

**Figure 16.** Structure of terthiophene.

**5. Photorefractive effect in photorefractive FLC blends containing**

FLCs are more crystalline than liquid, in comparison with nematic LCs; therefore several sophisticated techniques are required to prepare fine FLC films. Preparation of a uniformly aligned, defect-free SS-FLC using a single FLC compound is very difficult. In most cases, mixtures of LC compounds are typically used to obtain fine SS-FLC films. The composition of FLC mixture contains a base LC, which is a mixture of SmC phase forming LC compounds, and a chiral dopant. The chiral dopant introduces a helical structure into the LC phase. Utilization of an FLC as a photorefractive material requires the addition of photoconductive compounds to the FLC. However, the introduction of such non-LC compounds to the FLC often hinders the formation of a uniformly aligned SS-state. Thus, appropriate design of the photoconductive compounds is crucial. The photorefractive effect of FLC blends containing photoconductive chiral dopants has been investigated [17-19]. Terthiophene was selected as a photoconductive chromophore because it is a well-known semiconductor compound and has a rod-like structure (Figure 16), which increases the solubility into the rod-like structured LC material. The structures of the LC compounds, the electron acceptor TNF, and the photocon‐ ductive chiral compounds are shown in Figure 17. A ternary mixture of LC compounds was selected as a base LC. The mixing ratio of 8PP8, 8PP10, and 8PP6 was 1:1:2 since the 1:1:2 mixture exhibits the SmC phase over the widest temperature range. The textures of the FLC blends in 10 μm gap cells were observed using polarizing optical microscopy. The alignment of the FLC molecules is dominated not only by the properties of the FLCs but also by the affinity of FLC molecules with the alignment layer (polyimide). A homogeneous, anisotropic film can be obtained through interactions between LC molecules and the alignment layer. The FLC cell is fabricated by the precise assembly of indium tin oxide glasses coated with polyimide alignment layer into a cell of 10 μm gap determined by the diameter of the spacer bead. The appropriate preparation conditions for the fabrication of the LC cell differ from FLC to FLC. The thickness of the polyimide coating (Hitachi Chemicals LX-1400) was 20 to 30 nm and the surface of the polyimide was rubbed with a polyester velvet roll under specific conditions. Typical examples of textures observed in the 3T-2MB and 3T-2OC samples under a polarizing microscope are shown in Figures 18 and 19. On increasing the concentration of the photocon‐ ductive chiral dopant, defects appeared in the texture. The uniformly aligned state with few defects was obtained for samples with 3T-2MB concentrations lower than 8 wt.% (Figure 18). However, the 3T-2OC sample retained the uniformly aligned state with few defects for 3T-2OC concentrations less than 6 wt.% (Figure 19). The spontaneous polarization of the 3T-2MB

. On the other hand, the spontaneous polarization of the 3T-2OC

intermolecular interactions) of the 3T-2MB sample may be advantageous for the formation of the uniformly aligned SS-state. A texture with a pattern of strips was observed in the 3T-2MB samples (Figure 18). It indicates that a complete SS-state was not formed in the 3T-2MB sample in the 10 μm gap cell and the helical structure existed. Zig-zag defect, which is the typical defect

. The smaller spontaneous polarization (and thus smaller

**photoconductive chiral compounds**

**5.1. Photoconductive chiral dopants**

138 Ferroelectric Materials – Synthesis and Characterization

samples was less than 1 nC/cm2

samples was approximately 5 nC/cm2

**Figure 17.** Structures of the smectic LCs (8PP8, 8PP10, and 8PP6), photoconductive chiral dopants (3T-2MB and 3T-2OC), and the sensitizer TNF.

 

 

 **Figure 18.** Textures of FLC mixtures containing 3T-2MB in a 10 μm gap LC cell observed under a polarizing micro‐ scope. The strengths of the external electric field were +1.0, 0, and 1.0 V/μm. The direction of the applied electric field is shown in Fig. 5. The 3T-2MB concentrations were in the range of 2 wt.% to 10 wt.% with the addition of 0.1 wt.% TNF.

