Electroclinic effect nematics

When the helical structure of the chiral nematic phase is unwound by the influence of limiting walls, we can observe a linear-in-field light modulation which is caused by a small molecular tilt [85]. The effect is analogous to the electroclinic effect observed in the smectic A phase as the pretransitional phenomenon in the vicinity of the transition. 

The combination of a computer and an electronically addressable two-dimensional spatial light modulator is a good device in performing highspeed computations. The modulators could be used for basic logic operations while a computer could be used for programming and memory [57]. The operation speed ranges from 10 ms (pixel for nematics) to 0.5 /iS (pixel in electroclinic effect in smectic A liquid crystals), see Chapter 7. 

Alternatively, an external (electric) field can be used to change the orientation of the LC director inside the network. The network will then reorient and produce a shape change. This effect can be observed either in LC actuators made from highly swollen nematic systems [7,9,21,22,185] or in bulk LCEs with ferroelectric phases (see Sect. 3). In LCEs with ferroelectric phases, the electroclinic effect... 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

When we apply an electric field across an FLC cell there will always be a dielectric torque acting on the director, in addition to the ferroelectric torque. This is of course the same torque as is present in all liquid crystals and in particular in nematics, but its effect will be slightly different here than it is in a nematic. This is because in the smectic C phase the tilt angle 0 is a hard variable, as earlier pointed out. It is rigid in the sense that it is hardly affected at all by an electric field. This is certainly true for a nonchiral smectic C and also for the chiral C, except in the immediate vicinity of Tc a. where we may not neglect the electroclinic effect. Hence we will consider the tilt angle 6 uninfluenced by the electric field, and this then only controls the phase variable <(> of Fig. 60. 

Table 8.7 shows that the parameters of the prototype light valve (CdS-nematic) are much worse than that of the a Si-FLC device. The operation speed of the latter comes closer to the solid electrooptical crystal modulator (PROM), but with a considerably higher resolution. Liquid crystal light valves on a Si-FLC operate using the Clark-Lagerwall mode [21], the electroclinic eflFect [22], or the deformed helix ferroelectric effect [24]. The operation speed in the two mentioned cases could be 10-100 times faster than mentioned in Table 8.7. 

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