Dual-frequency-addressable photochromic cholesteric mixture with photo- and electro-switchable optical properties

Cholesteric liquid crystalline mixture with photo-, electro-controllable optical properties was prepared and the features of its switching behavior were demonstrated. The working principle of the obtained switchable cell is based on using dual-frequency-addressable nematic mixture that enables effective control of selectively reflected planar and scattered focal conics texture by applied field frequency modulation. The nematic mixture was doped with the chiral substance in order to induce a cholesteric structure formation with selective light reflection in visible spectral range. Azobenzene-containing photochrome was also introduced into this mixture in order to stimulate photo-optical response. An application of low-frequency electric field as well as UV irradiation induces short-wavelength shift of the selective light reflection. Under action of an electric field, a transition to scattered focal conics texture takes place. Amplitude of the light-induced shift is ca. 50 nm, while the electric field leads to a shift ca. 120 nm. On the contrary, high-frequency electric field induces fast (less than 1 s) restoration of perfect highly reflected planar texture. UV irradiation leads to not only shift of the selective light reflection band, but alters the electrooptic response by increase of frequency necessary for the restoration of the planar state and considerably changes the switching kinetics. The possibility of photopatterning of the obtained electrooptic cells by UV irradiation through a mask is demonstrated. It is shown, that application of electric field of different strength and frequencies enables smart control of the photorecorded image contrast.