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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.

We have studied the tunability of the reflection notch of a cholesteric filter containing a negative dielectric anisotropy LC in a planar alignment. For this purpose, we studied physical, optical and electro-optical characteristics of mixtures containing chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806. Interestingly, smectic A phases were seen at room temperature for S811 loadings >20% by weight of ZLI-2806. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) studies confirmed the formation of a cholesteric phase above room temperature. A phase diagram was constructed by varying S811from 9-50% by weight in the mixture. Reflection notchs were not seen at room temperature for compositions of S811 >20%. On heating, the selective reflection notch of the cholesteric phase appeared and blue shifted with temperature. Various methods of tuning the reflection notch were examined. Thermal tuning from 2200 nm to 450 nm was observed over the temperature range 23 to 55° C. Application of a DC field led to electrical tuning (~50 nm) of the notch. The notch was also tuned (>500 nm) optically by exposing a dye doped cholesteric cell to laser lines at either 532 or 647 nm.

We report on the large wavelength range and light-initiated thermal tuning of the reflection of a liquid crystal (LC) formulation (S811/ZLI-2806) near a smectic (SmA) to cholesteric (CLC) phase transition enabled by the use of a high order parameter heat transfer dye (anthraquinone, AQ). Upon irradiation with a 647 nm Krypton ion (Kr+) laser line, absorption by AQ generates heat that is transferred to the surrounding LC host. In the S811/ZLI-2806 formulation examined here, the optically generated increase in temperature serves to transition the phase from SmA to CLC. As has been documented, the SmA→CLC transition is typified by a pitch contraction that blueshifts the position of the CLC reflection, in this case a shift from 2500 to 700 nm that can occur in less than 100 s. The tuning range and speed are dependent on the laser power and the amount of dye in the cell.

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