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J. Phys. Chem. C 2007, 111, 17646-17652
Photorefractive Effect of Ferroelectric Liquid Crystals with an Applied Alternating Electric Field Takeo Sasaki,* Norihisa Moriya, and Yukiko Iwasaki Department of Chemistry, Faculty of Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: June 15, 2007; In Final Form: September 11, 2007
The photorefractive effect of ferroelectric liquid crystals (FLCs) with application of an alternating electric field was investigated. The consecutive switching motion of the director of a FLC under an alternating electric field was modulated by the photoinduced internal electric field at the interference fringe. The grating based on the difference in the switching motion of the FLC was formed, and it was confirmed to operate as a diffraction grating.
Introduction The photorefractive effect is defined as the optical modulation of the refractive index due to charge redistribution.1-3 Interference of two laser beams in a photorefractive material induces a charge generation at the light positions of the interference fringe. In an organic medium, the mobility of positive charges (holes) is much larger than that of electrons.3 Holes are transported by diffusion and drift, leading to a separation of holes and electrons. Electrons, on the other hand, are trapped at light positions. As a result, the light positions are negatively charged and the dark positions are positively charged, providing a periodic space-charge field (internal electric field) along the interference fringe. The refractive index of the corresponding area is changed by electro-optic effects. Thus, a refractive index grating is formed at the interference fringe. There has been extensive interest in the photorefractive effect of organic materials since the 1990s. For example, a number of organic materials such as glassy polymers,3-8 low-molecular-weight liquid crystals,9-19 liquid crystalline polymers,20 and amorphous compounds21,22 have been investigated for their photoreactive effects. The photorefractive effect of surface-stabilized ferroelectric liquid crystals (SS-FLCs) doped with photoconductive compounds has been investigated.14-22 Figure 1 shows the mechanism of the photorefractive effect in FLCs. The internal electric field alters the direction of spontaneous polarization in the area between the light and the dark positions of the interference fringe, which causes a periodic change in the orientation of the FLC molecules. Recently, the photorefractive effect of a mixture of FLC and a photoconductive compound under an alternating electric field (ac field) was reported.19 The switching motion of the SS-FLC was modulated by an internal electric field and formed a grating based on the difference in the switching motion of the FLC director. When an ac field is applied to the SS-FLC, the FLC director performs a consecutive rotational switching motion along a conical surface (Figure 2a).23,24 When laser beams interfere in a FLC doped with a photoconductive compound under application of an ac field, the rotational motion is * To whom correspondence should be addressed. E-mail: sasaki@rs. kagu.tus.ac.jp.
Figure 1. Schematic illustration of the mechanism of the photorefractive effect in FLCs upon application of a dc electric field: (a) two laser beams interfere in the surface-stabilized state of the FLC/ photoconductive compound mixture; (b) charge generation occurs at the bright area of interference; (c) electrons are trapped at the trap site in the bright area, whereas holes migrate by diffusion or drift in the presence of an external electric field, generating an internal electric field between the bright and dark positions; (d) the orientation of the spontaneous polarization vector (orientation of mesogens) is altered by the internal electric field.
modulated by the photoinduced internal electric field (Figure 2). The internal electric field vector is directed along the interference fringe wave vector, which is different from the direction of the applied alternating electric field. Thus, the total electric field on the FLC molecules is altered by the appearance of the internal electric field. The consecutive rotational motion of the FLC director in the area between the light and the dark
10.1021/jp0746291 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
Effect of Ferroelectric Liquid Crystals
Figure 2. Schematic illustration of the mechanism of the photorefractive effect in FLCs upon application of an ac electric field: (a) electro-optical switching of a FLC; (b) positive and negative charges appear at the light positions of the interference fringe; (c) an internal electric field develops in the area between the light and dark positions of the interference fringe; (d) the rotational motion of FLC molecules in the corresponding area is biased by the internal electric field.
