Temperature Dependence on the Grating Formation in a Low-T g

Jan 21, 2009 - South Korea; Materials Center, Samsung AdVanced Institute of Technology, P.O. Box 111, Suwon 440-600,. South Korea; and EnVironment ...
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J. Phys. Chem. B 2009, 113, 1592–1597

Temperature Dependence on the Grating Formation in a Low-Tg Polymeric Photorefractive Composite Jin-Woo Oh,*,† Won-Jae Joo,‡ In Kyu Moon,§ Chil-Sung Choi,† and Nakjoong Kim*,† Department of Chemistry, Hanyang UniVersity, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, South Korea; Materials Center, Samsung AdVanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea; and EnVironment and Energy DiVision 35-3, Korea Institute of Industrial Technology, Hongcheon, Ipjangmyeon, Cheonansi, Chungnam 330-825, South Korea ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: December 19, 2008

We investigated a dependence of the grating formation on the temperature in polymeric photorefractive (PR) composite, in terms of magnitude and buildup speed of the PR grating. For polymeric PR materials, the temperature is one of the most important factors together with the external electric field because it is closely related on photocharge generation efficiency, mobility of generated carrier, electro-optic coefficient tensor, and so on. Above the glass transition temperature, the diffraction efficiency of degenerate four-wave mixing decreased with increasing the temperature; it can be explained with the magnitude of space-charge field and the electro-optic behavior at different temperatures. The space-charge field decreased linearly with increasing temperature due to a decrease in the photocharge generation efficiency and an increase in the hole detrapping by the high dark conductivity. Also as we expected, the PR grating buildup speed, which is strongly dependent on the photoconductivity, steeply decreased with increasing the temperature, and its tendency was similar to the temperature dependence of the phase shift. Introduction The photorefractive (PR) effect is an electro-optic index change brought on by a space-charge field (ESC) arising from a light-induced redistribution of charges.1 Since the first polymeric PR composite2 was reported in 1991, several polymeric composites with high PR performance were developed3-6 that compete with and in some aspects even surpass the performance level of the best currently known inorganic materials, while showing the advantages of good optical quality, high structural flexibility, good reproducibility, and low cost, in contrast to inorganic materials.7 Therefore, polymeric PR composites are today considered a highly promising class of new materials for optical applications such as real-time holography, including optical correlation and real-time image processing, phase conjugation, neural networks, high-density holographic storage, and more.1,8-13 The PR effect in polymeric composites is a complicated process, in which the space-charge field forms through photocharge generation, carrier transport, and trapping, followed by the refractive index change due to the electro-optic (EO) effect and chromophore reorientation in the space-charge field.14 Therefore, the PR composites should possess both photoconductivity and optical nonlinear properties.15 Since optical nonlinearity of PR composites is based on the alignment of the rodlike chromophores toward electric field within the composite, it should be sensitive to the temperature, especially the temperature difference with the glass transition temperature (Tg) of the composites. Thus, by adjusting the Tg or the temperature of the measurement (T), it should be possible to achieve larger * Corresponding authors: Tel +82 2 2220 0935; Fax +82 2 2295 0572; e-mail [email protected] (J.-W.O.), [email protected] (N.K.). † Hanyang University. ‡ Samsung Advanced Institute of Technology. § Korea Institute of Industrial Technology.

