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

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 >20 wt %. 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 2300 to 500 nm was observed over the temperature range of 23-55 degrees C. Polarized optical microscopy, differential scanning calorimetry, and x-ray studies were utilized to confirm the temperature-dependent phase behavior. Tuning of similar to 50 nm by the application of a direct current electric field was also observed with no onset of electrohydrodynamic instabilities for voltages up to similar to 300 V. 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. (C) 2008 American Institute of Physics.