Static One-Dimensional Distortions in Cholesteric Liquid Crystals

We report new experimental observations on a cholesteric planar texture of pitch P and thickness L submitted to a static magnetic →H or a high-frequency electric field →E. Two configurations are considered: (a) →H parallel to the helical axis: if χₐ>0, L~P, an array of stripes is observed at threshold. This is to be compared to the case χₐ>0, LP, where a square gridlike pattern is nucleated in the same conditions. (b) →E perpendicular to the helical axis: if εₐ<0, L0, a static array of stripes is again observed, the stripes being perpendicular to →E. This configuration is equivalent to the configuration χₐ<0, →H perpendicular to the helical axis. The threshold conditions derived by Helfrich and Hurault in the case LP, χₐ>0, →H parallel to the helical axis seem to be also valid in those two cases. A more detailed theoretical treatment is devoted to the second geometry.