Tunable defect modes in chiral liquid crystals based on laser-induced modulation of helix

Abstract

A structural defect was introduced within the helical lattice of cholesteric liquid crystals to realize a tunable photonic defect mode in the selective reflection band, or the polarization sensitive photonic band-gap of the cholesteric liquid crystal. The cholesteric liquid crystal material was locally polymerized via a two-photon polymerization process and a finite unpolymerized region was left between two polymerized layers to act as the structural defect in which the helical pitch is different from the polymerized bulk. The structural defect was functionalized by first of all, rinsing the unpolymerized cholesteric liquid crystal, and then infiltrating a photo-chromic dye-doped cholesteric liquid crystal material into the gap. A defect mode was observed in the selective reflection band of the cholesteric liquid material at a wavelength depending on the contrast between the pitch at the defect and the bulk. The defect mode was tunable by applying heat or irradiating light at λ ~ 400 nm on the sample, which caused the shortening of the helix pitch of the cholesteric liquid crystal at the defect, therefore increasing the pitch contrast between the bulk and the defect. Reversible tuning of the defect mode was realized in an electrode-free cell.