Spiral Pitch Control in Cholesteric Liquid Crystal Layers with Hybrid Boundary Conditions

• Published in: Crystals (Basel) Vol. 13; no. 1; p. 10
• Main Authors: Palto, Serguei P., Rybakov, Dmitry O., Umanskii, Boris A., Shtykov, Nikolay M.
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.01.2023
• Subjects: Boundary conditions; Cholesteric liquid crystals; Crystal defects; Deformation; Electric fields; Electrodes; Free energy; Mathematical models; Metastable state; Motion picture directors & producers; photonic crystals; Pitch (inclination); selective reflection spectra
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst13010010

The optical spectra of the cholesteric liquid crystal (CLC) layers under conditions of hybrid anchoring show a short-wave shift under a pulsed electric field. This behavior is anomalous because it is associated with a decrease in the pitch of the cholesteric spiral, which is atypical at conditions when the electric field is perpendicular to the axis of the CLC spiral. An analytical model of the phenomenon is discussed, according to which the spiral pitch under hybrid boundary conditions can be greater than the natural pitch in an unlimited volume of CLC. An in-plane electric field, being localized near the homeotropic-alignment surface, can be treated as effectively influencing the azimuthal anchoring and leading to a variety of metastable states with both increased and decreased pitch. These metastable states with local minima of free energy prevent the spiral from unwinding, and corresponding bands of selective reflection can even be shifted to the short-wave region of the spectrum. The observed effect is also studied numerically. It is shown by numerical simulations that the localized electric field from short-pitch electrodes can also modify zenithal anchoring, which should allow for defect-free controlling of the spiral pitch and spectral stop-band location.