Voltage-induced birefringence

Benoit (1) performed a calculation similar to that of O Konski at a somewhat earlier date and included equations for the rise and decay of the birefringence under the action of a rectangular voltage pulse. Since the rise time depends on both permanent and induced dipole moments, it is a complex function involving more than one time constant and rather difficult to deal with experimentally. The decay time of birefringence, however, depends only on the molecular dimensions and for a rigid rod Benoit obtained the simple formula... 

We discuss the splay-bend distortion induced by an electric voltage applied to a cell similar to that shown in Fig. 11.16 using two transparent electrodes at z = 0 and z = d. The distortion is easy to observe optically for the cell birefringence. The splay-bend cell behaves like a birefringent plate discussed in Section 11.1.1 but now the plate birefringence is controlled by the field. The optical anisotropy... 

To find the value of the elastic moduli ratio Kss/Ku it is also possible to measure either the birefringence 6 = And/X And is the optical path difference and the A is the wavelength of light), or the capacitance C versus voltage U curve in the electrically induced Frederiks transition [55]. 

FIGURE 6.29. Electrooptical modulation by a blue phase, (a) Normalized field-induced birefringence at voltage 170 V and frequency 50 Hz (the position of an analyzer was adjusted at each temperature), (b) Response time versus temperature. 

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. ... 

With field-induced birefringence, variable colors are transmitted when the voltage exceeds the critical value. These angle-dependent color effects are unavoidable and constitute one of the main disadvantages of the induced birefringence effect. In the twisted nematic... 

Fig. 9—Various threshoid phenomena for nematic fiuids with negative dieiectric anisotropy and perpendicuiar alignment. The dashed horizontal line is the threshold voltage for induced birefringence. The curved solid line describes the frequency dependence of the threshold voltage for domains. The sloped dashed lines are the threshold plots for chevron formation. The material is MBBA at 25 C (Ref. [86]).

The different electro-optic phenomena have been classified into those that involve only dielectric forces and those that depend upon the interaction of conduction and dielectric torques. The field-effect phenomena possess several common properties. The resistivity of the materials may be as high as chemically practical, i.e., p 10 ohm-cm. For the induced birefringence, twisted nematic, and guest-host color switching effects, the threshold voltages are less than 3 or 4... 

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