LCD-technique                                  

Liquid crystal displays (LCD's) are widely used in applications for displaying information; for example mobile telephones, electronic notebooks, computers and watches as well as in optical shutters or light valves for the modulation of light.

A liquid crystal cell typically consists of a thin layer of liquid crystal material bounded by two substrates (glass or plastic) bonded together around the edges. The inner surfaces of the plates are typically coated with a thin transparent conducting oxide film (eg ITO) in order to enable an electric field to be applied across the active liquid crystal layer.

Typically, liquid crystal materials consist of elongated, rod-shaped molecules that resemble a liquid in that they are free to flow, but differ from it in that within a certain temperature range, the molecular shape gives rise to a long range stacking order. Other molecular shapes that display this phenomenon include discs and rectangular formed molecules.

The long range stacking sequence displays several different phases depending upon the temperature and geometry of the liquid crystal molecules. The most commonly used phase as far as liquid crystal displays are concerned is the nematic structure in which the molecules possess a natural tendency for the molecular director axes to align parallel with each other whilst their centre of masses remain randomly distributed throughout the material. 

A lowering of the temperature may produce a further ordering of the structure by causing the molecular centre of masses to align in layers, although the molecules still remain randomly distributed within each individual sheet of crystal. This produces both the Smectic A and Smectic C phases in which the average molecular axes are aligned respectively perpendicular to and with a small tilt angle to the planes of these layers. Further temperature reduction beyond this point finally produces the crystalline, solid phase in which the material also possesses stacking order within the individual layers themselves and are hence held rigidly in an ordered lattice. In general, liquid crystal materials operate in the nematic phase at room temperature. 

Typically, a thin polyimide layer about 10nm thick is deposited on top of the transparent conducting oxide layers covering the inner-surfaces of the two substrates and simplistically it can be considered that a series of microscopic scratches lying in a uniform direction are formed on the surface by gentle rubbing of a cloth. The liquid crystal molecules at the polyimide surface thereafter show a tendency to align parallel along the microscopic groves with a small tilt angle. Other alignment techniques also exist such as oblique vacuum deposition procedures. These orientation methods generate different molecular tilt angles but the cell still functions in essentially the same physical manner. 

The liquid crystal alignment at both sides of the cell are hence defined during cell manufacturing. By careful control, any twist-angle can therefore be induced in the helical structure across the liquid crystal layer. For example, with a twist-angle of exactly 90°, the standard 90° twisted nematic (TN) cell is formed. Twist-angles of less than 90° form the low-twist (LT) cell whilst by definition, super-twist cells (STN) are ones possessing twist-angles exceeding 180°.

Pure nematic materials can display induced twisting helixes with both the positive and negative senses of rotation depending upon the structure of the cell. However, liquid crystal eutectic mixtures are often doped with small quantities of cholesteric components that possess a natural helical stacking structure with a specific direction of orientation. This defines the spontaneous spiralling direction for the overall cell and hence prevents the formation of domains or regions of crystal where a reverse twist of 270° occurs instead of the required 90° structure. 

In the limit of large cell thickness, such a helical structure is capable of rotating the plane of linearly polarised light with nearly 100% efficiency. Light rotation occurs due to the inherent anisotropic index of refraction present in the molecules of the liquid crystal material. Incident radiation experiences two refractive indices depending upon the oscillation direction; no corresponding to the ordinary ray with the electric field vector oscillating perpendicular to the molecular director and ne for the extra-ordinary ray oscillating parallel to this axis. A phase factor or retardation is therefore introduced between the two light components upon passage through the material, corresponding to a rotation of the polarisation.

The birefringence or anisotropic index of refraction, Dn, is defined according to Dn = ne - no. The vast majority of liquid crystal materials possess positive anisotropic indices of refraction, typically lying between + 0.05  and + 0.30 respectively. In general, Dn is both wavelength and temperature dependent. However, in practice, the variations are relatively modest, D n becoming somewhat smaller both with increasing wavelength and temperature until the clearing point is reached where upon rapid molecular rotations average out the refractive indices for both light components and the anisotropic index of refraction for the material approaches zero.