Birefringence field induced

The perpendicular texture is optically isotropic to light propagating perpendicular to the cell walls. Consequently, with crossed polarizer and analyzer, no light is transmitted through the analyzer. Dur- 

The transmitted light intensity is maximum when 0 = 45 . Normally, the angle 0 is not well-defined because of the cylindrical symmetry that results when a perpendicularly aligned fluid is deformed by the electric field (see Fig. 2b). However, as described in the surface 

The maximum value of An(V) is the index of refraction anisotropy, An = n II — n . Typically, An is about 0.2 to 0.3 This anisotropy is so large that, for monochromatic radiation, the transmitted light intensity undergoes many maxima and minima as the voltage increases above threshold. With white light, variable colors can be observed as a function of voltage. 

To first order, the frequency response of induced birefringence is constant in amplitude from low frequency to the molecular dispersion frequency in the dielectric constant where the dielectric anisotropy changes. This property is typical of all the field effects. 

For a nematic material with positive dielectric anisotropy, induced birefringence can also be observed. However, the liquid crystal must be in the uniform parallel orientation at zero volts.Above the threshold voltage, the director aligns itself parallel to the applied field. With crossed polarizer and analyzer, the voltage dependence of the light intensity is reversed from that described previously for a fluid of negative dielectric anisotropy.  

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NONLINEAR DYNAMIC SUSCEPTIBILITIES AND FIELD-INDUCED BIREFRINGENCE IN MAGNETIC PARTICLE ASSEMBLIES... 

A. Dynamic Field-Induced Birefringence in the Framework of the Egg Model Interplay between Mechanical and Magnetic Degrees of Freedom of the Particles... 

B. Higher Harmonics Generation in Field-Induced Birefringence by a Suspension of Rigid Dipoles Zero Bias Field... 

Nonlinear Dynamic Susceptibilities and Field-Induced Birefringence in Magnetic Particle Assemblies 419... 

Fig. 6 Magnetic field-induced birefringence and transmission for a poly-(IV-5-COEN) sample in various magnetic fields...

The rod-to-coil transition, i.e. a pure single chain process was first considered from spectroscopic results in the visible and infrared which, however, could not prove alone the absence of aggregation. A more convincing experiment, in which experimental results contain some structural information, is the field-induced birefringence. From a field-induced birefringence experiment in yellow and red solution at concentration as low as 5 x 10 to 5 X 10 g cm, it has been argued that a rod-like conformation was observed when the solution turned... 

In electric-field-induced birefringence (or electrical birefringence, quadratic electrooptic effect, Kerr effect), an isotropic transparent substance becomes birefringent when placed in an electric field . The sample assumes the characteristics of a uniaxial crystal, the optical axis of which is parallel to the direction of the applied field. When the sample is illuminated normal to , the resulting two indices, and wj (which can be thought of as He and ), are associated with the parallel and perpendicular orientations, respectively. 

As discussed in the above section, blue phases allow Bragg reflection of visible Ught Therefore the transmittance is less than 100%. The precise treatments of the optics of blue phases are complex and are given by Belyakov, Dmitrienko, Homreich, et al. [32-35]. Here we only give some qualitative discussions, mainly for the purpose of understanding blue phase display based on electric field induced birefringence. 

Second, we consider the optical anisotropy of the blue phases. Generally speaking, the refractive indices of a crystal form an ellipsoid, as discussed in Chapter 2. Now the blue phases have cubic symmetries. On a macroscopic scale, the refractive index ellipsoid must have the same cubic symmetries. Cubic symmetries contain four-fold rotational symmetry around three orthogonal axes. Therefore the refractive index ellipsoid must be a sphere, that is, the refractive index in any direction is the same at macroscopic scale. Due to this optical isotropy, when a blue phase sample is sandwiched between two crossed polarizers, the transmittance is zero. This is the dark state of the blue phase display based on field induced birefringence. 

An optically isotropic liquid crystal (LC) refers to a composite material system whose refractive index is isotropic macroscopically, yet its dielectric constant remains anisotropic microscopically [1]. When such a material is subject to an external electric field, induced birefringence takes place along the electric field direction if the employed LC host has a positive dielectric anisotropy (Ae). This optically isotropic medium is different from a polar Uquid crystal in an isotropic state, such as 5CB (clearing point = 35.4°C) at 50 C. The latter is not switchable because its dielectric anisotropy and optical anisotropy (birefringence) both vanish in the isotropic phase. Blue phase, which exists between cholesteric and isotropic phases, is an example of optically isotropic media. 

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