AC electric field induced helix unwinding in planar texture of a ferroelectric liquid crystal

The helix unwinding induced by an AC electric field in a thick ferroelectric S*_C sample with planar orientation has been investigated experimentally. Four patterns of instabilities have been observed below the unwinding critical field E_c. The corresponding thresholds and are found to be dependent on frequency. The formation of two instabilities (λ pattern and rhombic lattice pattern) is discussed on the basis of the present theory. The behaviour above the critical field is briefly described.