Electro-thermal tuning in a negative dielectric cholesteric liquid crystal material

Abstract

The thermal and electrical tunability of a cholesteric liquid crystal containing a negative dielectric anisotropy liquid crystal in a planar alignment was studied. The physical, optical, and electro-optical characteristics of mixtures containing different ratios of chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806 were examined. A smectic A phase was seen at room temperature for S811 loadings >20wt%. Below 20%, a room temperature cholesteric phase was observed. Upon heating mixtures with composition S811 >20%, the selective reflection notch of the cholesteric phase appeared and blueshifted with temperature. Thermal tuning from 2300to500nm was observed over the temperature range of 23–55°C. Polarized optical microscopy, differential scanning calorimetry, and x-ray studies were utilized to confirm the temperature-dependent phase behavior. Tuning of ∼50nm by the application of a direct current electric field was also observed with no onset of electrohydrodynamic instabilities for voltages up to ∼300V. Bandwidth broadening but not tuning was obtained with the application of alternating current fields. Electrical tuning is likely due to pitch contraction brought about through the annealing of defects.