of the orientational enhancement effects in low glass transition temperature 共Tg兲 systems,4 a plasticizer is often included to obtain composites with a Tg close to room temperature. One of the best-performing polymer composites, that has been extensively investigated, is the one based on the hole transporting polymer poly共n-vinyl carbazole兲 共PVK兲, doped with the dicyanoester derivative 4-piperidinobenzylidenemalonitrile 共PDCST兲 as NLO chromophore, the liquid plasticizer butyl benzyl phathalate 共BBP兲 and the sensitizer C60.5–7 In this letter, we report on the observation of PR properties, via two-beam coupling 共2BC兲 experiments, in polymer composites based on PVK, PDCST, and BBP without the presence of any sensitizer. This behavior is observed only when, prior to the PR characterization, samples are suba兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2005/87共26兲/261111/3/$22.50
jected to an electric field for several minutes. We show that this treatment under an electric field leads to a significant enhancement of the photoconductivity and the birefringence of the samples, that are responsible for the observation of PR effect without the presence of any sensitizer. Our first observations of the enhancement of photoconductivity and birefringence were made using the standard composite with sensitizer. Samples with C60 were prepared as previously described.5–7 Solutions containing PVK 共49.5%兲/PDCST 共35%兲/BBP 共15% 兲 / C60 共0.5%兲 were prepared in chlorobenzene, and then were cast at 45 ° C onto indium tin oxide coated glass plates and dried overnight in an oven at a temperature of 95 ° C. Finally, the plates were assembled at 120 ° C, yielding films of thickness around 100 m. Teflon spacers were used to ensure a uniform thickness. The absorption coefficient 共␣兲 was 29 cm−1 at the operating wavelength of 633 nm, and the Tg measured by differential scanning calorimetry was about 27 ° C.5 Unsensitized composites based on PVK 共50%兲/PDCST 共35%兲/BBP 共15%兲 were prepared in a similar way. In this case, a lower Tg value was obtained 共⬃23 ° C兲. The electric field treatment, prior to the PR characterization, consisted on the application of a dc electric field for several minutes. After some trials at various fields and temperatures, performed in a temperature-stabilized setup, a field strength of 20 V / m for 10 min at a temperature of 32 ° C was chosen. In what follows, we will refer to the samples that have been subjected to this electric-field treatment as conditioned samples. It was observed that this field treatment leads to an increase of the composite Tg 共33 and 38 ° C for
87, 261111-1
© 2005 American Institute of Physics
261111-2
Appl. Phys. Lett. 87, 261111 共2005兲
Quintana et al.
TABLE I. Photoconductive and birefringence properties 共at 20 V/µm and 50 mW/cm2兲 and photorefractive properties 共from 2BC experiments at 60 V/µm and 200 mW/cm2兲 at 633 nm and at room temperature 共24°C兲 of sensitized 共with C60兲 and unsensitized 共without C60兲 PVK/PDCST/BBP composites. Conditioned samples have been subjected 共prior to these experiments兲 to an electric field of 20 V/µm for 10 min at temperature of 32°C.
Composite Sensitized unconditioned 共standard兲 Sensitized conditioned Unsensitized unconditioned Unsensitized conditioned
dark 共pS/cm兲
ph 共pS/cm兲
M
共ne − no兲 10−3
⌫ 共cm−1兲
⌫net 共cm−1兲
1 共ms兲
0.33± 0.3 0.13± 0.01 0.60± 0.06 0.11± 0.01
2.4± 0.3 2.8± 0.3 0 0.04± 0.02
0.9± 0.1 1.0± 0.1 0 0.26± 0.08
1.2± 0.1 1.5± 0.2 1.2± 0.1 3.6± 0.4
140± 20 170± 20 — 120± 10
110± 20 140± 20 — 110± 10
27± 4 22± 3 — 1000± 100
unsensitized and sensitized composites, respectively兲. At present, the physical interpretation of this observation is still unclear. Additional experiments that provide detail information about the changes in structure and polarization of the different components of the sample under the dc field need to be done. First, we studied the effect of the field treatment on photoconductivity and birefringence. The photoconductivity measurements were made by a simple dc technique. Current flowing through the sample at 20 V / m was measured with a Keithley 600B electrometer in the dark and under illumination at 633 nm provided by a 35 mW Melles Griot He–Ne laser with an intensity of 50 mW/ cm2. Then, the dark conductivity 共dark兲, the conductivity in the presence of light 共light兲 and the photoconductivity 共ph = light − dark兲 were obtained. Concerning conductivity, we also obtained the conductivity contrast defined by M=
ph ph + dark
共1兲
because is preferable than ph for PR characterization since the modulation of the internal space charge field in PR experiments is proportional to this parameter.8 Birefringence was measured by a simple transmission ellipsometric technique.9 The measured transmission 共T兲 due to birefringence is given by T = sin2共⌬nl/兲,
共2兲