positions of the interference fringe is biased by the internal electric field. Consequently, a grating based on the spatial difference in rotational motion (switching motion) of the FLC director is produced. Herein, details of the photorefractive effect of a FLC doped with a photoconductive compound under an ac field are presented. Experimental Section Sample Preparation. The FLC mixture used in this study was commercially obtained (Felix-SCE8, Clariant Japan Co.). The physical properties of SCE8 are listed in Table 1. A photoconductive compound, 9-ethyl-3-carbazolcarboxyaldehyde diphenylhydrazone (CDH, 2 wt %), and a sensitizer, 2,4,7trinitro-9-fluorenone (TNF, 0.1 wt %), were mixed with the FLC. The structures of these compounds are shown in Figure 3. The FLCs and dopants were dissolved in dichloroethane, and the solvent was evaporated. The mixture was then dried under vacuum at room temperature for 1 week. The samples were
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17647 injected into a liquid crystal (LC) cell (Figure 3) equipped with 1 cm2 indium tin oxide (ITO) electrodes and a polyimide alignment layer (LX-1400, Hitachi Chemicals Co., parallel rubbing). The thickness of the FLC films was determined by the gap of the cell. Cells with a gap of 10 µm were used in the current work. The helical structure of the SmC* phase uncoils in the cell and forms a surface-stabilized state. In order to form a highly homogeneous surface-stabilized state, the samples were heated to the isotropic phase temperature and deliberately cooled to the SmC* phase at a rate of 0.1 °C/min. Although a cell gap of 10 µm is much thicker than typical FLC devices (2 µm), the FLC mixture used in this work formed a surface-stabilized state with few defects (Figure 4). The UV-vis absorption spectra of the FLC/CDH/TNF mixture in a LC cell and chloroform solution are shown in Figure 5. Measurements. The phase transition temperatures were determined by differential scanning calorimetry (DSC; DSC822e, Mettler) and microscopic observation (FP-80, FP-82, Mettler; BX-50 polarizing microscope, Olympus). The photorefractive effect was measured with a two-beam coupling experiment. A linearly polarized (p-polarized) beam from an Ar+ laser (165LGS-S, Laser Graphics; 488 nm, continuous wave) was divided in two using a beam splitter and then interfered to the sample film. The intensity of the laser was 2.5 mW for each beam (1 mm diameter). The laser incidence geometry is shown in Figure 3. The sample was maintained (30 °C) using a thermocontroller (DB1000, Chino Co.). An alternating triangularwaveform electric field of 0 ( 1.5 V/µm (peak to peak), 0 Hz to 5 kHz, was applied to the sample using a function generator (FG-273A, Kenwood), and changes in the transmitted beam intensities were monitored using photodetectors (ET-2040, Electro-Optics Technology, Inc.) and recorded on a computer. The refractive index grating formation time in the FLC was measured based on the simplest single-carrier model of photorefractivity2,3 in which the gain transient was exponential. Results and Discussion Two-Beam Coupling in FLCs with Application of dc Field. When laser beams interfere in a surface-stabilized state of a photoconductive FLC with application of a dc electric field an orientational grating is formed. The change in molecular orientation occurs in the area between the light and the dark positions of the interference fringe. Therefore, the phase of the induced grating is shifted from the interference fringe. In such a phase-shifted grating, laser beams have a unique mode of propagation.2 The interfering laser beams are energetically coupled through the phase-shifted grating, and the transmitted intensity of one beam through the material appears to increase, whereas that of the other appears to decrease. This phenomenon is known as asymmetric energy exchange in two-beam coupling.1-3 Figure 6 shows a typical example of the asymmetric energy exchange observed in the SCE8/CDH/TNF sample under an applied dc electric field of 0.1 V/µm. Interference of the divided beams by the sample resulted in increased transmittance of one beam and decreased transmittance of the other. The change in the transmitted intensities of the two beams was completely symmetric, as can be seen in Figure 6. This result
TABLE 1: Physical Properties of FLC FLC
Ps at 25 °C (nC/cm2)
SCE8
-4.5
a
phase transition temp.a (°C) -Sc* 60 SA 80 N* 104 I
response timeb (µs)
rotational viscosity (mPas)
tilt angle (deg)
50
76
20
b
C, crystal; Sc*, chiral smectic C phase; SA, smectic A phase; N* chiral nematic phase; I, isotropic phase. Response time to a 10 V/µm electric field at 25 °C in a 2 µm cell.
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Figure 3. Beam incidence geometry and structures of photoconductive compounds CDH, ECz, and electron trap reagent TNF.
Sasaki et al.
Figure 6. Typical example of asymmetric energy exchange observed in SCE8 mixed with 2 wt % CDH and 0.1 wt % TNF. A dc electric field of 0.1 V/µm was applied. The sample angle R was 50°, and the intersection angle φ was 20°. The shutter was opened at t ) 0 s and closed at t ) 2.4 s.
Figure 4. Textures of SS states for FLC films with thicknesses of 10 and 15 µm observed using a polarizing microscope.
