Coexisting nematic and smectic-A phases in a twisted liquid-crystal cell

Twisted homogeneously planar-aligned nematic liquid-crystal cells are coole d into the smectic-it phase. The expected defective structure does not form . Instead the cells still show good optical-guiding characteristics. Explor ation 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 l iquid 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 tw isted nematic. As the cell is cooled so the smectic-A regions grow in thick ness with the nematic regions progressively thinning but with increased twi st gradient. A theoretical model of these novel results is presented.