Cholesteric elastomers: deformable photonic solids.

A mechanical strain applied to a monodomain cholesteric elastomer modulates and eventually unwinds the helical director distribution. There are similarities with the classical problem of an electric field applied to a cholesteric liquid crystal, but also differences. Frank elasticity is of minor importance unless the gel is very weak. The interplay is rather between the director being helically anchored to the rubber elastic matrix and the external mechanical field. Stretching perpendicular to the helix axis induces the uniform unwound state via the elimination of sharp, pinned twist walls above a critical strain. Below the critical strain the coarsening of the director distribution is not accompanied by an increase but rather by an affine decrease in the pitch. Unwinding through conical director states occurs when the elastomer is stretched along the helical axis. Finally we consider cholesteric elastomers in a classical device geometry with an electric field applied along the pitch axis and hence transverse to the director orientation.