Figure 7. Switching behavior of SCE8 mixed with 2 wt % CDH and 0.1 wt % TNF under a 100 Hz, (0.5 Vm/µm triangular wave electric field. (a) Transmittance of the light through the FLC under polarizing microscope. (b) Magnitude of the applied electric field. Figure 5. Absorption spectrum of CDH and TNF in a FLC (SCE8) medium measured in a 10 µm gap LC cell. The concentrations of CDH and TNF were 2 and 0.1 wt %, respectively. The absorption of FLC was subtracted. The inset shows the absorption spectrum of a chloroform solution of CDH (2.5 × 10-5 mol/L) and TNF (1.3 × 10-6 mol/L).
suggests that the phase of the refractive index grating is shifted compared with that of the interference fringe.1-3 Neither photochemical reactions nor thermal effects can induce the symmetric change.3 Furthermore, the grating formation was within the Bragg diffraction regime,2 and no higher order diffraction was observed under this condition. Thus, a phaseshifted orientational grating was determined to form at the interference fringe upon application of a dc electric field. The effects of the beam incidence conditions and material properties on the photorefractivity of FLCs under a dc field have been previously described in detail.15,16
Two-Beam Coupling in FLCs upon Application of ac Field. The FLC molecules displayed consecutive switching motion upon application of an alternating triangular-waveform electric field of (0.5 V/µm, 100 Hz in the two-beam coupling experiment. The switching was confirmed using a polarizing microscope equipped with a photodetector (Figure 7). Figure 8a shows the transmitted intensities of the laser beams through the FLC/CDH/TNF mixture (identical sample as that used for the dc experiment) upon application of an alternating electric field as a function of time. Interference of the divided beams in the sample resulted in increased transmittance of one of the beams and decreased transmittance of the other. The incidence beam conditions were the same as those used in the dc experiment. Although the transmitted intensity of the laser beam oscillates due to the switching motion of the FLC molecules, the average intensities of the beams were symmetrically
Effect of Ferroelectric Liquid Crystals
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17649
Figure 9. Gain coefficient of the two-beam coupling under an ac electric field as a function of temperature. An ac electric field of (0.5 V/µm, 100 Hz was applied.
Figure 8. (a) Typical example of the asymmetric energy exchange observed in two-beam coupling experiments. A triangular waveform ac electric field of (0.5 Vm/µm, 100 Hz was applied in this case. The sample angle R was 50°, and the intersection angle φ was 20°. The shutter was opened at t ) 0 s and closed at t ) 2 s. (b) An example of the curve fit.
changed, as shown in Figure 8. This indicates that a grating based on the spatial difference in the rotational switching motion of FLC molecules was formed and acted as a diffraction grating. The symmetric change in the transmittance of the two beams proves that the phase of the motion-mode grating shifted from that of the interference fringe. No higher order diffraction was observed under this condition. The spacing of the gratings was calculated to be 0.9 µm (1100 lines/mm). Furthermore, the signal arising from the diffracted beam was fitted using the square of a single-exponential function3
γ(t) - 1 ) (γ - 1)[1 - exp(-t/τ)]2
(1)
where γ(t) represents the transmitted beam intensity at time t divided by the initial intensity (γ(t) ) I(t)/I0), γ represents γ(t) at the stationary state, and τ is the formation time. The gain coefficient was calculated from the γ value obtained from the fitted curve (Figure 8b). The two-beam coupling gain coefficient, Γ, was calculated assuming Bragg diffraction as follows1,2
Γ)
gm 1 ln L 1+m-g
(
)
(2)
where g is the ratio of the intensities of the signal beam behind the sample with and without a pump beam and m is the ratio of the beam intensities (pump/signal) in front of the sample. Figure 9 shows the temperature dependence of the gain coefficient obtained under an ac electric field. The asymmetric energy exchange was observed only at those temperatures at which the FLC/CDH/TNF mixture exhibits a ferroelectric phase. This
Figure 10. Gain coefficient of the two-beam coupling as a function of the applied ac electric field strength. The frequency of the field was 100 Hz.
