Photorefractive Effect of Ferroelectric Liquid Crystals

The photorefractive effect of low-molecular-weight nematic liquid crystals has been investigated, and the photoinduced changes in refractive index in such crystals has been shown to be caused by changes in the orientation of liquid crystal molecules induced by the internal electric field. The effect of the reorientation of liquid crystal molecules on the apparent refractive index is particularly strong. Despite the apparent advantages of organic photorefractive materials, their response time is too slow (usually ~100 ms) for practical applications, attributable to the slow reorientation of chromophores. The use of high-Tg polymers has been investigated as one means of addressing this problem. In such stiff materials, the reorientation of chromophores is restricted, and refractive index modulation is solely due to the Pockels effect. Although rapid response times have been achieved in these materials, the reported diffraction efficiencies are lower than in low-Tg materials due to the absence of a supplementary orientational mechanism.
In order to obtain fast photorefractive response, ferroelectric liquid crystals (FLCs) were investigated. FLCs are classified as chiral smectic C (SmC*) liquid crystals that form a layered structure. In the SmC* phase, the long axis of each FLC molecule tilts from the layer normal. Spontaneous polarization (bulk polarization) appears when the FLCs in the SmC* phase are sandwiched between glass plates within a thickness of 2 ~ 10 micro-meter (in a surface stabilized state). When a DC electric field is applied to an FLC, the direction of the spontaneous polarization is governed by the polarity of the applied DC field. The tilting direction of the molecular long axis of FLC also switches in accordance with the direction of the spontaneous polarization. Since the switching of FLC molecules comes from the response of bulk polarization, the switching is extremely fast. Figure 1 shows a schematic illustration of the mechanism of the photorefractive effect in FLCs. In this scheme, the internal electric field alters the direction of spontaneous polarization in the area between the light and dark positions of the interference fringe, which induces a periodic change in the orientation of FLC molecules. This is different from the processes that occur in other photorefractive materials in that the molecular dipole, rather than the bulk polarization, responds to the internal electric field.

Figure 1

Ferroelectrics / Physical Effects　 (M. Lallart (Ed.) INTECH), Chapter 21, 487-506 (2011).
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