Molecular motion in a smectic liquid crystal showing V-shaped switching as studied by optical second-harmonic generation

The molecular motion during the V-shaped switching in a homogeneously aligned smectic-C*-like liquid crystal (LC) cell has been investigated by means of second-harmonic generation (SHG) and SHG interferometry. At normal incidence, SHG intensity profiles were observed as a function of applied electric field for four combinations of input-output polarizations of light. For the analysis of SHG results, the V-shaped switching system was considered as a ferroelectric uniform state with C₂ symmetry (collective model). It was shown that the observed SHG behavior was successfully simulated using the collective model. By using SHG interferometry measurements, it was found that azimuthal rotation of LC molecules is limited within half the cone, where molecules could distribute. Moreover, it was also found that LC molecules undergo counterazimuthal rotation in the upper and lower halves of the LC layer in chevron structure. The stabilization of the uniform molecular alignment was attributed to the escape from the formation of polarization charges and a surface molecular constraint. The mechanism of the collective motion was discussed on the basis of the softening of the system due to the frustration of ferroelectric and antiferroelectric interactions.