Dynamic Amplification of Optical Signals by Photorefractive Ferroelectric Liquid Crystals http://dx.doi.org/10.5772/60776 141

 **Figure 19.** Textures of FLC mixtures containing 3T-2OC in a 10 μm gap LC cell observed under a polarizing micro‐ scope. The strengths of the external electric field were +1.0, 0, and 1.0 V/μm. The direction of the applied electric field is shown in Fig. 5. The 3T-2OC concentrations were in the range of 2 wt.% to 10 wt.% with the addition of 0.1 wt.% TNF.

**Figure 18.** Textures of FLC mixtures containing 3T-2MB in a 10 μm gap LC cell observed under a polarizing micro‐ scope. The strengths of the external electric field were +1.0, 0, and 1.0 V/μm. The direction of the applied electric field is shown in Fig. 5. The 3T-2MB concentrations were in the range of 2 wt.% to 10 wt.% with the addition of 0.1 wt.% TNF.


μ

μ

140 Ferroelectric Materials – Synthesis and Characterization

 

 

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### **5.2. Asymmetric energy exchange in photorefractive FLC blends**

The photorefractive effect of the FLC blends was measured by two-beam coupling experi‐ ments. A typical example of the asymmetric energy exchange observed for a mixture of the base LC, 3T-2MB, and TNF at 25 °C with an applied electric field of 1 V/μm is shown in Figure 20. When the laser beams interfered in the sample, the increase in transmitted intensity of one of the beams and the decrease in the transmitted intensity of the other beam were observed. When the polarity of the applied electric field was reversed, these transmittance characteristics were also reversed. Asymmetric energy exchange was not observed when the external electric field was not applied. This indicates that the beam coupling was not caused by a thermal grating or gratings formed through photochemical mechanisms. It was found that approxi‐ mately 40% of the energy of the L2 laser beam migrated to the L1 beam.

The gain coefficients of the samples were measured as a function of the applied electric field strength (Figure 21(a)). The gain coefficient of 1200 cm–1 was obtained in the 10 wt.% 3T-2MB sample with application of 1.5 V/μm. This gain coefficient is much higher than the values for FLCs reported previously [16]. The higher transparency of the LC blend was considered to contribute to the large gain coefficient. The weak external electric field strength required for the photorefractive effect in FLCs is advantageous for the photorefractive applications. The response time decreased with an increase of the electric field strength due to the increased charge separation efficiency. The shortest formation time of 0.9 ms was obtained with appli‐ cation of an external electric field of 1.9 V/μm (Figure 21(b)). The large gain and fast response are advantageous for the realization of optical devices such as real-time image amplifiers and accurate measurement devices.

**Figure 20.** Typical results for two-beam coupling experiments with a ternary mixture base LC, 3T-2MB, and TNF meas‐ ured at 25 °C. The pump beam was incident at 1 s and closed at 4 s.

**5.2. Asymmetric energy exchange in photorefractive FLC blends**

142 Ferroelectric Materials – Synthesis and Characterization

mately 40% of the energy of the L2 laser beam migrated to the L1 beam.

accurate measurement devices.

 

ured at 25 °C. The pump beam was incident at 1 s and closed at 4 s.

The photorefractive effect of the FLC blends was measured by two-beam coupling experi‐ ments. A typical example of the asymmetric energy exchange observed for a mixture of the base LC, 3T-2MB, and TNF at 25 °C with an applied electric field of 1 V/μm is shown in Figure 20. When the laser beams interfered in the sample, the increase in transmitted intensity of one of the beams and the decrease in the transmitted intensity of the other beam were observed. When the polarity of the applied electric field was reversed, these transmittance characteristics were also reversed. Asymmetric energy exchange was not observed when the external electric field was not applied. This indicates that the beam coupling was not caused by a thermal grating or gratings formed through photochemical mechanisms. It was found that approxi‐

The gain coefficients of the samples were measured as a function of the applied electric field strength (Figure 21(a)). The gain coefficient of 1200 cm–1 was obtained in the 10 wt.% 3T-2MB sample with application of 1.5 V/μm. This gain coefficient is much higher than the values for FLCs reported previously [16]. The higher transparency of the LC blend was considered to contribute to the large gain coefficient. The weak external electric field strength required for the photorefractive effect in FLCs is advantageous for the photorefractive applications. The response time decreased with an increase of the electric field strength due to the increased charge separation efficiency. The shortest formation time of 0.9 ms was obtained with appli‐ cation of an external electric field of 1.9 V/μm (Figure 21(b)). The large gain and fast response are advantageous for the realization of optical devices such as real-time image amplifiers and

