Organic doped sol – gel glasses for electro-optics and display applications

Gel-glass dispersed liquid crystal (GDLC) films may be used as electro-optic devices. Films scatter light according to the number of droplets and the relative refractive indices of the LC and the silica matrix. LCs are birefringent; therefore their refractive index depends on the LC orientation and the optical angle of incidence. If the film is coated with transparent electrodes, and an electric field is applied, a reorientation of the LC director in the droplet occurs, producing a variation of the LC refractive index as `seen’ by the incoming light. If the refractive index of the sol – gel substrate matches the new LC index, the material changes from an opaque, scattering state to a transparent state. This feature can be used for preparing devices for visual presentation, i.e. displays. Unaltered GDLCs switch from white opaque to colourless transparent states. The contrast ratio, viewing angle and switching time of GDLC displays are presented. If these materials are to be used for displays, colour needs to be incorporated for many applications. The use of GDLCs associated to gel-glasses doped with fluorescent dyes for preparation of emissive `all sol – gel’ displays is presented, with the GDLC acting as an optical shutter for the RGB emission of the fluorescent dyes located at the backplane. A number of fluorescent dyes with high quantum yield for the chosen UV excitation, and appropriate colour coordinates, have been investigated.

D. Levy