We demonstrate a fast switchable grating based on ferroelectric liquid crystals and orthogonal planar alignment by means of photo alignments. Both 1D and 2D gratings have been constructed. The proposed diffracting element provides fast response time of around 20 μs, contrast of 7000:1 and high diffraction efficiency, at the electric field of 6 V/μm. The saturated electro-optical (EO) states up to very high frequency (≈5 kHz) are the real advantage of the proposed switchable grating, which opens several opportunities to improve the quality of existing devices and to find new applications.

We study both theoretically and experimentally the electro-optical properties of vertically aligned deformed helix ferroelectric liquid crystals (VADHFLC) with subwavelength pitch that are governed by the electrically induced optical biaxiality of the smectic helical structure. The key theoretical result is that the principal refractive indices of homogenized VADHFLC cells exhibit the quadratic nonlinearity and such behavior might be interpreted as an orientational Kerr effect caused by the electric-field-induced orientational distortions of the FLC helix. In our experiments, it has been observed that, for sufficiently weak electric fields, the magnitude of biaxiality is proportional to the square of electric field in good agreement with our theoretical results for the effective dielectric tensor of VADHFLCs. Under certain conditions, the 2$$\pi${}$ phase modulation of light, which is caused by one of the induced refractive indices, is observed without changes in ellipticity of incident light.

Electro-optical properties of deformed helix ferroelectric liquid crystal (DHFLC) cells are studied by using a general theoretical approach to polarization gratings in which the transmission and reflection matrices of diffraction orders are explicitly related to the evolution operator of equations for the Floquet harmonics. In the short-pitch approximation, a DHFLC cell is shown to be optically equivalent to a uniformly anisotropic biaxial layer where one of the optical axes is normal to the bounding surfaces. For in-plane anisotropy, orientation of the optical axes and birefringence are both determined by the voltage applied across the cell and represent the parameters that govern the transmittance of normally incident light passing through crossed polarizers. We calculate the transmittance as a function of the electric field and compare the computed curves with the experimental data. The theoretical and experimental results are found to be in good agreement.

We disclose the vertically aligned deformed helix ferroelectric liquid crystal whose Kerr constant ($K_{\mathrm{kerr}}\approx 130\mathrm{nm}/{\mathrm{V}}^2$ at $\lambda =543\mathrm{nm}$) is around one order of magnitude higher than any other value previously reported for liquid crystalline structures. Under certain conditions, the phase modulation with ellipticity less than 0.05 over the range of continuous and hysteresis-free electric adjustment of the phase shift from zero to $2\pi$ has been obtained at subkilohertz frequency.

The structural peculiarities and electro-optic performance of liquid crystal (LC)–colloidal nanoparticle (NP)–polymer (P) composites formed by photoinduced phase separation are considered. We classify these materials under two groups according to two limiting cases of polymer morphology. The first group corresponding to small polymer concentration comprises LCs filled with NPs that are stabilized with a polymer network. It is found that, in addition to the light scattering caused by the LC orientational defects, the refractive index mismatch between LC and NP aggregates may significantly affect the electro-optic contrast and its angular characteristics. The second group is represented by polymer dispersed liquid crystals (PDLCs) filled with NPs. It is established that, in the process of photoinduced phase separation of the LC-NP-prepolymer mixture, the nanoparticles are mainly involved with the polymer, serving as building blocks for the polymer matrix. When the aggregation rate of the NPs is high or their size is large, the NPs enhance light scattering in the polymer. For low aggregation rate, NPs modify the effective refractive index and/or the absorption coefficient of the polymer phase without producing any noticeable optical inhomogeneity. Additionally, we found that ${\mathrm{TiO}}_2$ NPs may cause a photochromic effect, which manifests itself in color changes in the course of the photoinduced phase separation. For PDLCs with optically transparent polymer matrices modified by NPs, it is shown that doping with NPs can be used to control the refractive index ratio of the LC and polymer. In this way one can modify the contrast and substantially reduce the off-axis haze of the PDLC. The observed effects show LC-NP-P composites as materials of considerable promise for LCD and other electro-optic applications.

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by either photoreorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At the early stages of irradiation, the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photosaturation. In the regime of photoselection, the photosaturated state of the film is optically isotropic. But, in the case of the photoreorientation mechanism, anisotropy of this state is uniaxial with the optical axis dependent on the preferential alignment of azochromophores. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.