photoconductivity and faster chromophore orientation, and therefore, faster PR response time.15 In our previous paper, we first reported on the temperature dependency of photocharge generation efficiency (φ) at the Tg region.16 With increasing temperature the φ increased below Tg but decreased above Tg. This behavior could be attributed to temperature dependence of heat capacity and the dielectric constant of PR composite. From our results, we can confirm that the φ be strongly dependent on the temperature. The relation of the mobility (µ) of the photogenerated carrier with temperature is given by µ(T) ) µ0 exp[-(T0/T)2].17 It is certain that the mobility be increased with temperature. Considering only the temperature dependence on photocharge generation efficiency and the mobility, it seems as if sample temperature more directly affects the response time than the magnitude in the formation of space-charge field. But, there are lots of the other effects induced by temperature, for example, the increment of the dark current with temperature. Therefore, it is certain that the formation time and magnitude of the space-charge field are strongly influenced by temperature. For EO behavior in the polymeric PR composite with low Tg, the index grating is mainly induced through the chromophore reorientation along applied electric field. The order parameter of reoriented chromophore under an electric field is well-defined by the oriented gas model.10 On the other hand, it is no easy to expect the response time quantitatively, since there are too many difficult factors considered for determining the response time, such as free volume effect as well as size and shape of chromophore. Simply, we can expect that the rotational mobility will be increased with temperature. Also, according to the oriented gas model, the order parameter should be decreased with increasing temperature at a given applied electric field. In this paper, we investigated the temperature dependency of the grating formation in a low-Tg polymeric PR composite. The space-charge field, diffraction efficiency, gain coefficient,

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and phase shift were measured at different temperatures, and they were analyzed with the temperature dependence of photocharge generation efficiency, photoconductivity, and electrooptic behavior. Furthermore, the photorefractive grating buildup speed was analyzed with the temperature dependence of photoconductivity. To measure the magnitude of space-charge field, we used the characterization method presented in our previous paper.18 For space charges in the polymeric materials, the magnitude is determined by several factors related to traps, such as trapping and detrapping.19,20 The diffraction efficiency was calculated on the basis of the measured value of spacecharge field and electro-optic behavior, and it was compared with that obtained by degenerated four-wave mixing. Experimental Section Materials and Device Fabrications. In this work, a low-Tg photorefractive composite was prepared by doping the optically anisotropic chromophore, 2-{3-[(E)-2-(dibutylamino)-1-ethenyl]5,5-dimethyl-2-cyclohexenyliden} malononitrile (DB-IP-DC), into the photoconducting polymer matrix, poly[methyl-3-(9carbazolyl)propylsiloxane] (PSX-Cz) sensitized by 2,4,7-trinitro9-fluorenone (TNF). PSX-Cz and DB-IP-DC were synthesized using previously described methods.21 TNF was obtained from Kanto Chem. Co. Inc. and was used after purification. The composition of polymeric composite was PSX-Cz:DB-IP-DC: TNF ) 69:30:1 by wt %. For sample preparation, the mixture (total 100 mg) was dissolved in 400 mg of dichloromethane, and the solution was filtered through a 0.2 µm membrane. The composite was cast on a patterned indium tin oxide (ITO) glass substrate, dried slowly for 12 h at ambient temperature, and then heated in an oven to 90 °C for 24 h to completely remove the residual solvent. The composite was then softened on a hot plate at 100 °C and next sandwiched between ITO glasses with Teflon film spacer of 100 µm to yield a film with a uniform thickness.22 The thickness of the active layer was near 100 µm. Measurements. We prepared a sample holder, which allows the temperature of sample to be adjusted from 26 to 40 °C with error range of (0.2 °C. There was a 1.5 cm diameter hole at the center of the heating plate of the sample holder. The laser beams illuminating the sample passed through this hole. The sample temperature was monitored using a thermometer (Fluke 50S), whose probe was placed in contact with the glass plate of the sample. The experimental data could be obtained at each temperature after the sample temperature was stabilized for 1 h. The photorefractive property of the composite was characterized by the two-beam coupling (2BC) and degenerated fourwave mixing (DFWM) methods using the 100 µm thick film. For 2BC measurement, two coherent laser beams were irradiated at sample in the tilted geometry with the incidence angle of 30° and 60° with respect to the normal sample. The p-polarized laser beam at the wavelength of 632.8 nm (He-Ne laser) with an intensity of 20 mW/cm2 was used. The asymmetric energy coupling between two laser beams is evaluated by the gain (γ) defined as γ ) I2(I1 * 0)/I2(I1 ) 0), where I1 and I2 are pump beam and a signal beam, respectively. And the gain coefficient (Γ) employed as the measure of photorefractive performance was calculated from the measured value of γ according to the following equation.23

Γ)

ln βγ 1 L ln(1 + β - γ)

[

]

(1)

where L is the beam path length and β is the intensity ratio of incident laser beams (I1/I2 ) 1.17 in our work).

