Freedericksz-effect

The many different kinds of LCDs " based on electrically controlled birefringence (ECB) are variants of the Freedericksz effect first reported in the 1930s. These related display types based on the same electro-optical effect, but a slightly different display configuration are denominated by a multitude of names. However, current versions of this type of display are now often referred... 

Distortions due to magnetic and electric fields static theory 3.4.1 The Freedericksz effect... 

In other words, deformation occurs only above a certain critical field This is referred to as the Freedericksz effect. The threshold condition can be used for a direct determination of the splay modulus n. 

Fig. 3.4.3. (a) The usual experimental configuration for the optical observation of the Freedericksz effect. Li t is incident normal to the film. However, for reasons discussed in the text, this arrangement is unsuitable for observing a twist deformation, b) Oblique configuration which enables the optical detection of a twist deformation. The magnetic field is perjjendicular to the plane of the paper... 

The Freedericksz effect in UgUy anisotropic nenuttics periodic... 

We shall now extend the theory of the Freedericksz effect to study the dynamical behaviour when the magnetic field is switched on or off suddenly. The analysis is particularly simple for a twist deformation (fig. 3.4.1 b)) because the torsion exerted on the director does not result in a translational motion of the centres of gravity of the molecules. Neglecting director inertia in (3.3.2) we obtain the following equation of motion for this geometry ... 

Dynamics of the Freedericksz effect Neglecting inertial effects, (3.3.1) reduces in the present case to... 

Reorientation of a nematic liquid crystal above a certain threshold value of the electric held of an optical wave, known as the photoinduced Freedericksz transition (PFT)—recently discovered and investigated both experimentally and theoretically -—exhibits a certain number of special features which are absent in the case of the rf analog of the Freedericksz effect. 

Quasi-Freedericksz Effect Displays in Nematic Liquid Crystals... 

These values have been taken from [21], and wa e measured using ftie magnetic Freedericksz effect. They are lower ftian the oftier values quoted, whidi may reflect some uncertainty ovCT the magnetic susceptibility anisotropy. 

Basic Freedericksz effect geometries involve pure splay, twist, and bend deformations, FIGURE 5. In each case the deformation is not uniform across the slab and can be expressed as ... 

For small distortions Al(t)/A,oc (t). Interference figures are usually observed with the aid of an analyser. Constructive or destructive interference of transmitted light is th observed depending on whether N(t) is an odd or even integer. Similar transient optical behaviour is also characteristic fin the other two dynamic Freedericksz effect geometries [42,43,95]. A typical optical set-up to measure the birefringence of the liquid crystal film is schematically shown in FIGURE 6 [44,95]. 

FIGURE 6 A scheme of epical observation of the Freedericksz effect N - nonatic film between glass plates, L - light source, P -polariser, A - analyso-, and D photodetection system. 

---