finding suggests that the ferroelectricity or the switching movement of SmC* is necessary for beam coupling under an ac electric field. Figure 10 shows the gain coefficient plotted as a function of the electric field strength. The asymmetric energy exchange was observed at electric field strengths higher than (0.1 V/µm. Furthermore, no asymmetric energy exchange was observed without an external electric field. This eliminates the possibility that the beam coupling resulted from either a thermal grating or a photochemically formed grating. The gain coefficient was independent of the external electric field strength for fields higher than (0.1 V/µm. This behavior differs from that reported previously for the photorefractive effect of FLCs under an applied dc electric field, wherein the gain coefficient decreased with increasing strength of the external dc electric field.15,16 The refractive index grating formation time was measured based on the simplest single-carrier model for photorefractivity in which the gain transient is exponential.3 The grating formation time τ, was obtained from the fitted curve. As seen in Figure 11, the grating formation time decreased with increasing applied electric field strength, as a result of the increased efficiency of charge generation. The grating formation time was determined to be 30-40 ms. Frequency Dependence of the Gain Coefficient and Grating Formation Time. The gain coefficients are plotted as a function of the frequency of the applied external electric field in Figure 12. The gain coefficient was observed to increase with the increase in frequency in the range from 1 to 100 Hz. The FLC used in this study exhibited switching based on polarization reversal at a frequency lower than 500 Hz. The gain coefficient reached a maximum at frequencies of 60-500 Hz. However,
17650 J. Phys. Chem. C, Vol. 111, No. 47, 2007
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Figure 11. Grating formation time as a function of the ac electric field strength. The frequency of the ac field was 100 Hz. Figure 14. Example of the result of the two-beam coupling experiment under an ac electric field for SCE8 without photoconductive dopant. An ac electric field of (0.5 V/µm, 100 Hz was applied. The shutter was opened at t ) 0 s and closed at t ) 2 s.
Figure 12. Gain coefficient as a function of the frequency of the applied electric field. The frequency was varied from 1 to 5000 Hz. The strength of the field was (0.5 V/µm.
Figure 13. Grating formation time as a function of the frequency of the applied electric field. The frequency was varied from 1 to 4000 Hz. The strength of the field was (0.5 V/µm.
the magnitude of the gain coefficient decreased as the frequency was increased to higher than 500 Hz. This is thought to be because the FLC cannot perform a complete switching motion at frequencies higher than 500 Hz; therefore, the FLC molecules showed vibrational motion. Moreover, at frequencies from 500 to 2000 Hz, some of the FLC molecules perform a rotational switching motion whereas others undergo vibrational motion. When the frequency exceeds 2000 Hz, the magnitude of the gain coefficient reached a constant value, regardless of the frequency. Thus, the gain coefficient at frequencies lower than 500 Hz can be attributed to the photorefractive effect based on the complete switching of FLC molecules and that at frequencies higher than 2000 Hz represents the photorefractive effect based on the vibrational motion of FLC molecules caused by the alternating electric field. Figure 13 shows the frequency dependence of the grating formation time. The grating formation time was shortened as the frequency increased. In addition, the frequency dependence on the formation time was not correlated with the dependence on the gain coefficient. As the frequency
Figure 15. Dependence of the TNF concentration on the gain coefficients of a FLC doped with photoconductive dopants. (a) SCE8 doped with 2 wt % CDH. (b) SCE8 doped with 2 wt % ECz. An electric field of (0.5 V/µm, 100 Hz was applied.
of the applied electric field increased, the switching and vibrational motion of the FLC molecules was thought to be accelerated, and the affect on the internal electric field becomes apparent. The asymmetric energy exchange in FLCs under a static dc field is dominated by the direction of the dc field. The increase/ decrease of the beam intensity switches when the direction of the field is reversed. This incident originates from the direction of the charge separation. Thus, an internal electric field is thought to be difficult to form under an ac electric field. However, asymmetric energy exchange was observed with good reproducibility, suggesting that an internal electric field is formed even under an ac electric field. In addition, no energy exchange was observed in a FLC lacking a photoconductive
Effect of Ferroelectric Liquid Crystals
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17651 of the charge separation. In this case, the magnitude of the internal electric field is dominated by the concentration of the ionic species. On the other hand, the difference in the mobilities of ECz and TNF is small, and thus, less effective charge separation is induced, resulting in the independence of the internal electric field on the concentration of ionic species. If ionic conduction is the major contributor to the formation of the space-charge field, the anisotropical mobility of ionic species in the LC medium may affect formation of the field. As shown in Figure 3, the interference fringe is formed across the smectic layer, so that migration of ionic species occurs in the interlayer direction. The asymmetric structure of the surface-stabilized state of the FLC may lead to asymmetric mobility of cations and anions, which is thought to be one possible model to explain the charge separation under an ac electric field. However, the mechanism of formation of the space-charge field under an ac field requires further investigation. Conclusions The photorefractive effect of a ferroelectric liquid crystal upon application of an alternating electric field (ac field) was investigated. The rotational motion of FLC molecules under the influence of an alternating electric field was modulated by an additional photoinduced electric field built at the interference fringe of two laser beams. This results in formation of a hologram comprised of the periodic difference in the molecular motion of the FLC molecules. Asymmetric energy exchange was observed upon application of an ac field. This grating is thought to be based on the spatial difference in the molecular motion of the FLC molecules. The response time was in the order of a few tens of milliseconds and dominated by formation of the internal electric field. Faster responses are expected by utilizing FLC materials with higher photoconductivities.