The diffraction efficiency of photorefractive grating was determined by the DFWM experiments. Photorefractive grating was formed by the irradiation of two s-polarized beams with the intensity of 20 mW/cm2 and a spot size of 6 mm in order to minimize the beam coupling between the writing beams, which causes the variation of photorefractive grating throughout the sample. Then the recorded photorefractive grating was read out by a p-polarized counter-propagating probe beam with the intensity of 0.06 mW/cm2 and a spot size of 1.5 mm. The sample geometry such as incident angles of two writing beams and the wavelength of the laser employed in the DFWM method are the same as those in 2BC measurement. The magnitude of the diffraction efficiency (η) was determined from the measured transmitted and diffracted intensities of the reading beam,22 using the relation

η ) IR,diffracted /(IR,diffracted + IR,transmitted)

(2)

The PR grating buildup times of the photorefractive composite were calculated by fitting the evolution of the growth of the diffraction signal.24 The magnitude of the space-charge field was measured using the method reported previously.18 The basic scheme of this method can be summarized as follows: The chromophore group, which had previously been aligned along the external electric field, was reoriented by the newly formed space-charge field. A change in the birefringence was induced by the reorientation and was closely associated with the space-charge field. Using numerical analysis based on the oriented gas model and the index ellipsoid method, the magnitude of the space-charge field can be determined from the birefringence change. Details on this method have been well described in ref 18. In order to characterize the electro-optic behavior of a given chromophore, the standard two crossed polarizers setup was used. The polarization axes of the analyzer and polarizer were set to 45° and -45° with respect to the incident plane, respectively, and the sample was tilted by 30° from the propagation direction of the prove beam. Results and Discussion The number of available charges, which can be indicated as photocharge generation efficiency, is an important parameter in the buildup of the space-charge field in the PR composite.25 Figure 1 shows the temperature dependences of photocharge generation efficiency of PR composite in the temperature region 26-38 °C at an electric field of 30 V/µm. The photocharge generation efficiency had the maximum value (2.52 × 10-4) at 26 °C and then decreased by 41% with an increase in the temperature by 12 °C. In our previous paper, we first reported on the temperature dependency of photocharge generation efficiency (φ) at the Tg region.16 With increasing temperature the φ increased below Tg but decreased above Tg. This behavior could be attributed to temperature dependence of heat capacity and the dielectric constant of PR composite. From our results, we can confirm that the φ be strongly dependent on the temperature. As shown the inset of Figure 1, the glass transition temperature of PR composite was about 26 °C. The photoconductivity (σph) is related to the number density (n) of free charges produced by light absorption and the charge mobility (µ) by10

σph ) neµ )

( )

φRIqτ eµ hV

(3)

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Figure 1. Temperature dependence of the photocharge generation efficiency (φ) at an electric field of 30 V/µm. Inset shows glass transition curves of PR composite. The Tg is obtained from the second cooling by DSC (at the cooling rate of 10 °C/min under a nitrogen atmosphere).

Oh et al.

Figure 3. Comparison of measured and calculated space-charge fields (9, measured data; O, calculated data). The line is a guide to the eye.

