Coexisting Nematic and Smectic-APhases in a Twisted Liquid-Crystal Cell

Twisted homogeneously planar-aligned nematic liquid-crystal cells are cooled into the smectic- $A$ phase. The expected defective structure does not form. Instead the cells still show good optical-guiding characteristics. Exploration of the cells using a half-leaky guided-mode arrangement reveals that the liquid-crystal phase separates into three or more regions. Adjacent to both the upper and lower boundaries is a region of highly twisted nematic liquid crystal. In the center of the cell is one or more homogeneous smectic- $A$ regions with smectic layers normal to the cell surfaces, separated by twisted nematic. As the cell is cooled so the smectic- $A$ regions grow in thickness with the nematic regions progressively thinning but with increased twist gradient. A theoretical model of these novel results is presented.