N. M. Shtykov

A Study of a Ferroelectric Liquid Crystal Using Second Optical Harmonic Generation

Abstract The phase-matched generation of the optical second harmonic was investigated in the chiral smectic C* phase of a classical ferroelectric liquid crystal (DOBAMBC). The phase-matching was achieved either by incidence angle or by temperature tuning. The temperature behaviour of the second order nonlinear susceptibility was studied.

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.

Fine structure of the repolarization current in the B 2 phase of a banana compound

A detailed study of the field induced polarization switching has been carried out over the whole temperature range of the antiferroelectric B2 phase of a banana-shaped compound, the tetradecyl homologue (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n -alkylphenylimino)benzoate]s. The fine structure of the current response oscillograms was attributed to the field switching of two antiferroelectric subsystems, most probably the racemic and homogeneously chiral domains which, according to the Boulder group, appear as a result of the spontaneous breaking of mirror symmetry. The temperature and field dependences of polarization as well as switching times and the corresponding viscosity of the substance have been measured.

A field-reversal method for measuring the parameters of a ferroelectric liquid crystal

A method for measuring the spontaneous polarization P s, the switching time, the rotational viscosity γφ, and the d.c. conductivity σ is presented. The possibilities of estimating the azimuthal angle φ0, the dielectric anisotropy δε and the dielectric permittivity ε⊥ in the same experiment are also discussed. It is explicitly shown that the switching delay, though primarily dependent on the material and the applied field, is also dependent on the geometry of the cell.

Optical Second Harmonic Generation in Various Liquid Crystalline Phases

The zero-field and field-induced optical second harmonic generation (SHG) was investigated for the nematic and smectic A phases of various liquid crystals. The components of the cubic non-linear susceptibility tensor were measured for substances with different molecular structure. The phase-matched SHG was observed for all the compounds investigated. The directions of the phase synchronisms as well as the corresponding non-linear susceptibilities were determined for the ee-o and oe-o interactions. The zero-field phase-matched SHG was observed for the oe-o interaction. It was accounted for by a multipolar mechanism.

Fast electric field switched 2D-photonic liquid crystals

We demonstrate field-induced 2D-photonic liquid crystals (LC). The 2D spatially periodic modulation of the LC director field is achieved using a geometry with two crossed interdigitated systems of electrodes located at opposite sides of the LC layer. With a special method of dual-field driving, a very fast switching between different spatially periodic LC director distributions is achieved. The director field distribution and potential use of these photonic crystals for fast switched multidirectional lasing is discussed.

Liquid crystal on subwavelength metal gratings

Optical and electrooptical properties of a system consisting of subwavelength metal gratings and nematic liquid crystal layer are studied. Aluminium gratings that also act as interdigitated electrodes are produced by focused ion beam lithography. It is found that a liquid crystal layer strongly influences both the resonance and light polarization properties characteristic of the gratings. Enhanced transmittance is observed not only for the TM-polarized light in the near infrared spectral range but also for the TE-polarized light in the visible range. Although the electrodes are separated by nanosized slits, and the electric field is strongly localized near the surface, a pronounced electrooptical effect is registered. The effect is explained in terms of local reorientation of liquid crystal molecules at the grating surface and propagation of the orientational deformation from the surface into the bulk of the liquid crystal layer.

Modeling laser generation in cholesteric liquid crystals using kinetic equations

The paper presents the results of calculating the threshold pumping power necessary for laser generation in cholesteric liquid crystals doped with fluorescent dyes. Calculations have been performed with allowance for (i) specific spectral properties of the chiral cholesteric medium as a photonic structure and (ii) the spatially distributed character of feedback in the active medium. The dependences of the threshold pumping power on the sample thickness, dye concentration, and leading front widths of long pumping pulses have been studied.

A fast anharmonic mode in electrooptical switching of liquid crystal structures based on chiral nematics

Polarization, spectral, and relaxation features of a new electrooptical effect in oriented layers of chiral nematic liquid crystals (LCs) are considered. The physics behind this electrooptical effect is the induction of higher order spatial harmonics in the helical distribution of the director field, which ensures the high speed of electrooptical response. It is shown that the spectral properties of the electrooptical response can be effectively controlled by varying the optical anisotropy of the LC and the pitch of the helical structure.

Amplification of the emission of a liquid-crystal microlaser by means of a uniform liquid-crystal layer

The amplification of the emission of a microlaser based on a cholesteric liquid crystal by means of a uniformly oriented layer of a laser-dye-doped nematic liquid crystal is demonstrated. The nematic liquid crystal is characterized by the anisotropy of optical properties, including the amplification coefficient, which opens the possibility of creating amplifiers controlled by a low electric field.