Figure 4. Photocharge generation efficiency dependence of the spacecharge field at various temperatures. Figure 2. Temperature dependence of total, photo-, and dark conductivity obtained at an electric field of 30 V/µm. The photoconductivity was calculated as the difference between the total conductivity under the illumination intensity of 20 mW/cm2 and the dark conductivity. The line is a guide to the eye.

where e is the elementary charge, R is the absorption coefficient, Iq is the optical intensity, τ is the lifetime of charge, and hV is the energy of photon. From eq 3 we can confirm that the photoconductivity is governed by the charge generation and transport. Figure 2 shows total conductivity, photoconductivity, and dark conductivity (σd) as a function of the temperature. The photoconductivity was calculated as the difference between the total conductivity in the presence of light and the dark conductivity in the absence of light. In Figure 2, the photoand dark conductivity increase considerably with an increase in the temperature. The temperature dependency of conductivity is stronger at temperatures above Tg than around Tg, which is reflected in differences of activation energies in these temperature regions. As seen from the comparison between dark and photoconductivity, the increment of photoconductivity is smaller than that of the dark conductivity due to the decreasing of the photocharge generation, as shown Figure 1. The charge generation dependence of the photoconductivity is much greater than that of the dark conductivity. In amorphous organic materials, however, clarifying the conductivity mechanism is not an easy task, since the conductivity is described with a complicated function of many parameters such as energetic and positional disorder,26 trapping, detrapping, recombination of impurities, and various kinds of traps.27 The above results will be explained in detail in the space-charge field section.

Figure 3 shows the comparison between the measured spacecharge field and the one calculated by Kukhtarev’s model. By our previously reported method,18 we have measured the variations of space-charge field with the temperature. Above Tg, the magnitude of space-charge field decreased linearly by 37% with an increase in the temperature from 26 to 38 °C. This behavior could be attributed a decrease in the photocharge generation efficiency and an increase in the dark conductivity by a high hole detrapping rate. The influence of the photocharge generation efficiency on the space-charge field is shown in Figure 4. At all temperature regions, the space-charge field was linearly dependent on the photocharge generation efficiency. In addition, we calculated the space-charge field using Kukhtarev’s model28 to explain in detail the relation of space-charge field formation and conductivity in polymeric PR composite. In this model the magnitude of the space-charge field ESC is given by eq 4

|ESC(T)| ) m

σph(T) σph(T) + σd(T)

E0Eq(T)

√E02 + Eq2(T)

(4)

where m is the modulation depth, E0 is the projection of applied electric field along the grating wave vector, and the saturation field Eq(T) ) eΛNT(T)/[2πε0ε(T)], where Λ is the grating constant and NT is the PR trap density. Equation 4 assumes that the charge drift in the electric field dominates the diffusion of charges.15 The calculated results of space-charge field are in

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Figure 5. Ratio σph/(σph + σd) as a function of temperature. The line is a guide to the eye.

Figure 7. Temperature dependence of the diffraction efficiency obtained at an electric field of 30 V/µm. Inset shows the temperature dependence of the refractive index modulation amplitude ∆n. The line is a guide to the eye.

birefringence is considerably decreased. When describing chromophore alignment in the electric field at temperatures above Tg, it is conventional to apply the oriented gas model,29 which assumes freely rotating, noninteracting molecules. In the lowfield limit, the electric field-induced change in birefringence can be written as the following equation.14,30

∆nBR(T) )

CBR(T) ) Figure 6. Birefringence of composite under electric field at different temperatures. In these measurements, applied field was 30 V/µm and the line is a guide to the eye.

good agreement with the experimental data, as shown in Figure 3. Generally, at low temperatures, σd , σph, and the obtainable space-charge field is limited by the saturation and biasing fields. Because of the exponential dependence of dark conductivity on temperature, the dark conductivity becomes comparable to photoconductivity at high temperature, and the magnitude of the space-charge field determined by σph/(σph + σd) decreases. From Figure 5, we can see that the conductivity contrast σph/ (σph + σd) decreased with increasing temperature due to a much stronger increase in dark conductivity compared with photoconductivity (Figure 2), and the magnitude of space-charge field is directly reduced by large dark conductivity (Figure 3). That is, the reduction of trapped charge density by detrapping may reduce the space-charge field, and the saturation field (Eq) is known to decrease with temperature because of the lower trap density (NT).23 The temperature dependence of the steady-state birefringence (∆nBR) for our composite is shown in Figure 6 (I ) 13 mW/ cm2; E ) 30 V/µm). The ∆nBR of composite was determined from the variation of the transmitted intensity (T) through crossed polarizers upon the application of the electric field, as described by the following equation:

