Polarization, spectral and relaxation properties of a new electrooptic effect in aligned layers of chiral nematic liquid crystals are considered. The physical mechanism of the effect is based on the emergence of high spatial harmonics in the helical structure of the director field, which determine a high rate of the electrooptic response. An effective control of spectral properties of the corresponding devices is performed using variations of the optical anisotropy and the helical pitch of a liquid crystal.