Combined electric and photocontrol of selective light reflection at an oblique helicoidal cholesteric liquid crystal doped with azoxybenzene derivative.

An oblique helicoidal cholesteric liquid crystal ${\mathrm{Ch}}_{\mathrm{OH}}$ represents a unique optical material with a single-harmonic periodic modulation of the refractive index and a pitch that can be tuned by an electric or magnetic field in a broad range from submicrometers to micrometers. In this work, we demonstrate that the oblique helicoidal cholesteric doped with azoxybenzene molecules can be tuned by both the electric field and light irradiation. The tuning mechanism is explained by the kinetics of trans-cis photoisomerization of the azoxybenzene molecules. At a fixed voltage, UV irradiation causes a redshift of the reflection peak by more than 200 nm. The effect is caused by an increase of the bend elastic constant of ${\mathrm{Ch}}_{\mathrm{OH}}$ under irradiation. The demonstrated principle has the potential for applications such as smart windows, sensors, tunable lasers, and filters.