where ⌬n is the induced birefringence, l is the optical path length inside the sample, and is the wavelength. Samples were tilted 30° 共internal angle = 18°兲 and the external field strength was 20 V / m. The birefringence of the poled polymer, assuming uniaxial symmetry, is defined as the semiaxis difference 共ne − no兲 of the index ellipsoid and related to the birefringence experienced in the ellipsometer by the approximated equation10 ne − no =
⌬n . sin2
measured, in accordance with results reported in Ref. 12. Detail studies of the temperature and electric field dependence of dark and ph will be reported in a more extended paper. As previously mentioned, the field treatment leads to an increase in Tg, that justifies the observed reduction in dark. Finally, it should be noted that the relative increase in the conductivity contrast produced by the field treatment is small 共about 10%兲 because the photoconductivity is primarily due to C60 sensitizer. In order to investigate the effect of the field treatment without the interference of C60, we also measured dark and light for conditioned and unconditioned samples of the unsensitized PVK 共50%兲/PDCST 共35%兲/BBP 共15%兲 composite 共␣ = 6.0 cm−1 at 633 nm兲. Values of dark, ph and M corresponding to this composite are also included in Table I. As for sensitized composites, also in this case conditioned samples show a lower dark value than unconditioned ones. In addition, dark of unsensitized composites is larger than that of sensitized ones. This is also due to the fact that Tg is lower for unsensitized composites. It is also observed that ph is null for the unconditioned composite, while a relatively high photoconductivity that leads to a value of the conductivity contrast about 0.26 is obtained for the conditioned composite. Figure 1 shows the conductivity contrast as a function of the field treatment temperature for unsensitized samples. It is observed that conductivity contrast increases with temperature. The treatment temperature of 32 ° C seems to be a good choice because higher temperatures produce sample breakdown, while treatments at lower temperatures have less efficacy. Birefringence measurements are also included in Table I. Samples were subjected to an electric field of 20 V / m until
共3兲
It should be noted that both experiments, photoconductivity and birefringence, were performed at room temperature 共24 ° C兲, below the electric-field treatment temperature. Values of dark, ph, and M for conditioned and unconditioned standard composites 关PVK 共49.5%兲/PDCST 共35%兲/ BBP 共15% 兲 / C60 共0.5%兲兴 are shown in Table I. It can be seen that the field treatment produces an increase of photoconductivity due to an increase of light and a decrease of dark. Several groups have studied the relation between dark and Tg in PVK-based composites11 and in organic glasses.12 In our case, a decrease of dark with increasing Tg has been
FIG. 1. Conductivity contrast at room temperature 共24 ° C兲 vs field treatment temperature for an unsensitized conditioned PVK/PDCST/BBP composite.
261111-3
Appl. Phys. Lett. 87, 261111 共2005兲
Quintana et al.
birefringence reached the steady state, 90 s for the unsensitized conditioned composite and 10 s for the others three cases. Results show that the field treatment increases birefringence and that this increase is significantly higher for the unsensitized conditioned composite. The fact that 共ne − no兲 is lower for sensitized composites suggests that C60 would inhibit chromophore orientation. Local distortions in the electric field produced by C60 ions would affect chromophore orientation. Very recently, similar results were reported in other composites also sensitized with C60.13 In addition, for unsensitized composites, 共ne − no兲 gets larger only when samples have been conditioned. This indicates that the field treatment affects the PDCST chromophores in such a way that they have a better facility to get aligned when an electric field is applied again. The PR properties were studied by performing 2BC experiments at room temperature 共24 ° C兲. We used a 35 mW He–Ne laser operating at 633 nm and the same geometry configuration of previous works.6,7,14 It consists of two p-polarized beams of equal intensity 共100 mW/ cm2 per beam兲 intersecting the sample with external angles of 30° and 60° with respect to the sample normal, that generate a grating spacing of 1.6 m. The experiments were performed under the application of an external electric field of 60 V / m perpendicular to the sample. A typical 2BC run consisted of the following steps. First, in the presence of the 30° beam, the electric field was applied to the sample. Then, the pump 共60° beam兲 was turned on and the transmitted output powers of both beams were monitored until steady-state energy transfer was reached. No gain coupling