Bookshelf alignment

Fig. 13.6 SSFLC cell. The structure of the cell with bookshelf alignment of smectic layers (a) and the cone of the director n motion with two stable states 3 in the electrode plane yz (b). Note that in sketch (a) the director in the cell plane yz is turned to the reader through angle + (shown by thicker right parts of the rod-like molecules) in agreement with sketch (b). The double-head arrow shows the optimum angular position of polarizer P...

Mauzac et al. [28] oriented their networks at the isotropic to liquid crystalline phase transformation by applying an electric poling held [28]. However, the bookshelf alignment is not perfect. Finally Gebhard and Zentel [29] realized ultrathin free-standing ferroelectric Aims using a method of Skarp [30]. The prepolymer was spin-coated on an NaCl plate and photo-cross-... 

Illustration of the helical ground state (b) and the field-induced unwound state (a and c). The boundary plates are shown so that they are corresponding to the bookshelf alignment. 

If zigzag defects already exisL they can be removed by an adequate AC electric field treatment which causes a change from the chevron structure to a slightly bent layer structure [24,25] which is termed "quasi-bookshelf." Alternatively, a quasi-bookshelf alignment has btwn obtained for spe dasses of FLC materials [26]. To summarize, there are at present four different optical structures which avoid the occuneooe of zigzag defects and are considered for applkation ... 

Unfortunately, the ideal bookshelf structure is difficult to make. Usually the electrodes are covered by polymer layers and rubbed unidirectionally. This provides a good alignment of the director along the electrodes and the bookshelf ... 

Theoretical consideration. We shall consider a simple and instructive theory of the switching of a helix free antiferroelectric phase [36]. The smectic layers normal h is aligned along the rubbing direction z in the plane of the cell (bookshelf geometry in Fig. 13.25). The tilt has the amplitude 9 and its phase O changes... 

The orientations of the molecules of the FLC materials are classified by the presence or absence of a helical structure. The most famous FLC device is the SSFLC (Surface Stabilized FLC) [1], in which the helical structure of the FLC material is unwound. While a variety of molecular orientations have been applied in SSFLC devices, three molecular orientations appear to be the most useful in practical FLC displays.These are the bookshelf-layered structure and the Cl-uniform (CIU) and the C2-uniform (C2U) orientations [2]. Each of these structures shows monostability or bistability, depending on the material and its alignment properties. The monostable orientations are applicable to active matrix FLC displays while the bistable orientations are applicable to passive matrix FLC displays. FLC displays with a helical orientation have also been investigated. One useful FLC mode with the helical orientation is the DHF (deformed-helix ferroelectric) mode [3]. This mode is monostable and is thus suitable for an active matrix drive method. 

Now the question is why and how molecules choose one-half of the cone for the switching. If the smectic layer is of bookshelf type, there is no reason to choose either half of the cone. However, there exists a chevron structure, as was confirmed using a separate cell by X-ray analysis. Then molecules can choose one-half (actually less than 180°) of the azimuthal angle on the cone, since molecules have a tendency to align themselves parallel to a substrate surface, as illustrated in Figure 9.33 E = 0). If the volume of the upper and lower halves of the chevron structure is not the same, SHG would be observed. This situation is illustrated in Figure 9.33 The liquid crystal molecules rotate toward the opposite directions in the upper and lower halves of the chevron structure. 

The FLC SLM is constructed as a thin layer (approx. 2 pm thick) of smectic C FLC sandwiched between two transparent electrode arrays, as in Fig. 5 a. The molecules are usually arranged in the bookshelf geometry and are all aligned to the glass electrodes by alignment layers which the FLC molecules follow. 

The quasi-bookshelf smectic layer structure is realized and the twisted state of the director alignment is inherently avoided. Hence the relatively high contrast ratio of 20-30 is easily obtained. 

Self-recovery from alignment damage caused by mechanical and thermal shoeks is observed during operation. This results from the reversible layer switching between the bookshelf and the quasibookshelf structures, which occurs in the network of characteristic stripe defeets parallel to the layer normal. 

A homogeneous planar surface alignment is imposed by shearing the sample (or by appropriate alignment layers), whereby the smectic layers develop perpendicular to the two confining glass plates, forming the so-called bookshelf structure. 

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