**1. Introduction**

Ferroelectric liquid crystals (FLCs) have attracted significant interest from both fundamental and practical perspectives [1, 2]. Even though liquid crystals (LCs) are liquid, some of them exhibit ferroelectricity. The response of FLCs to an applied electric field is very fast, in the range of a few tens of microseconds to a few milliseconds. The fast response of FLCs is advantageous for display applications; however, the treatment of FLCs requires sophisticated techniques because FLCs are very viscous. Displays that employ FLCs are not commercially available nowadays, due to the difficulty of fabrication into wide area displays. Recently, the photorefractive effect in FLCs was investigated, and FLCs were found to exhibit very high performance. The photorefractive effect is a phenomenon in which a dynamic hologram is

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formed in a material. When two laser beams interfere in a photorefractive material, a refractive index grating is formed. This phenomenon is applicable to devices related to diffraction optics, including 3D displays, optical amplification, optical tomography, novelty filters, and phaseconjugate wave generators [3]. The unique feature of the photorefractive effect is the asym‐ metric energy exchange in two-beam coupling; when two laser beams interfere in a photorefractive material, the energy of one of the interfering beams is transferred to the other beam. The asymmetric energy exchange can be used to coherently amplify signal beams [4]; therefore, it has the potential to be used as a transistor for electrical circuits in a wide range of optical technologies (Figure 1). Optically transparent materials that possess both photovoltaic and electro-optic properties also exhibit the photorefractive effect. There have been a lot of studies to develop photorefractive materials, such as inorganic photoconductive ferroelectric crystals, organic photoconductive ferroelectric crystals, organic photoconductive polymers, amorphous organic photoconductive materials, photoconductive amphiphilic compounds, and photoconductive LCs [5-8]. Since 1991, organic materials have attracted significant interest because they exhibit large photorefractivity and a shorter response time compared with that of inorganic crystals [3, 7, 8]. The photorefractive effect induces a change in the refractive index by a mechanism that involves both photovoltaic and electro-optic effects (Figure 2). An organic photorefractive material is composed of a photoconductive compound, an electro-optic compound, and an electron-trap reagent. Thus, the active wavelength can be determined by the selection of the photoconductive compound. With interfering two laser beams in an organic photorefractive material, charge generation occurs at the bright positions of the interference fringe and the generated charges diffuse within the material. The distances of the diffusions of positive and negative charges are different in an organic material because the mobilities of the positive and negative charges are different. That leads to a formation of charge-separated state. The higher mobility charge diffuses over a longer distance than the lower mobility charge. While the charges with low mobility stay in the bright areas, the charges with high mobility diffuse to the dark areas. Thus, the bright area and dark area are charged with opposite polarities, and an internal electric field (space charge field) is formed in the area between the brightest and the darkest positions. The refractive indices of the areas are changed by the electro-optic effect. In such a way, a refractive index grating is produced. A high electric field of 10–50 V/μm is typically applied to a polymer film, aside from the internal electric field, to obtain photorefractivity. The polymer material is typically 100 μm thick, so that a voltage of 1–5 kV is required to apply on the film to achieve photorefractivity, which is almost comparable to the breakdown voltage of the polymer film. This electric field is necessary for increasing the charge generation efficiency.

It is expected that a multiplex hologram display devices can be developed using photorefrac‐ tive polymers [9, 10]. It was shown that clear 3D images were recorded in the photorefractive polymer film. However, if polymer photorefractive materials are put into practical uses, a high voltage (3–5 kV) required to activate the photorefractive effect in polymer materials must be improved. There have been reports on the photorefractive effect of LCs [11]. LCs can easily be driven by a low electric field because LCs are liquid in nature. The most well-known LC phases are nematic and smectic phases (Figure 3). LCs that show nematic phase are used in liquid crystal displays (LCDs). However, LCs that show smectic phases are very viscous so that they are seldom utilized in practical applications. The LCs of which the photorefractivity was firstly investigated were nematic LCs [11]. It was found that a large photorefractivity was obtained with the application of low activation voltage (a few volts). The photorefractivity of ferroelec‐ tric liquid crystals doped with photoconductive compounds has been reported [12-14]. FLCs are considered to be candidates for practical photorefractive materials. A sub-millisecond refractive index grating formation time and a large gain coefficient are easily obtained in photorefractive FLCs.