 -

 

> 

μ

 

**Figure 20.** Typical results for two-beam coupling experiments with a ternary mixture base LC, 3T-2MB, and TNF meas‐

**Figure 21.** Electric field dependence of the (a) gain coefficients and (b) refractive index grating formation times (re‐ sponse time) for mixtures of the base LC, 3T-2MB (10 wt.%), and TNF (0.1 wt.%) measured at 25 °C.

### **5.3. Temperature dependence of the asymmetric energy exchange for photorefractive FLC blends**

The temperature dependence of the gain coefficient for a photorefractive FLC blend with 3T-2MB is shown in Figure 22. Asymmetric energy exchange was observed at temperatures below the SmC\*-SmA phase transition temperature. Figure 23 shows temperature dependence of the helical pitch of the 3T-2MB samples observed under polarizing microscopy. The helical pitch diverged when the temperature approached the phase transition temperature. It has been reported that the asymmetric energy exchange for an FLC sample was observed only in the temperature range where the sample exhibits spontaneous polarization [8]. Thus, asymmetric energy exchange was observed only in the temperature range where the sample exhibits ferroelectric properties (i.e., the SmC\* phase).

**Figure 22.** Temperature dependence of gain coefficients for mixtures of the base LC with 3T-2MB concentrations of (a) 6 wt.% and (b) 10 wt.%.

**Figure 23.** Temperature dependence of helical pitch lengths for mixtures of the base LC with 3T-2MB concentrations of (a) 6 wt.% and (b) 10 wt.%.

### **5.4. Effect of the photoconductive chiral dopant concentration**

 The gain coefficients of samples with various 3T-2MB and 3T-2OC concentrations are plotted as a function of the external electric field magnitude in Figure 24. The gain coefficient increased with the strength of the external electric field up to 1.6 V/μm. The decrease in the gain coefficient at high external electric field is due to the realignment of the FLC molecules being restricted because the strength of the external electric field exceeds that of the internal electric field. The gain coefficient increased with the concentration of the photoconductive chiral dopants. The increase in the concentration of charge carriers in the FLC medium and an increase in the magnitude of spontaneous polarization contributes to a larger gain coefficient. The magnitude of the gain coefficient was independent of the concentration of TNF. It shows that charges are drifted through electric conduction based on a hopping mechanism, where electron holes hop between the photoconductive chiral dopants. The molecular weight of the photoconductive chiral dopant is similar to that of the LC molecules; therefore, in a 10 wt.% doped sample, approximately 10 photoconductive molecules are dispersed in 90 LC molecules. A cube, wherein each side includes 5 LC molecules, contains 125 LC molecules. Thus, the average distance between the photoconductive chiral dopant molecules in the LC is no more than 3 LC molecules. In this case, charge transport based on a hopping mechanism may be feasible. The gain coefficient of the 10 wt.% 3T-2MB sample was 1200 cm–1 with an applied electric field of only 1.6 V/μm (Figure 24(a)), which is twice as high as that of the 8 wt.% 3T-2OC sample at a similar external electric field (Figure 24(b)).

The response time of the samples is plotted as a function of the external electric field magnitude in Figure 25. The response time decreased with the increase of the electric field strength due to increased charge separation efficiency. The shortest formation time of 0.93 ms was obtained for the 10 wt.% 3T-2MB sample with an applied electric field of 2.0 V/μm. It was found that the response was faster than that of the 3T-2OC samples. The gain coefficient is lower and the response speed slower for the 3T-2OC sample even though the magnitude of the spontaneous polarization in the 3T-2OC samples (5 nC/cm2 ) is higher than that in the 3T-2MB sample (less than 1 nC/cm2 ). The transparency of the FLC film is more important for the photorefractive effect than the magnitude of spontaneous polarization.

(a) 6 wt.% and (b) 10 wt.%.

