Devices birefringence effect

For the determination of the birefringence effect, polarizer and analyzer are turned by an angle of 45° in their fittings. In this way, the maximum phase difference caused by the total reflection on the mirror device, is added to the phase difference caused by the flowing polymer. This mirror effect, however, can simply be subtracted. It can be calculated with the aid of an exact theory and it can also be measured, when the plate is at rest. For further details the original paper should be inspected (17). 

SEE Supertwist birefringence effect device effect in a 180° to 270° twisted nematic structure that uses a combination of interference and polarization guiding to achieve an optical effect with sharp threshold behavior, making it particularly suitable for multiplexing. 

Compared to the conventional transmissive TN LCD, the above transflective TN LCD only requires one additional transflector between the bottom polarizer and the TN LC layer. Naturally, this transflective LCD device configuration can also be extended to an STN-based transflective LCD [42]. In contrast to the so-called polarization rotation effect in TN LCD, the STN LCD uses the birefringence effect of the super-twisted nematic LC layer [43]. Therefore, a larger twist angle (180 270°), a thicker LC cell gap, and a different polarizer/analyzer configuration are required. 

Birefringence effect can be exploited for all-optical switch devices. A simple all-optical switch based on photinduced birefringence effect has been demonstrated in azo dye (DRl) doped polymer (PMMA) films. The all-optical switching effect has... 

In addition to the above three effects, there are two others i.e, memory birefringence and depolarization scattering, which exist in the PLZT materials and have been proposed for device applications. These are described in reference 5. 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

The Kerr effect is the result of applying an electric field to produce birefringence. This phenomena is commonly observed for both colloidal and polymeric liquids and is used in the characterization of the structure of these materials. Alternatively, by using an AC electric field, a modulation of the polarization of light can be affected. Such devices have rarely been used as modulators but do have the potential of allowing higher frequencies than the more common photoelastic devices. 

Retarders are usually devices which rotate the polarization plane of radiation or convert linearly polarized radiation into a elliptically or circularly polarized one. Their basic physical function consists in decomposing the electric vector of the linearly polarized radiation into two mutually orthogonally polarized components between which a phase difference retardation) is created. Depending on the physical phenomenon that causes the retardation effect practical retarders based on birefringence and total internal reflection are known and used. 

Ceramic PLZT has a number of structures, depending upon composition, and can show both the Pockels (linear) electro-optic effect in the ferroelectric rhombohedral and tetragonal phases and the Kerr (quadratic) effect in the cubic paraelectric state. Because of the ceramic nature of the material, the non-cubic phases show no birefringence in the as-prepared state and must be poled to become useful electro-optically (Section 6.4.1). PMN-PT and PZN-PT are relaxor ferroelectrics. These have an isotropic structure in the absence of an electric field, but this is easily altered in an applied electric field to give a birefringent electro-optic material. All of these phases, with optimised compositions, have much higher electro-optic coefficients than LiNb03 and are actively studied for device application. 

The most exciting aspect of this research is the possibility to use these properties (photoinduced birefringence, surface gratings and two-beam coupling) in photonic devices design, given the simplicity of the material and of the necessary preliminary manipulation in order to obtain the desired effects. 

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