Features of electro-optical characteristics of composite liquid crystal media (a review)

Main patterns of structure formation of composite liquid crystal (LC) media and their classification according to the percentage content of liquid crystal and polymer are considered. Their properties are compared with the properties of homogeneous LC layers and the opportunities of their practical use in optical modulators are discussed. It is shown that, at small (10 wt %) monomer concentrations in the composite, its polymerization leads to formation of a thin-wall network which separates the liquid crystal into domains and provides an uniform orientation in the bulk. The polymer network increases the elasticity of the layer and decreases the relaxation time, but the devices usually work in polarized light and use the same principle as the devices filled with pure LC; i.e. the phase of the light or its polarization changes due to a change in the effective refraction index. However, the division of the LC volume into relatively autonomous domains also allows one to create a polarization-independent device based on the scattering effect. By increasing the relative content of the monomer, it is possible to ensure formation of a porous polymer matrix with inclusions of isolated from each other LC droplets. Such polymer-dispersed LC in its initial state either scatter the light of any polarization and becomes transparent state when an electric field is applied, or, with the use of special methods, the switch-off and switch-on states are swapped ("reverse mode" devices). The main advantages of the composite media are independence of polarization, mechanical strength, and small relaxation times, while the main disadvantages are increased power consumption, high polarization-independent optical losses, and significantly lower contrast. Possible ways to increase the contrast are described.