Figure 16. Absorption spectra of mixtures of FLC (SCE8) and CDH in a 10 µm gap cell. The concentrations of photoconductive compounds were fixed at 2 wt %, and the TNF concentration was varied from 0 to 0.5 wt %. The reflection at the cell surface and light scattering of the LC are not subtracted.
dopant under an ac electric field (Figure 14). This observation indicates that photoconductivity is necessary for two-beam coupling under an ac field. It is necessary to consider both hole transportation by the hopping mechanism and ionic conduction in order to explain formation of the space-charge field under an ac field. In addition, a number of experimental results have supported a large contribution of ionic conduction to the FLC medium. The two-beam coupling gain coefficients of mixtures of FLC (SCE8) and photoconductive compounds under a dc field was investigated as a function of the concentration of TNF (electron acceptor). When an electron donor with a large molecular size (CDH) relative to the TNF was used as the photoconductive compound, the gain coefficient was strongly affected by the concentration of TNF (Figure 15a). However, when ethylcarbazole (ECz) with a molecular size almost the same as that of TNF was used, the gain coefficient was less affected by the TNF concentration (Figure 15b). As shown in Figure 16, the difference in the change in absorbance at 488 nm upon addition of TNF was not significant when comparing CDH and ECz. Therefore, the results shown in Figure 15 cannot be explained based on this difference. In addition, the photorefractive effect in SCE8 doped with 2 wt % ECz under an ac field was very small. These findings suggest ionic conduction plays a major role in formation of the space-charge field. The mobility of the CDH cation is smaller than that of the TNF anion, and this difference in mobility is thought to be the origin
Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes (1) Solymar, L.; Webb, J. D.; Grunnet-Jepsen, A. The Physics and Applications of PhotorefractiVe Materials; Oxford: New York, 1996. (2) Yeh, P. Introduction to PhotorefractiVe Nonlinear Optics; John Wiley: New York, 1993. (3) Moerner, W. E.; Silence, S. M. Chem. ReV. 1994, 94, 127. (4) Kippelen, B.; Peyghambarian, N. AdVances in Polymer Science, Polymers for Photonics Applications II; Springer: New York, 2002; pp 87-156. (5) Meerholz, K.; Volodin, B. L.; Kippelen, B.; Peyghambarian, N. Nature 1994, 371, 497. (6) Kippelen, B.; Marder, S. R.; Hendrickx, E.; Maldonado, J. L.; Guillemet, G.; Volodin, B. L.; Steele, D. D.; Enami, Y.; Sandalphon; Yao, Y. J.; Wang, J. F.; Ro¨ckel, H.; Erskine, L.; Peyghambarian, N. Science 1998, 279, 54. (7) Hattemer, E.; Zentel, R.; Mecher, E.; Meerholz, K. Macromolecules 2000, 33, 1972. (8) Wright, D.; Gubler, U.; Moerner, W. E.; CeClue, M. S.; Siegel, J. S. J. Phys. Chem. B 2003, 107, 4732. (9) Khoo, I. C.; Li, H.; Liang, Y. Opt. Lett. 1994, 19, 1723. (10) Wiederrecht, G. P.; Yoon, B. A.; Wasielewski, M. R. Science 1995, 270, 1794. (11) Wiederrecht, G. P.; Yoon, B. A.; Svec, W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 3358. (12) Wiederrecht, G. P.; Waiselewski, M. R. J. Am. Chem. Soc. 1998, 120, 3231. (13) Ono, H.; Kawamura, T.; Frias, N. M.; Kitamura, K.; Kawatsuki, N.; Norisada, H. AdV. Mater. 2000, 12, 143. (14) Wiederrecht, G. P.; Yoon, B. A.; Wasielewski, M. R. AdV. Mater. 2000, 12, 1533. (15) Sasaki, T.; Katsuragi, A.; Mochizuki, O.; Nakazawa, Y. J. Phys. Chem. B 2003, 107, 7659.
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Sasaki et al. (21) Hofmann, U.; Grasruck, M.; Leopold, A.; Schreiber, A.; Schloter, S.; Hohle, C.; Strohriegl, P.; Haarer, D.; Zilker, S. J. J. Phys. Chem. B 2000, 104, 3887. (22) Ostroverkhova, O.; Moerner, W. E. Appl. Phys. Lett. 2003, 82, 3602. (23) Skarp, K.; Handschy, M. A. Mol. Cryst. Liq. Cryst. 1988, 165, 439. (24) Fukuda, A.; Takezoe, H. Structure and Properties of Ferroelectric Liquid Crystals; Corona: Tokyo, 1990.
