SSFLC cell

Figure 8.8 Structure and phase sequence of (R)-MHPOBC is shown. One of most famous smectic LCs, antiferroelectric switching in SSFLC cells was first discovered with this material.

Figure 8.34 Left Gold focal conics of MHOBOW coexisting with accordion domains in 4-p.m SSFLC cell. Cell has not seen electric field. Right Same area after brief application of field above threshold for causing textural change of focal conics from gold SmA-like to bistable blue SmC -like. Transition from gold to bistable blue is still incomplete in this photomicrograph clear domain walls between two textures are easily seen.

Figure 8.37 Ferroelectric switching and lack of brush rotation illustrated for SiiiCaPf phase exhibited by KYOBOW in SSFLC cell.

Figure 1. The geometry of a parallel-aligned SSFLC cell. Note that the spacing between the glass bounding plates (= 1.5 pm) and the smectic layer spacing (= 35 A) are not to scale.

Fig. 5.10.9. The SSFLC cell The bookshelf geometry of a thin film of smectic C sandwiched between two glass plates (a) field up (normal to the plane of the diagram) and (b) field down .

It can be seen clearly from Fig. 15 that MSFLC cells offer many advantages. For example, a large gap of 10 Xm can be used [109, 130] instead of 2 pm used in SSFLC cells [131]. The polymeric nature of the LC materials offers excellent shock resistance and mechanical stability. No rubbing of the substrate is needed. We believe that practically useful electro-optical cells can be made utilizing this concept if the FLC-coil diblock copolymers are properly designed, synthesized, and processed. 

We can answer the last question if consider a constraction of the so-called surface stabilised ferroelectric liquid crystal cell or simply SSFLC ceU [9]. Such SSFLC cell is only few micrometers thin and, due to anchoring of the director at the surfaces, the intrinsic helical stmcture of the SmC is unwound by boundaries but a high value of the spontaneous polarisation is conserved. The cell is con-stracted in a way to realise two stable states of the smectic C liquid crystal using its interaction with the surfaces of electrodes, see Fig. 13.6a. First of all, in the SSFLC cell, the so-called bookshelf geometry is assumed the smectic layers are vertical (like books) with their normal h parallel the z-axis. Then the director is free to rotate along the conical surface about the h axis as shown in Fig. 13.6b (Goldstone mode). It is important that, to have a bistability, the director should be properly... 

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...

We can use Eqs. (13.13) and (13.14) and find parameters a, %j and p in the SmA phase. For this we need slow, automatically made temperature scans through the A —> C phase transition with simultaneous measurements of SSFLC cell capacitance, i.e. Xsm(T) and the electrooptical response i.e., field induced angle 9(7 at frequency 0.1-1 kHz. Then the asymptotic behaviour of capacitance at temperature T > provides us the value of dielectric constant and susceptibility Xx = (e - and the ratio XsJ c = PX us the coupling constant p in... 

Fig. 13.12 Clark-Lagerwall effect in thin SSFLC cell. Application of the electric field E between the ITO electrodes causes up-down switching of spontaneous polarization accompanied by conical motion of the director n. The projection of the n-vector on plane xy is C-director forming an angle

As to dynamics of the response of a SSFLC cell to the alternating field, it is controlled by Eq. (13.38) with the viscous j dipldt torque added. When the helical stmcture of the SmC is unwound in a thin cell (a typical case) one can neglect the elastic torque d

anchoring energies Wf and W are reasonably weak, the electric field torque would solely be balanced by the viscous torque ... 

At high frequencies of the a.c. field, the total polarisation of the entire sample is switched very fast and the ground, antiferroelectric state may be bypassed. Then the switching occurs between the two ferroelectric states as in an SSFLC cell. With increasing frequency (for example, from 100 Hz to 10 kHz) the double hysteresis loop is substituted by a single loop typical of ferroelectrics as shown in Fig. 13.16 by the solid and the dashed curves. 

Fig. 1.4 Sketch of the surfaee-stabilized feiroelectric liquid crystal (SSFLC) cell structure. Due to the surface-stabilization, the helical structure of the SmC phase is unwound as only two director orientations on the tilt eone can be realized. These two director states correspond to either UP or DOWN polarization (redrawn after [15])...

There are more possibilities for modulating light propagation using ferroelectric liquid crystals, particularly ferroelectric LC polymers, besides the ferroelectric switching in an SSFLC cell as shown in Fig. 32b. 

FIGURE 32 (a) Ferroelectric switching in an SSFLC cell (b) reorientation of a single mesogenic moiety. 

Four states, CIU (Cl-uniform), CIT (Cl-twisted), C2U (C2-uniform), and C2T (C2-twisted), were found in SSFLC cells by investigating many samples aligned by various alignment films involving parallel rubbing [18]. These four states are expressed by a combination of the above two classifications. 

Figure 8.7. Idealized configuration of the SSFLC cell. Depending on the polarity of the applied dc field E, the director n switches between two different positions on the tilt cone, corresponding to two different positions of the optical axis in a plane parallel to the cell plates. In real devices the smectic layer planes are not perfectly perpendicular to the cell plates but show a chevron structure with a kink inside the cell.

The Clark-Lagerwall Effect. This effect is observed in thin surface-stabilized FLC (SSFLC) cells where the smectic layers are perpendicular to the substrates, the thickness is less than the helical pitch (delectric field of opposite polarity switches the direction of the spontaneous polarization between the UP and... 

Figure 23. The Clark-Lagerwall effect. In a thin SSFLC cell the electric field of opposite polarity. switches the spontaneous polarization between the UP and DOWN positions, which correspond to the LEFT and RIGHT positions of the director.

DOWN positions which correspond to the LEFT and RIGHT positions for the director that moves along the surface of a cone, the axis of which is normal to the layers and parallel to the cell substrates. In the LEFT and RIGHT positions the director remains parallel to the substrates and the SSFLC cell behaves as a uniaxial phase plate. The total angle of switching equals the double tilt angle 6. 

For weak anchoring conditions, two regimes of switching exist in SSFLC cells, separated by threshold field E [180] ... 

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