Electrooptical Switching

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. 

FIGURE 7.33. Tilt angle relaxation times (dynamics of the soft-mode) as functions of the temperature for polymer (7.iv). 

1 mS at an enhanced temperature (about 100 C) [170]. The same substance allows the color switching in the guest-host mode [173]. 

Helix unwinding and free relaxation from the unwound state back to the helical structure requires a much longer time (r 1-100 s, within the range Tc a — T = 40-10 °C, respectively) because, in this case, a lot of defects are expelled from the sample and appear again. This process is governed by some apparent flow viscosity which is much higher than 7 .  

In conclusion, we can state that the field behavior of polymer ferroelectric liquid crystals is very similar to that of the low molecular mass compounds. It can be interpreted in the framework of the theory discussed above (Sections 7.1, 7.2) with the high viscosity coefficients 7, (and others) taken into account. The other physical parameters (spontaneous polarization, the electroclinic coefficient, and elastic moduli) are of the same order of magnitude as for their low molecular counterparts. 

---

It is also possible to switch a single picosecond pulse out of the train of mode-locked pulses using an electrooptic switch. It is possible to obtain a single pulse having duration in the picosecond regime or even less. Pulses with durations in the regime of a few hundred femtoseconds (10 s) are also available (Fig. 4e). 

This chapter gives an overview of the origin of NLO effects and focuses on the design of chromophores for two current applications of technological interest poled polymer materials for electrooptic switching and two-photon absorption. 

Interfacial monolayer, multilayer and polymer species which exhibit interesting examples of light and electrically stimulated functions such as isomerization and proton transfer in ISAs are also presented in this chapter. Such materials may represent the precursors for electrooptic switches and addressable molecular-based machines. 

M. Wahle, O. Kasdorf, H. Kitzerow, Y. Liang, X. Feng, K. Mullen, Electrooptic switching in graphene-based liquid crystal cells. Mol. Cryst. Liq. Cryst. 543, 187-193 (2011)... 

J.R. Bruckner, J.H. Porada, C.F. Dietrich, 1. Dierking and F. Giesselmann, A lyotropic chiral semctic C liquid crystal with polar electrooptic switching. Angewandte Chemie International Edition 52, 8934-8937 (2013)... 

A combination of two dyes (with positive and negative dichroism) has been used [177] for color switching in the guest-host effect. The degree of dye order was 5dye = 0.677 and 0.754 for anthraquinone D /D > 1) and tetrazine (T) /D ) dyes, respectively. High values of color contrast were thus achieved on electrooptical switching. 

FIGURE 4.45. Electrooptical switching in PDLC films, (a) and (b) off and on states (c) PDLC with good viewing angles. The refractive indices of the side-chain nematic polymer are similar to those of low-molecular mass nematic in droplets. 

FIGURE 7.17. Influence of a space charge on electrooptical switching in the Clark-LagerwaU effect. 

An unusual effect was observed in an antiferroelectric liquid crystal (AFLC) [116-125]. The switching is associated with the appearance of the third state, in addition to the two bistable up and down states known for the Clark-Lagerwall effect. The corresponding hysteresis of electrooptical switching is shown in Fig. 7.23. We can see that the third state at the zero field is stable, and can be transformed either into up or down states if not applying a rather high switching field. 

FIGURE 7,23. Tristable electrooptical switching. Above hysteresis of the apparent tilt angle, which correlates with the average value of the polarization in the cell versus the applied voltage. Below the microscopic interpretation of the antiferroelectric state in the helical and imwound versions. 

FIGURE 7.26. Electrooptical switching of FLC cells for different polar anchoring energies Wp. Short pulses of the electrical field E with different signs are applied... 

Angle selective enhancement of beam deflection in high-speed electrooptic switches. IEEE Photonics Technology Letters, Vol. 19, No. 9, pp. 701. 

Molecular ordering and electrooptic switching in chiral smectic phases are discussed in detail elsewhere (see Chap. VII, Sec. 9 of this Volume). In the case of ferroelectric switching in chiral smectic C phases, the direetor n is free to move about a cone of angles which is centered on the horizontal axis. Each molecule has a ferroelectric dipole P, which is perpendicular to its length. This is depicted in Fig. 3. 

In Sec. 2.6.4 we derived the scaling law for the cone mode viscosity with respect to the tilt angle 6. In this section we want to penetrate a little deeper into the understanding of the viscosities relevant to the electrooptic switching dynamics. Let us therefore first review the previous result from a new perspective. With 0 = const, an electric torque will induce a cone motion around the z axis (see Fig. 75a). We can describe this motion in different ways. If we choose to use the angular velocity of the c director, that is, with respect to the z axis, then we have to relate it to the torque component... 

Phases having biaxial symmetry (tilted smectic phases) exhibit dielectric biaxiality in particular. At frequencies of 1 MHz and below, the biaxiality becomes important and critically influences the electrooptic switching behavior of the SmC phase. It is therefore important to be able to measure the biaxiality at these frequencies. Being a symmetrical second rank tensor, the dielectric permittivity can always be diagonalized in a proper frame and described by three components along the principal directions. The three principal values can then be expressed by a single subscript and can be determined by three independent measurements performed at three different orientations of the director relative to the measuring electric field. In practice, this may not be that... 

An extension of the idea to achieve fast electrooptical switching by using oligomers resulted in the synthesis of chiral twin dinaers 35. composed of ferroelectric mesogens... 

The hooiopolyiner and ibc copolymer conuining S% azo dye monomer show nearly identical phase-transition temperatures. With 15% dye content tte transition temperatures deaease. The electrooptical switching starts at 60 in comparison with 8(fC for the homopolymer, and is faster at the same temperatures. This copolymer exhibits switching in three different smectic phases, as shown in Figure 42. 

---