**5.4. Effect of the photoconductive chiral dopant concentration**

sample at a similar external electric field (Figure 24(b)).

 

**Figure 23.** Temperature dependence of helical pitch lengths for mixtures of the base LC with 3T-2MB concentrations of

The gain coefficients of samples with various 3T-2MB and 3T-2OC concentrations are plotted as a function of the external electric field magnitude in Figure 24. The gain coefficient increased with the strength of the external electric field up to 1.6 V/μm. The decrease in the gain coefficient at high external electric field is due to the realignment of the FLC molecules being restricted because the strength of the external electric field exceeds that of the internal electric field. The gain coefficient increased with the concentration of the photoconductive chiral dopants. The increase in the concentration of charge carriers in the FLC medium and an increase in the magnitude of spontaneous polarization contributes to a larger gain coefficient. The magnitude of the gain coefficient was independent of the concentration of TNF. It shows that charges are drifted through electric conduction based on a hopping mechanism, where electron holes hop between the photoconductive chiral dopants. The molecular weight of the photoconductive chiral dopant is similar to that of the LC molecules; therefore, in a 10 wt.% doped sample, approximately 10 photoconductive molecules are dispersed in 90 LC molecules. A cube, wherein each side includes 5 LC molecules, contains 125 LC molecules. Thus, the average distance between the photoconductive chiral dopant molecules in the LC is no more than 3 LC molecules. In this case, charge transport based on a hopping mechanism may be feasible. The gain coefficient of the 10 wt.% 3T-2MB sample was 1200 cm–1 with an applied electric field of only 1.6 V/μm (Figure 24(a)), which is twice as high as that of the 8 wt.% 3T-2OC

 

The response time of the samples is plotted as a function of the external electric field magnitude in Figure 25. The response time decreased with the increase of the electric field strength due to increased charge separation efficiency. The shortest formation time of 0.93 ms was obtained for the 10 wt.% 3T-2MB sample with an applied electric field of 2.0 V/μm. It was found that the response was faster than that of the 3T-2OC samples. The gain coefficient is lower and the

 -

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144 Ferroelectric Materials – Synthesis and Characterization

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**Figure 24.** Electric field dependence of the gain coefficients for (a) mixtures of the base LC, 3T-2MB (2-10 wt.%), and TNF (0.1 wt.%), and (b) mixtures of the base LC, 3T-2OC (2-8 wt.%), and TNF (0.1 wt.%) measured at 25 °C.

**Figure 25.** Refractive index grating formation times (response time) for (a) mixtures of the base LC, 3T-2MB (2-10 wt. %), and TNF (0.1 wt.%), and (b) mixtures of the base LC, 3T-2OC (2-8 wt.%), and TNF (0.1 wt.%) measured at 25 °C.

### **5.5. Dynamic amplification of optical images in photorefractive FLC blends**

The most straightforward application of the photorefractive effect is the amplification of optical signals, which is one of the important elements of optical technologies. Light amplifi‐ cation based on stimulated emission and non-linear optical effects are well known. Distinct from these phenomena, the photorefractive effect enables selective amplification. The photo‐

refractive effect is based on the formation of holograms in a material; therefore, it distinguishes a specific light signal from other light signals based on the difference in wavelength, polari‐ zation, and phase. Optical image amplification was demonstrated (Figure 26), where a computer-generated image was displayed on a spatial light modulator (SLM) and irradiated with a 473 nm diode-pumped solid-state laser beam [18]. A signal laser beam carrying a 2D image was transmitted through the FLC sample. The image was monitored with a chargecoupled device (CCD) camera. A pump beam (a beam divided from the signal beam before SLM) was interfered with the signal beam. The amplification of the signal beam transmitted through the FLC was observed. The intensities of the optical signal beam with and without the pump beam are shown in Figure 27. The intensity of the signal beam was amplified sixfold to the value without the pump beam.

**Figure 26.** Optical image amplification experiment. A computer-generated image was displayed on the SLM. The SLM modulated the object beam (473 nm), which was irradiated on the FLC sample and interfered with the pump beam. The image transmitted through the FLC sample (10 wt.% 3T-2MB) was monitored with a CCD camera.