**Figure 1.** Transistor in electric circuit and photorefractive amplifier.

formed in a material. When two laser beams interfere in a photorefractive material, a refractive index grating is formed. This phenomenon is applicable to devices related to diffraction optics, including 3D displays, optical amplification, optical tomography, novelty filters, and phaseconjugate wave generators [3]. The unique feature of the photorefractive effect is the asym‐ metric energy exchange in two-beam coupling; when two laser beams interfere in a photorefractive material, the energy of one of the interfering beams is transferred to the other beam. The asymmetric energy exchange can be used to coherently amplify signal beams [4]; therefore, it has the potential to be used as a transistor for electrical circuits in a wide range of optical technologies (Figure 1). Optically transparent materials that possess both photovoltaic and electro-optic properties also exhibit the photorefractive effect. There have been a lot of studies to develop photorefractive materials, such as inorganic photoconductive ferroelectric crystals, organic photoconductive ferroelectric crystals, organic photoconductive polymers, amorphous organic photoconductive materials, photoconductive amphiphilic compounds, and photoconductive LCs [5-8]. Since 1991, organic materials have attracted significant interest because they exhibit large photorefractivity and a shorter response time compared with that of inorganic crystals [3, 7, 8]. The photorefractive effect induces a change in the refractive index by a mechanism that involves both photovoltaic and electro-optic effects (Figure 2). An organic photorefractive material is composed of a photoconductive compound, an electro-optic compound, and an electron-trap reagent. Thus, the active wavelength can be determined by the selection of the photoconductive compound. With interfering two laser beams in an organic photorefractive material, charge generation occurs at the bright positions of the interference fringe and the generated charges diffuse within the material. The distances of the diffusions of positive and negative charges are different in an organic material because the mobilities of the positive and negative charges are different. That leads to a formation of charge-separated state. The higher mobility charge diffuses over a longer distance than the lower mobility charge. While the charges with low mobility stay in the bright areas, the charges with high mobility diffuse to the dark areas. Thus, the bright area and dark area are charged with opposite polarities, and an internal electric field (space charge field) is formed in the area between the brightest and the darkest positions. The refractive indices of the areas are changed by the electro-optic effect. In such a way, a refractive index grating is produced. A high electric field of 10–50 V/μm is typically applied to a polymer film, aside from the internal electric field, to obtain photorefractivity. The polymer material is typically 100 μm thick, so that a voltage of 1–5 kV is required to apply on the film to achieve photorefractivity, which is almost comparable to the breakdown voltage of the polymer film. This electric field is necessary for increasing the

It is expected that a multiplex hologram display devices can be developed using photorefrac‐ tive polymers [9, 10]. It was shown that clear 3D images were recorded in the photorefractive polymer film. However, if polymer photorefractive materials are put into practical uses, a high voltage (3–5 kV) required to activate the photorefractive effect in polymer materials must be improved. There have been reports on the photorefractive effect of LCs [11]. LCs can easily be driven by a low electric field because LCs are liquid in nature. The most well-known LC phases are nematic and smectic phases (Figure 3). LCs that show nematic phase are used in liquid crystal displays (LCDs). However, LCs that show smectic phases are very viscous so that they are seldom utilized in practical applications. The LCs of which the photorefractivity was firstly investigated were nematic LCs [11]. It was found that a large photorefractivity was obtained

charge generation efficiency.

126 Ferroelectric Materials – Synthesis and Characterization


 

**Figure 3.** Structures of the nematic and smectic phases.