(

T ) sin2

2π l∆nBR λ

)

(5)

where λ is wavelength and l is a distance of light path. The birefringence of our sample strongly depends on the temperature relative to Tg. At temperatures several degrees above Tg, the

1 3 2 C (T) + CEO(T) E02 2ε0n 2 BR 3

(

)

ζ 2 Nf δR 45 ∞ kT

2

( )

CEO(T) ) Nf0 f∞2

κm 5kT

(6)

where N is the chromophore concentration, f0 and f∞ are local field factors, δR is the polarizability anisotropy, κ is the molecular first hyperpolarizability, and ζ is the dipole moment. According to eqs 6, the steady-state electric field induced ∆nBR is expected to behave as a second-order polynomial of (1/T), that is, a decrease as the temperature increases due to thermal disruption of the chromophore alignment.15 In our composite, the birefringence decreased by 30% with an increase in the temperature from 26 to 40 °C, as shown in Figure 6. This result can be used to predict the steady-state holographic contrast of the PR composite and to quantify the rotational freedom of the chromophores within the sample. Figure 7 shows the diffraction efficiency of our composite as a function of the temperature. The inset of Figure 7 shows the refractive index modulation amplitudes ∆n calculated using Kogelnik’s expression for the diffraction efficiency (η) in thick transmission holograms31

[

η ) sin2

πd∆n cos(θ2 - θ1) λ√cos θ1 cos θ2

]

(7)

where d is the composite thickness and θ1 and θ2 are the internal angles of incidence of the two writing beams. The diffraction efficiency considerably decreased with an increase in the temperature. It may be attributed to the decreases of the birefringence and/or the space-charge field with increasing temperature. However, we cannot be certain which one is more dominant in reducing the diffraction efficiency because the index

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Figure 8. Comparison of measured and calculated diffraction efficiencies (9, measured data; O, calculated data). The line is a guide to the eye.

Figure 9. Gain coefficient at an electric field of 30 V/µm as a function of the temperature.

contrast of the photorefractive grating is given by the combination of field-induced orientational birefringence factor and spacecharge field as in the following equation:

∆n ∝ f(T)|Esc(T)|

(8)

where f(T) is an orientational birefringence factor, which is a function of the internal angles of incidence and the coefficients CBR(T) and CEO(T) defined in eqs 6.14 Figure 8 shows the comparison between the measured diffraction and the calculated one on the basis of the oriented gas model and the index ellipsoid method. This calculation was accomplished in a similar way to Figure 5 in ref 18. When the photorefractive grating was recorded in the composite, the locally modulating refractive index could be calculated from electro-optic behavior of the sample and the magnitude of spacecharge field. The former was well-defined from the experimental data and the calculation based on the oriented gas model, and the latter was obtained experimentally. Then the diffraction efficiency can be determined from the spatial distribution of refraction index via the coupled mode theory.1 According to the results in ref 18, the index contrast calculated from the determined space-charge field is ca. 40% larger than one experimentally obtained by degenerated four-wave mixing measurement. For this reason, we normalized the measured and the calculated diffraction efficiencies to compare them. As can be seen in Figure 8, they show good agreement, and it becomes clear that the decrease of the space-charge field and the orientational birefringence cause the reduction of the diffraction efficiency, when the temperature increases. As can be seen in Figure 9, the two-beam coupling gain coefficient Γ follows the same tendency as the diffraction efficiency. This was expected, since the gain coefficient is directly proportional to the product of the refractive index modulation amplitude and the sine of the phase shift between the space-charge field and the interference pattern10,12

Γ)

4π (eˆ · eˆ )∆n sin ψ λ 1 2

(9)

eˆ1 and eˆ2 are the polarization vectors of the two writing beams, and ψ is the phase shift between the space-charge field and the interference pattern generated by the interaction beams. At several degrees above Tg the reduction of the gain coefficient is observed. As the temperature increases further, the gain

Figure 10. Photorefractive phase shift calculated from gain coefficient and index modulation amplitude as a function of the temperature at the applied field 30 V/µm. The line is a guide to the eye.

