Electrically and Optically Controlled Light Modulation and Color Switching Using Helix Distortion of Ferroelectric Liquid Crystals

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Abstract
The electro-optic effect associated with the helix distortion of chiral smectic C (SmC*) liquid crystals and its application to light modulation are investigated. Optically and electrically addressed spatial light modulators based on this effect are demonstrated. They exhibit an analog response, response times as short as 100 μs, good contrast ratio and resolution, and color switching capability. The analog response is shown to be a result of the gradual distortion of the helix at low voltages. Optically this corresponds to a linear rotation of the average optic axis and change of the average effective birefringence. From measurements of the wavelength of the maximum transmission the birefringence has been found to change as a function of dc voltage and frequency of ac voltage and magnitude. The agreement between theory and experiment suggests that color changes are due solely to birefringence changes associated with changes in the geometry of the helix. We derive expressions for the angle of rotation α of the average optic axis and birefringence Δn av for the case of small distortions. It is shown that α varies linearly with field in this regime while Δn av varies quadratically. The rotation of the average optic axis is shown to be in a plane.