Director reorientation, optical properties

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

The electro-optic effects described above all refer to director reorientation induced through the dielectric properties (Af) of the polymer. If, however, a change in the optical properties is induced by a current flow resulting from the anisotropic electrical conductivity (A a) of the material, then the so-called electro-hydrodynamic effects are observed. ... 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

The basis of the majority of specific liquid crystal electrooptical effects is found in the reorientation of the director (the axis of preferred orientation of the molecules) in the macroscopic volume of the material under the influence of an externally applied field or the fiow of the liquid. Anisotropy of the electrical properties of the medium (of the dielectric susceptibility and the electrical conductivity) is the origin for reorientation, whereas the dynamics of the process also depend on the viscoelastic properties and the initial orientation of the director of the mesophase relative to the field. The optical properties of the medium, its local optical anisotropy, are changed as a result of this reorientation of the director (either occurring locally or throughout the whole of the sample) and underlies all the known electrooptical effects. 

In general, the distortions on the electronic wave function of liquid crystal molecules caused by an applied field do not cause appreciable change to its contribution to the refractive indices (see Chapter 10). However, the orientation of the molecules can be dramatically altered by the apphed field. This process alters the overall optical properties of the medium and is the principal mechanism used in liquid-crystal-based electro-optical devices. As noted in Section 6.2.2, the electrically induced orientational refractive index changes could be Pockel or Kerr effect. In this and the next sections, we shall focus on nematic liquid crystals in which the director axis reorientation is a Kerr-like effect that is, the process is quadratic in the applied field. 

Smectic elastomers, due to their layered structure, exhibit distinct anisotropic mechanical properties and mechanical deformation processes that are parallel or perpendicular to the normal orientation of the smectic layer. Such elastomers are important due to their optical and ferroelectric properties. Networks with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal with respect to the normal smetic direction are mechanically deformed by uniaxial and shear deformations. Under uniaxial deformations two processes were observed [53] parallel to the direction of the mechanical field directly couples to the smectic tilt angle and perpendicular to the director while a reorientation process takes place. This process is reversible for shear deformation perpendicular and irreversible by applying the shear force parallel to the smetic direction. This is illustrated in Fig. 2.14. 

Reorientation of director L (or the optical axis) of the macroscopic volume of a liquid crystal under the effect of a field or flow of a liquid is Ae basis of most of the Imown electro- and magneto-optical effects. The anisotropy of the electrical and magnetic properties of the medium (dielectric constant Ae, diamagnetic susceptibility A%, electrical conductivity Aa) is the direct cause of orientation. The rearrangement processes are a function of the initial mientation of the molecules of the liquid crystal and its viscoelastic properties. The change in the c tical properties as a result of reorientation is the consequence of the optical anisotropy of liquid crystals. 

Structure is unwound and h points in a fixed direction, as in Figure 1.12d, then p will point in one direction. Clearly, this and other director axis reorientation processes are accompanied by considerable change in the optical refractive index and other properties of the system, and they can be utilized in practical electro- and opto-optical modulation devices. A detailed discussion of smectic liquid crystals is given in Chapter 4. 

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