Figure 11. Temperature dependence of the grating buildup time (τ) at an electric field 30 V/µm. Inset shows the correlation between the photoconductivity (σph) and the PR grating buildup speed (1/τ). The line is a guide to the eye.

coefficient decreases several times. From eq 9 and the index modulation amplitudes in the inset of Figure 7, we have calculated the photorefractive phase shift. The results from the calculations are shown in Figure 10. An increase in temperature produces a larger phase shift and coincides with the increased photoconductivity at higher temperature. The charges at high temperature have a higher mobility and able to move further away from the intensity maximum. The dependence of phase shift on the temperature can be understood according to the standard theory of photorefractivity.28 In the steady-state regime,

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the phase shift ψ of the space-charge field, in the case where the diffusion field is ignored, is given by

ψ ) arctan(E0 /Eq(T))

(10)

With increasing temperature, the trap density changes, and this change can be directly probed with measurement of the phase shift. As shown in Figure 10, the phase shift increased with increasing temperature since the trap density decreased with increasing temperature. The temperature dependence of photorefractive grating buildup time was also analyzed. The PR grating buildup rate is very important for real applications such as a real-imaging and real-data processing. The buildup time of the photorefractive composites was evaluated from the buildup of the beam intensity of the DFWM measurement. The time constants, τ1 and τ2, were calculated by fitting the evolution of the growth of the gain, g(t), with the following biexponential function32

g(t) ) a1{1 - exp(-t/τ1)} + a2{1 - exp(-t/τ2)}

(11) where τ1 and τ2 are the fast and slow time constants, respectively. Figure 11 shows the PR grating buildup times (τ1) as a function of the temperature at an electric field 30 V/µm. The temperature dependence of PR grating buildup speed shows similar behavior with phase shift.33 Above Tg, an elevated temperature causes a faster PR buildup speed, as was expected, since the charge mobility as well as the orientational effects increase with temperature. The inset of Figure 11 shows the photoconductivity dependence of the PR grating buildup speed. The PR buildup speed linearly increased with increasing photoconductivity strongly dependent on the charge mobility. Conclusions In this work, we investigated the dependence of the grating formation on the temperature in a low-Tg polymeric PR composite. Above 26 °C, the diffraction efficiency dramatically decreased with an increase in the temperature because the spacecharge field decreased by ca. 37% and the birefringence decreased by ca. 30%. The temperature dependence of spacecharge field could be attributed to a decrease in the photocharge generation efficiency and an increase in the dark conductivity by a high hole detrapping rate. Furthermore, the PR grating buildup speed, which was strongly dependent on the photoconductivity, increased considerably with an increase in the temperature and showed a similar tendency of phase shift. In conclusion, the temperature is an important factor to manifest the high PR performance. Moreover, for a particular application, careful temperature control of the sample will be essential to obtain repeatable and reproducible results.

Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (Grant R11-2007-050-010030) and by the Research fund of HYU (HYU-2008-T). References and Notes (1) Yeh, P. Introduction to PhotorefractiVe Nonlinear Optics; Wiley: New York, 1993. (2) Ducharme, S.; Scott, J. C.; Twieg, R. J.; Moerner, W. E. Phy. ReV. Lett. 1991, 66, 1846. (3) Cox, A. M.; Blackburn, R. D.; West, D. P.; King, T. A.; Wade, F. A.; Leigh, D. A. Appl. Phys. Lett. 1996, 68, 2081. (4) Lundquist, P. M.; Wortmann, R.; Geletneky, C.; Twieg, R. J.; Jurich, M.; Lee, V. Y.; Moylan, C. R.; Burland, D. M. Science 1996, 273, 1182. (5) Grunnet-Jepsen, A.; Thompson, C. L.; Twieg, R. J.; Moerner, W. E. Appl. Phys. Lett. 1997, 70, 1515. (6) Wu¨rthner, F.; Wortmann, R.; Matschiner, R.; Lakaszuk, K.; Meerholz, K.; De Nardin, Y.; Bittner, R.; Bra¨uchle, C.; Sens, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 2765. (7) Bittner, R.; Bra¨uchle, C.; Meerholz, K. Appl. Opt. 1998, 37, 2843. (8) Moerner, W. E.; Silence, S. M. Chem. ReV. 1994, 94, 127. (9) Zhang, Y.; Burzynski, R.; Ghosal, S.; Casstevens, M. K. AdV. Mater. 1996, 8, 111. (10) Nalwa, H. S.; Miyata, S. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: New York, 1997. (11) Meerholz, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 945. (12) PhotorefractiVe Materials and Their Applications, I & II; Topics in Applied Physics; Gu¨nter, P., Huignard, J.-P., Eds.; Springer-Verlag: Berlin, 1998; Vols. 61 and 62. (13) Ostroverkhova, O.; Moerner, W. E. Chem. ReV. 2004, 104, 3267. (14) Moerner, W. E.; Grunnet-Jepsen, A.; Thompson, C. L. Annu. ReV. Mater. Sci. 1997, 27, 585. (15) Ostroverkhova, O.; He, M.; Twieg, R. J.; Moerner, W. E. ChemPhysChem 2003, 4, 732. (16) Oh, J.-W.; Kim, N. Chem. Phys. Lett. 2008, 460, 482. (17) Gambino, S.; Samnel, I. D. W.; Barcena, H.; Burn, P. L. Org. Electron. 2008, 9, 220. (18) Joo, W.-J.; Kim, N. J.; Chun, H.; Moon, I. K.; Kim, N. J. Appl. Phys. 2002, 91, 6471. (19) Schildkraut, J. S.; Buettner, A. V. J. Appl. Phys. 1992, 72, 1888. (20) Oh, J.-W.; Lee, C.; Kim, N. J. Appl. Phys. 2008, 104, 073709. (21) Oh, J.-W.; Choi, C.-S.; Lee, C.; Kim, N. Mol. Cryst. Liq. Cryst. 2007, 471, 373. (22) Chun, H.; Moon, I. K.; Shin, D.-H.; Kim, N. Chem. Mater. 2001, 13, 2816. (23) Bittner, R.; Daubler, T. K.; Neher, D.; Meerholz, K. AdV. Mater. 1999, 11, 123. (24) Oh, J.-W.; Choi, C.-S.; Kim, N. Mol. Cryst. Liq. Cryst. 2006, 477, 133. (25) Steenwinckel, D. V.; Hendrickx, E.; Persoons, A. J. Chem. Phys. 2001, 114, 9557. (26) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker: New York, 1998; Vol. 59. (27) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford University Press: New York, 1999; Vol. 56. (28) Kukhtarev, N. V.; Markov, V. B.; Odulov, S. G.; Soskin, M. S.; Vinetskii, V. L. Ferroelectrics 1979, 22, 949. (29) Kuzyk, M. In Characterization Techniques and Tabulations for Organic Nonlinear Optical Materials; Kuzyk, M., Dirk, C., Eds.; Marcel Dekker: New York, 1998; Vol. 60. (30) Sandalphon; Kippelen, B.; Meerholz, K.; Peyghambarian, N. Appl. Opt. 1996, 35, 2346. (31) Kogelnik, H. Bell Syst. Tech. J. 1969, 48, 2909. (32) Diaz-Garcia, M. A.; Wright, D.; Smith, B.; Glazer, E.; Moerner, W. E. Chem. Mater. 1999, 11, 1784. (33) Malliaras, G. G.; Krasnikov, V. V.; Bolink, H. J.; Hadziioannou, G. Appl. Phys. Lett. 1995, 66, 1038.

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