Chiral Liquid Crystal Lenses Confined in Microchannels

It is known that the liquid crystalline smectic-A phase has geometric defects, called focal conic domains, which can be used as gradient-index lenses. Cholesteric (chiral nematic) phases also have circular topological defects that form lenses tunable with temperature. We have explored a weakly chiral system in which both types of defects can be present in the same material at different temperatures, and demonstrated a strategy for creating tunable lenses whose focal length can be changed with temperature. To study this behavior, we have confined liquid crystal in microchannels and measured the focal length of the lensing defects and their behavior near the phase transition. We have determined the carefully controlled experimental conditions that make the simultaneous presence of smectic focal conic domains and the circular cholesteric domains possible, such as the concentration of chiral dopant and the rate of heating and cooling. The transformation of focal conic domains into circular cholesteric domains is a new example of memory at the phase transition between smectic-A and nematic liquid crystals.