Polar and apolar light-induced alignment of ferroelectric nematics on photosensitive polymer substrates

Surface alignment of a recently discovered ferroelectric nematic liquid crystal (N_F) is usually achieved using buffed polymer films, which produce a unidirectional polar alignment of the spontaneous electric polarization. We demonstrate that photosensitive polymer substrates could provide a broader variety of alignment modes. Namely, a polyvinyl cinnamate polymer film irradiated by linearly polarized ultraviolet (UV) light yields two modes of surface orientation of the N_F polarization: (1) a planar apolar mode, in which the equilibrium N_F polarization aligns perpendicularly to the polarization of normally impinging UV light; the N_F polarization adopts either of the two antiparallel states; (2) a planar polar mode, produced by an additional irradiation with obliquely impinging UV light; in this mode, there is only one stable azimuthal direction of polarization in the plane of the substrate. The two modes differ in their response to an electric field. In the planar apolar mode, the polarization can be switched back and forth between two states of equal surface energy. In the planar polar mode, the field-perturbed polarization relaxes back to the single photoinduced "easy axis" once the field is switched off. The versatility of modes and absence of mechanical contact make the photoalignment of N_F attractive for practical applications.