B. A. Umanskii

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.

Electric field controlled polarization grating based on a hybrid structure "photosensitive polymer-liquid crystal"

An investigation was carried out on a hybrid structure consisting of a photosensitive polymer in contact with a nematic liquid crystal (NLC) layer. Before assembling the structure, holographic gratings of different periods were recorded in the photopolymer by two laser beams with opposite circular polarization. Such gratings are known for their unique asymmetric diffraction in case of elliptically polarized light and can be used in optical polarization devices. Due to a spatial modulation of the direction of the local optical axis, the photopolymer modulates the anchoring conditions for NLC in a similar way. As a result, a new grating is induced in NLC with the enhanced diffraction efficiency (in a particular experiment it was four orders of magnitude higher than that of the photopolymer). The NLC grating also shows the characteristic asymmetric diffraction for the reconstructing beams of opposite circular polarizations. The diffraction efficiency is easily controlled by an external electric field.

Planar amplifier for a microlaser on a cholesteric liquid crystal

A planar device is described, which combines an oscillator (microlaser) based on a dye doped cholesteric liquid crystal and an amplifier made of another laser dye solution in glycerin. Both the oscillator and amplifier are pumped longitudinally with the same pump beam. With that simple 4mm thick structure, the oscillator beam was amplified seven times.

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.

Simulation of light generation in cholesteric liquid crystals using kinetic equations: Time-independent solution

We report on the results of calculating the conditions for light generation in cholesteric liquid crystals doped with fluorescent dyes using kinetic equations. Specific features of spectral properties of the chiral cholesteric medium as a photonic structure and spatially distributed type of the feedback in the active medium are taken into account. The expression is derived for the threshold pump radiation intensity as a function of the dye concentration and sample thickness. The importance of taking into account the distributed loss level in the active medium for calculating the optimal parameters of the medium and for matching the calculated values with the results of experiments is demonstrated.

Mathematical model of compensation for dispersion of light polarization states in electrooptical systems based on chiral liquid crystals

Spectral dispersion of the polarization states occurs after passage of linearly polarized light through a thin layer of a chiral nematic crystal along the helical axis. It is clearly pronounced for crystals with a high optical anisotropy and a helical pitch somewhat larger than the light wavelength. The results of numerical analysis of the spectral dependence of polarization states at the output of the liquid crystal layer and the method for compensating for the dispersion are presented for the first time. It is shown that polarization dispersion can be eliminated using phase plates of two types, one of which should have a high anomalous birefringence dispersion. The possibility of developing fast light modulators operating in a wide spectral range is discussed.

Field Induced Helix Anharmonicity and Fast In-Plane Electrooptic Switching of Chiral Liquid Crystals

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.

Amplification of Laser Emission from Cholesteric Liquid Crystals by Planar Cells with Laser Dye Nematic and Isotropic Solutions

A possibility have been shown for amplification of weak emission intensity of microlasers based on cholesteric liquid crystals (CLC) by thin planar layers of both isotropic dye solutions and dye doped nematic liquid crystals. Very high gain index values (up to 500 cm−1) and absolute amplification of about 15 have been demonstrated. Especially important is the anisotropy of gain in nematics, which allows the field control of the light amplification by an electric field. The planar amplifiers using nematic liquid crystals can easily be integrated within single planar devices together with distributed feedback CLC microlasers. In such a hybrid structure, 6-times amplification of CLC microlaser intensity has been achieved.

Lasing in micro-grating liquid crystal systems

ABSTRACT A waveguide lasing effect has been observed and investigated in a dye-doped layer of a nematic liquid crystal (NLC) between two quartz plates. One of the plates has an electrode micro-grating, which allows (i) creating the feedback, (ii) guiding a part of the lasing emission into the quartz substrate and (iii) modulating the NLC refractive index by an electric field. At 0 V, a single Transverse Magnetic mode (TM)-polarised mode lasing has been observed. The emission goes out from the edge of the quartz plate in a narrow angular sector (±1.5°) at an angle of about 67.0° with respect to the NLC layer normal. At voltage applied, a number of additional lasing modes propagating at the same angle, but located at different wavelengths, appear. The experimental results are interpreted on account of numerical simulations based on the finite difference time domain method. Graphical Abstract