Deformed helix ferroelectric effect

The Deformed Helix Ferroelectric (DHF) effect was observed in the very first investigations of FLC structures [1, 92], but the first adequate explanation was given in [93-96]. The geometry of the FLC cell with a DHF effect is presented in Fig. 7.18. The polarizer (P) on the first substrate makes an angle / with the helix axis and the analyzer (A) is crossed with 

The light beam with the aperture a iZo passes parallel to the FLC layers through an FLC cell placed between the polarizer and analyzer. In an electrical field the FLC helical structure becomes deformed, so that the corresponding dependence of the director distribution cos as a function of coordinate 2 /110, oscillates symmetrically in E electric fields (Fig. 7.18). These oscillations result in a variation of the effective refractive index, i.e., electrically controlled birefringence appears. The effect takes place up to the fields of FLC helix unwinding 

The characteristic response times Tc (7.27) of the effect in small fields E/Eu -C 1 are independent of the FLC polarization Ps and the field E, and defined only by the rotational viscosity 7, and the helix pitch Rq 

The optical transmission of the DHF cell could be calculated as follows  

As shown in [4, 96] for small values of the applied field cosp E/Eu and changes its sign for the field reversal E = —E (Fig. 7.18). Thus according to (7.60) for 

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Fig. 13.11 Deformed Helix Ferroelectric effect. Scheme of observation of the effect (a) and the picture of distortion of the hehcal stmcture (b) in the zero, positive and negative field. P and A are polarizers and analyser, ITO means indium-tin oxide electrodes, Iq and I are intensities of incoming and outgoing beams. Note that at E = 0 the hehx is harmonic, for 0 < l l < anharmonic and for l l > l J unwound...

Helix Distortion and Deformed Helix Ferroelectric effect... 

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. 

This effect is observed in a geometry where the cholesteric axis h is homogeneously oriented in the plane of the cell (along x) and an electric field is applied to the electrodes of a sandwich cell along the z axis [137,138]. In this case, the helical structure, even the ideal one, is incompatible with the planar boundary conditions, and splayed and bended regions form near the boundaries. Thus, according to Eq. (38), the flexoelectric polarization arises in those regions which can interact with the electric field. The distortion is very similar to that observed in the ferroelectric smectic C phase (see Fig. 24) for a so-called deformed helix ferroelectric effect [139]. 

Besides the ambitious efforu to arrive at well-aligned SSFLC structures, some work has been dedicated to other electrooptic effects as are well known with LMM FLCs, such u the electroclinic (SMFLC). the antifenoclectric (AFLC), and the deformed helix ferroelectric effect (DHF). 

Further research work has been devoted to creating new FLC polymers with multifunctional properties. Recently, Scherowsky reported on fluorescent FLC polymers [136]. The endeavor to produce FLC polymers with strong SHG activity will be dealt with in Section VI. So tar we have discussed variouB applications with an eleclrooptic effect aiudogous to the SSFLC Qark-Lagerwall effect observed on LMM FLCs. htoy of these applications are conceivable with other electrooptic effects such as the antifer-roelectric. electrodink. or deformed helix ferroelectric effect, in particular if gray scale is required. 

Until the mid-1990s and after 20 years of intense research on nematic field-effect LCDs it was still uncertain whether LCDs and LC materials could indeed meet the short response time requirements and the optical quality required for LCD television. Therefore, parallel to nematic LCD research, strong efforts were made to find effects based on the inherently faster responding ferroelectric liquid crystals (FLCs). Unfortunately, FLCs proved to be difficult to surface-align, rendering them up to now commercially applicable only for niche products such as electronic eye shutters or time sequential LCD projection. FLC examples are the surface-stabilized ferroelectric (SSF)-LCD of Clark and Lagerwall [40] which initiated FLC-LCD development and the deformed helix ferroelectric (DHF)-LCD of Beresnev et al. [41], In 1995 a TFT-addressed black-white DHF-LCD television prototype with 20 ps response time and broad field of view was developed by the author and coworkers in collaboration with Philips [42] (Fig. 6.5a). 

The deformed helical ferroelectric (DHF) effect. If the voltage applied to the smectic C phase is lower than the untwisting field value, the helix is not completely suppressed but only distorted (Fig. 14). For a square voltage, there will be an alternation between two deformed helical states, and optically it appears as switching of the refractive index ellipsoid [6,121). In contrast to ferroelectric switching, the response time for the DHF effect is independent of... 

It should be noted that phenomenologically this effect is analogous to the deformed helix electrooptical mode observed in ferroelectric liquid crystals where coupling of an electric field with the spontaneous (instead of flexoelectric) polarization is used [83, 84]. 

The scattering effects observed during the deformation of the ferroelectric helix have not yet been satisfactorily investigated [115]. For instance, one should explain the correlation between temperature dependence of the helix pitch and intensity of the scattered light [113], as well as the effect of FLC physical parameters on the response times and hysteresis behavior of transmission-voltage characteristics. Moreover, these effects have not been studied in commercial FLC mixtures operating at room temperature. Nevertheless, these electrooptical modes might be useful for applications in nonpolaroid FLC displays for realization of the optical memory, etc. 

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