Twisting ferroelectrics

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Surface Stabilised Ferroelectric Super Twisted Nematic temperature (degrees centigrade)... 

The twist grain boundary smectic phase was discovered serendipitously at Bell Laboratories in 1987. Its discovery followed the back-tracking of a number of decisions made concerning the development of ferroelectric liquid crystals for display device applications. 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

The most studied chiral smectic phase is ferroelectric SmC phase [18], which is derived from Smectic C (SmC) phase. As shown in Fig. 5.2, the helical twist in SmC results from chiral organization of smectic layers as similar to the formation of N from nematic layers mentioned above. The molecules in each smectic layer... 

Surface-stabilized ferroelectric liquid crystal Splay-twist Supertwisted nematic Transmission electron microscopy Twisted nematic Thin film transistor Uniform lying helix Ultraviolet... 

SmC C2 X T(2) Optically active chiral analogy of SmC phase showing macroscopic periodicity with twist axis perpendicular to smectic layers. Quasi-long-range positional order along the layer normal and two-dimensional liquid-like structure within the layer plane. Single layers of the same symmetry may form diffcaent phases in the bulk ferroelectric (SmC ), antiferroelectric (SmCA ) and ferrielectric (SmCy ). 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

Using constraint director dynamics, McWhirter and Patey [206] also determine the shear and twist viscosities describing the coupling between the pressure and shear rate tensors and the Miesowicz viscosities (linear combinations of the former) and show that the latter are qualitatively similar to those of a ferroelectric tetragonal 1 lattice in accord with the fact that the short-range spatial correlations in the ferroelectric liquid state are similar to those of the tetragonal lattice structure [102]. 

Both CdS and a-Si have been successfully used as the photocondoctor 45°-twisted nematic layers and, on an experimental basis, ferroelectric layers have been used for the liquid crystal. CCD structures and silicon vidicon microdiode arrays have been used in place of the photocon-ductive layer. The device is useful both when the write beam is coherent (for example, a scanned laser) and when it is incoherent (for example, a CRT). In the latter case, the SLM can be used as an incoherent-to-coherent converter. The CRT-written device has also found application as a projection display. There exists a very large potential market for optically addressed SLMs in a variety of optical processing applications and for projection displays. 

S-W. Cheong, M. Mostovoy, Multiferroics a magnetic twist for Ferroelectricity, Nature Materials, 6, 13, (2007). 

This problem is overcome by Clark and Lagcrwall in their invention of the surface-stabilized ferroelectric liquid crystal (SSFLC) device [16], shown in Figure 4.9. The liquid crystal is sandwiched between two parallel substrates with the cell gap, h, thinner than the helical pitch, P, of the liquid crystal. The inner surface of the substrates is coated with alignment layers which promote parallel (to the substrate) anchoring of the liquid crystal on the surface of the substrate. The smectic layers arc perpendicular to the substrate of the cell, while the helical axis is parallel to the substrate. Now the helical twist is suppressed and unwound by the anchoring. 

Even though the first SmC materials were synthesized at the beginning of the 20th century [10], it took decades until the macroscopic chirality of the SmC phase was discovered. The existence of a hypothetical twisted smectic phase was first discussed by Saupe in 1969 [11]. Two years later, in 1971, Helfrich and Oh [12] detected the SmC phase as such for the first time due to its ability to selectively reflect light. The ferroelectricity of the SmC phase was then theoretically predicted, explained and experimentally proved by Meyer et al. [13] in 1975 for the first time. Five years later, Clark and Lagerwall published their groundbreaking work [14], which demonstrated the ferroelectric switching of the SmC phase if surface-stabilized. 

Study of potential chirality effects like the helical twist of the tilt-direction and ferroelectricity in the lyotropic analog of the SmC phase. 

A smectic C with a chiral agent can also exhibit a helical twist, but of just the out-of-plane component of the molecular alignment direction. This phase, called smectic C, is ferroelectric and has been used to make fast-switching display panels. 

As mentioned earlier, the helical structure of the smectic C phase should be untwisted by an electric or magnetic field, or suppressed by a surface effect, to observe ferroelectric properties of the phase. In the first publication on ferroelectric liquid crystals [5] an approach to nontwisted ferroelectric LC materials was suggested. By mixing two individual ferroelectric liquid crystals having opposite signs of P but different absolute values, one can compensate the helical twisting without zeroing the polarization. That has been done for low-molar-mass liquid crystals... 

Thus it passes through the top polariser and the cell appears bright. Because the switching in both directions is driven by the interaction of the spontaneous polarisation with the electric field (rather than the interaction between an induced polarisation and the electric field or a relaxation process when the electric field is removed), these surface stabilised ferroelectric hquid crystal (SSFLC) displays are much faster than twisted nematic and birefringent displays. 

Smectic C Chiral smectic C with twist axis normal to layers (symmetry C2, ferroelectric properties). 

Smectic H Chiral smectic H with twist axis normal to the layers. A ferroelectric. 

Mixing two ferroelectric liquid crystals with an opposite sign of chirality and polarization we can compensate for both the twist and polarization. If the two compounds are chemically different, the two compensation ( magic ) points occur for different composition of the mixture. Fig. 1.20 [56]. With zero twist we can have a spontaneously polarized liquid crystal, that is, a nonhelical ferroelectric. The reason is that different molecular mechanisms are responsible for the twist and polarization. The pitch is determined by a chiral interaction of all types of molecules, and the polarization depends not only on chirality but also on the steric and electrical dipoles as well. The simultaneous compensation for a helix and polarization occurs only in racemic mixtures. 

The process of the director reorientation in polymer ferroelectrics, as in their low-molecular counterparts, involves changes in the tilt 0) and azimuthal (f) angles. These two modes are characterized by quite different rates. The 6 process corresponds to the soft-mode distortion, and the corresponding time To diverges at the C A phase transition point. The process means the motion of the director over the conical surface around the normal to the smectic layer (the Goldstone mode). In the helical structure the latter involves the twisting-untwisting mode, tq and differ considerably from each other, because backbones participate in those modes to a different extent. This can be seen in the dielectric spectra [172], and in the pyroelectric and electrooptical response. 

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