 Dynamic amplification of moving optical signal was demonstrated using a photorefractive FLC blend [18]. A rotating image was displayed on the SLM with the frame rate at 30 fps. A 473 nm beam was irradiated on the SLM and the reflected beam was incident on the FLC sample. A pump beam was then interfered with the beam from the SLM in the FLC sample. The laser beam containing the moving animation image was amplified by the incident pump beam (Figure 28). This result shows that the response of the photorefractive FLC was suffi‐ ciently fast to amplify the moving optical image. If a typical photorefractive polymer with a response time of ca. 100 ms was used in place of the FLC sample, then the moving image would not be amplified. In that case, although a still image could be amplified, the intensity of the video-rate moving image would not be amplified.

Dynamic Amplification of Optical Signals by Photorefractive Ferroelectric Liquid Crystals http://dx.doi.org/10.5772/60776 147

**Figure 27.** Signal beam intensities with and without the pump beam.

refractive effect is based on the formation of holograms in a material; therefore, it distinguishes a specific light signal from other light signals based on the difference in wavelength, polari‐ zation, and phase. Optical image amplification was demonstrated (Figure 26), where a computer-generated image was displayed on a spatial light modulator (SLM) and irradiated with a 473 nm diode-pumped solid-state laser beam [18]. A signal laser beam carrying a 2D image was transmitted through the FLC sample. The image was monitored with a chargecoupled device (CCD) camera. A pump beam (a beam divided from the signal beam before SLM) was interfered with the signal beam. The amplification of the signal beam transmitted through the FLC was observed. The intensities of the optical signal beam with and without the pump beam are shown in Figure 27. The intensity of the signal beam was amplified six-

**Figure 26.** Optical image amplification experiment. A computer-generated image was displayed on the SLM. The SLM modulated the object beam (473 nm), which was irradiated on the FLC sample and interfered with the pump beam.

Dynamic amplification of moving optical signal was demonstrated using a photorefractive FLC blend [18]. A rotating image was displayed on the SLM with the frame rate at 30 fps. A 473 nm beam was irradiated on the SLM and the reflected beam was incident on the FLC sample. A pump beam was then interfered with the beam from the SLM in the FLC sample. The laser beam containing the moving animation image was amplified by the incident pump beam (Figure 28). This result shows that the response of the photorefractive FLC was suffi‐ ciently fast to amplify the moving optical image. If a typical photorefractive polymer with a response time of ca. 100 ms was used in place of the FLC sample, then the moving image would not be amplified. In that case, although a still image could be amplified, the intensity of the

The image transmitted through the FLC sample (10 wt.% 3T-2MB) was monitored with a CCD camera.

 

 

 

 

fold to the value without the pump beam.

146 Ferroelectric Materials – Synthesis and Characterization

 

video-rate moving image would not be amplified.

**Figure 28.** Optical image amplification experiment. A computer-generated animation was displayed on the SLM. The SLM modulated the object beam (473 nm), which was irradiated on the FLC sample, and interfered with the reference beam. The image transmitted through the FLC sample (10 wt.% 3T-2MB) was monitored with a CCD camera.

### **5.6. Dynamic holograms formed in FLC blends**

 Dynamic hologram formation was demonstrated in an FLC blend [19]. A moving image was displayed on an SLM and a laser beam was irradiated onto the SLM. The reflected beam was incident on the FLC sample and a reference beam was then interfered with the beam within the FLC sample. The interference condition was set to a Raman–Nath diffraction regime. The multiple scattering was observed in this condition. A red beam from a He–Ne laser (633 nm) was incident on the FLC sample and the diffraction was observed. A moving image was observed in the diffracted beam (Figure 29). No image retention was observed, which indicates that the hologram image (refractive index grating) formed in the FLC was rewritten with sufficient speed to project a smooth reproduction of the holographic movie. This result shows that a hologram image was formed at the interference area in the FLC material and that contributes to the optical image amplification.

**Figure 29.** Dynamic hologram formation experiment on an FLC sample. A computer-generated animation was dis‐ played on the SLM. The SLM modulated the object beam (488 nm), which was irradiated on the FLC sample and al‐ lowed to interfere with the reference beam. The readout beam (633 nm) was irradiated on the FLC and diffraction was observed.
