A model of stressed liquid crystals: close-packed, shaped liquid crystal droplets inside a sheared polymer matrix

Abstract

Stressed liquid crystals (SLCs) have been applied in fields such as optical phase array non-mechanical beam steering applications, adaptive optical tip-tilt correction, and fast displays because SLCs are capable of switching large phase shift in sub-millisecond time ranges. SLCs consist of liquid crystal micro-domains dispersed in a stressed polymer matrix. In this paper, we propose a model of close-packed, shaped liquid crystal droplets inside a sheared polymer matrix based upon the measurements of polarizing microscopy, fluorescence confocal microscopy, and visible-near-infrared spectroscopy. The light scattering of SLC films results mostly from the index mismatch between adjacent liquid crystal domains instead of the index mismatch between polymer matrices and liquid crystals as in traditional polymer dispersed liquid crystals. We show how the light scattering of SLC cells is greatly reduced upon shearing because the liquid crystal domains are aligned along the direction of shearing. The stretching of polymer matrices and the reshaping of liquid crystal domains upon shearing are confirmed by fluorescence confocal microscopy. The calculations of the electro-optic responses are based on the balance between the elastic torque and the electric field torque. Our experimental results support the calculations.