was observed in unsensitized unconditioned samples. After steady state energy transfer was achieved, the gain 共␥0兲 was measured to determine the gain coefficients 共⌫兲. Net gain coefficients were calculated by subtracting the absorption coefficient of the sample at 633 nm 共⌫net = ⌫ − ␣兲. As in previous works,6,7,14 the speed of the PR effect was quantified by fitting the 2BC gain with a double exponential function and considering only the short time constant 1 obtained from the fit. ⌫, ⌫net, and 1 values are given in Table I. For standard samples 共sensitized and unconditioned兲 these results are different than those already reported in literature6 共⌫ = 73 cm−1 and = 230 ms at 50 V / m and 647 nm兲, due to the different excitation wavelength and applied voltage used in the experiments. Concerning the conditioned composite with C60, as expected, it presents a slightly higher gain and a shorter response time than those obtained for the standard composite. The most interesting results were obtained for unsensitized composites: the conditioning treatment led to PR net gain coefficients 共at 60 V / m兲 similar to those of the standard composite. Preliminary studies of the electric field dependence of the gain for conditioned samples indicate that, at low fields 共below 60 V / m兲, ⌫ values of unsensitized composites are superior to those of sensitized ones. The contrary occurs at high fields 共above 60 V / m兲. A detailed study of the electric field dependence of the photoconductiv-
ity, the birefringence and the PR properties is presently being performed and will be reported elsewhere. Concerning the speed of the PR process, as observed in Table I the response time of the unsensitized conditioned composite is significantly longer than those of sensitized one. Previous works6,7 have reported that for standard composites, the PR speed is limited by the photoconductivity, that is by the formation of the internal electric field, rather than by the orientation of the NLO chromophores. If the same is assumed for unsensitized and conditioned samples, the longer response time measured might be explained by the reduced photoconductivity this type of composite shows. In conclusion, we have shown that the PVK 共50%兲/ PDCST 共35%兲/BBP 共15%兲 composite has a significant PR response after conditioning in a 20 V / m electric field for about 10 min at a temperature of 32 ° C. The 2BC net gain coefficient 共117 cm−1 at 60 V / m兲 is similar to that obtained for the standard composite with 0.5% of C60 sensitizer at the same field strength. We have also shown that this result is due to an increase of photoconductivity and birefringence of the composite during conditioning under the electric field. PR response times 共1.0 s for 60 V / m兲 are longer than those of the standard composite 共27 ms for 60 V / m兲. This work was supported by the Spanish Government through Grant Nos. BQU2002-04513-C02-01 and BQU2002-04513-C02-02 and by the Generalitat Valenciana 共Spain兲 through Grant Nos. CTIDIA/2002/33 and GV99147-1-15. The authors also thank Vicente Esteve for technical support. 1
Photorefractive Materials and Their Applications, edited by P. Günter and J.-P. Huignard 共Springer, New York, 1988兲, Vols. 1 and 2. 2 L. Solymar, D. J. Webb, and A. Grunnet-Jepsen, The Physics and Applications of Photorefractive Materials 共Clarendon, Oxford, 1996兲. 3 O. Ostroverkhova and W. E. Moerner, Chem. Rev. 共Washington, D.C.兲 104, 3267 共2004兲. 4 W. E. Moerner, S. M. Silence, F. Hache, and G. C. J. Bkorklund, J. Opt. Soc. Am. B 11, 320 共1994兲. 5 A. Grunnet-Jepsen, G. L. Thompson, and W. E. Moerner, J. Opt. Soc. Am. B 15, 905 共1998兲. 6 D. Wright, M. A. Díaz-García, J. D. Casperson, M. DeClue, and W. E. Moerner, Appl. Phys. Lett. 73, 1490 共1998兲. 7 M. A. Diaz-Garcia, D. Wright, J. D. Casperson, B. Smith, E. Glazer, W. E. Moerner, L. I. Sukhomlinova, and R. J. Twieg, Chem. Mater. 11, 1784 共1999兲. 8 N. V. Kukhtarev, V. B. Markov, S. G. Odulov, M. S. Soskin, and V. L. Vinetskii, Ferroelectrics 22, 949 共1979兲. 9 G. B. Jung, K. Honda, T. Mutai, O. Matoba, S. Ashihara, T. Shimura, K. Araki, and K. Kuroda, Jpn. J. Appl. Phys., Part 1 42, 2699 共2003兲. 10 J. D. Shakos, M. D. Rahn, D. P. West, and K. Khand, J. Opt. Soc. Am. B 17, 373 共2000兲. 11 T. K. Däubler, R. Bittner, K. Meerholz, V. Cimrová, and D. Neher, Phys. Rev. B 61, 13515 共2000兲. 12 O. Ostroverkhova, M. He, R. J. Twieg, and W. E. Moerner, Chem. Phys. Chem. 4, 732 共2003兲. 13 S. Tay, J. Thomas, M. Eralp, G. Li, R. A. Norwood, A. Shülzgen, M. Yamamoto, S. Barlow, G. A. Walker, S. R. Marder, and N. Peyghambarian, Appl. Phys. Lett. 87, 171105 共2005兲. 14 J. Ortiz, F. Fernández-Lázaro, A. Sastre-Santos, J. A. Quintana, J. M. Villalvilla, P. Boj, M. A. Díaz-García, J. A. Rivera, S. E. Stepleton, C. T. Cox, Jr., and L. Echegoyen, Chem. Mater. 16, 5021 共2004兲.
