Within the framework of phenomenological approach, a generalized concept of a non-local order parameter, that have the form of a traceless correlation tensor function or a tensor integral operator, is introduced. The main relationships, which determine the equilibrium orientational states and the phase transitions of a deformed nematic liquid crystal, are described. The resulting expression for the free energy is quadratic with respect to the gradients of the order parameter tensor. The limiting transition of the non-local order parameter to the local one leads to an expression that complements the classical Oseen–Frank theory of elasticity. The considered model includes eight independent elastic constants. Three of them relate to "bulk" constants analogous to Frank constants, another three ones relate to "surface" constants, and the remaining two constants determine the anisotropic effect of the gradient of the scalar order parameter on the director field. The constants are necessary for the consideration of a liquid crystal state near defects, as well as for the description of orientation effects caused by the inhomogeneity of order parameter. It is shown that in the free energy expression, the "surface" terms contribute to the director orientation in the case of a non-constant scalar order parameter, even in the approximation of rigid boundary conditions.

Comparative results of experimental studies and numerical analysis are presented using the developed original technique for simulating an installation for measuring ultralow absorption based on single-beam photothermal common path interferometry (PCI). The possibility of decrease the detection limit for volume absorption by the PCI method is shown to be significantly lower than $10^{-6}\ \text{cm}^{-1}$ , absorption on the surface and in thin films is less than 1 ppm.

High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [ χ (2) > 10 −6 m V −1 ] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10 −6 m V −1 ]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 10 13 W m −3 ). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.

The effect of nonlinear light action on a thin (∼10 µm) films of the nematic liquid crystal deposited onto the absorbing substrate is experimentally investigated. The dynamics of the orientational and thermocapillary effects is directly studied. The two types of orientational processes were found out. The first one appears for several hundreds of milliseconds when the light beam irradiation is turned on or off. The second one develops much slowly and does not relax during the light beam irradiation.

Optical nonlinearity of semiconducting organic films based on regioregular polythiophene (RR-P3HT) was explored by means of laser frequency conversion in a fiber-coupled experimental setup. Features of harmonic generation in this medium were revealed and compared with other nonlinear media. Possible enhancement of nonlinear susceptibility of RR-P3HT in high-gradient light fields is discussed as well.

We report on theoretical and experimental study of pulsed lasing driven by synchronous pumping of an Yb-fiber laser. We show that even simple sine-wave modulation of the pump power with moderate depth (~50%) can result in shaping of a regular train of discrete laser pulses. Moreover, generated pulses are typically much shorter than pumping pulses. The pulse shortening factor rises with the modulation depth of pumping. It reached 5 in our setup. Thus, ~200-ns laser pulses were generated with sub-MHz sine-wave modulation of the pump power. Theoretical modelling predicts possibility of further pulse shortening (up to a factor of 11) by increasing maximal pump power and modulation index. Such shortening can be attributed to self-induced gain discrimination of the temporal pulse profile.

The nonlinear optical properties of a nematic liquid crystal (NLC 1289) were experimentally studied according to the intensity and polarization of the pump radiation at a wavelength of 1.56 mu m. It is shown that the maximum generation efficiencies of the second and third harmonics correspond to different polarization characteristics of the main radiance. In the investigated range (10(6) horizontal ellipsis 2 center dot 10(7)W cm(-2)) the nonmonotonic dependence of the second harmonic intensity on the pump radiance power was demonstrated for the first time. In addition to this, explanation of this effect is also proposed.

In this study, the beam mode conversion in a nematic liquid crystal film deposited on the end-face of a single-mode optical fibre is considered. The infrared laser radiation delivered by this fibre induces a defect structure in the initially homeotropic liquid crystal. This leads to the formation of an output light beam with the annular intensity distribution in its cross-section. The obtained beam mode maintains its transverse profile in free space and can be interpreted as an optical vortex.

A generalized notion of a nonlocal tensor order parameter is introduced within the framework of the phenomenological approach. This parameter has the form of a traceless tensor correlation function or a tensor integral operator. Based on this form, the governing relations are written, which determine the steady states and phase transitions of the deformed liquid crystal. Linear relations for eigenfunctions of the introduced operator are derived. A principal drawback of currently available models of liquid crystals based on the local presentation of the tensor order parameter (equality of two Frank constants in the case of a quadratic form of the strain part of the free energy) is eliminated. Particular examples are considered, which demonstrate the model workability and the absence of contradictions in the model as well as its adequacy when describing small-scale structures.

Nonlinear optical coupling between two single-mode fibers terminated coaxially in a nematic liquid crystal (NLC) was explored for the first time. Light-induced reorientation of nematic molecules can result in the stable self-collimation of light transmitted through the gap between fibers. Thus, high coupling efficiency can be achieved despite large fiber spacing. We demonstrated a coupling efficiency of up to ∼0.7, achieved with spacing equal to four diffraction lengths. This feature opens up possibilities for the development of novel in-line fiber-optic elements based on NLCs. For instance, a polarization controller was proposed and considered.

Triggering capability of a laser radiation transmission through a fiber-coupled nematic liquid crystal (NLC) was experimentally investigated. This capability arises from light-induced reorientation of NLC molecules that enables self-focusing and self-confinement of propagating laser radiation. It was shown, that triggered optical coupling of two single-mode optical fibers terminated coaxially in NLC can be achieved through a self-induced light guide. It allows implementation of a miniature all-fiber devise, which can act as a precision power-sensitive optical trigger.

A new formulation of Maxwell's equations based on the introduction of two vector and two scalar potentials is proposed. As a result, the electromagnetic field equations are written as a hyperbolic system that contains, in contrast to the original Maxwell system, only evolution equations and does not involve equations in the form of differential constraints. This makes the new equations especially convenient for the numerical simulation of electromagnetic processes. Specifically, they can be solved by applying powerful modern shock-capturing methods based on the approximation of spatial derivatives by upwind differences. The cases of an electromagnetic field in a vacuum and an inhomogeneous material are considered. Examples are given in which electromagnetic wave propagation is simulated by solving the formulated system of equations with the help of modern high-order accurate schemes.

Предлагается новая формулировка уравнений Максвелла, основанная на использовании двух векторных и двух скалярных потенциалов. Их введение позволяет записать уравнения электромагнитного поля в виде гиперболической системы, содержащей, в отличие от оригинальных уравнений Максвелла, только уравнения эволюционного типа и не включающей уравнения, имеющие характер дифференциальных ограничений. Это делает новые уравнения особенно удобными при численном моделировании электромагнитных процессов, позволяя, в частности, использовать для их решения мощные современные методы сквозного счета, основанные на аппроксимации пространственных производных разностями против потока. Рассматривается как случай электромагнитного поля в вакууме, так и в неоднородной материальной среде. Даются примеры моделирования распространения электромагнитных волн путем решения сформулированной системы уравнений с помощью современных схем высокого порядка точности. Библ. 22. Фиг. 7.

Summary form only given: The temperature impact on the optical harmonic generation in a fiber-coupled nematic liquid crystal (NLC) pumped by femtosecond laser radiation was explored. A strong thermal enhancement of the third harmonic generation (THG) was revealed. Upon heating to a certain temperature (near the upper limit of the mesophase), the NLC-based convertor features a spike of the THG efficiency in excess of 10%. This effect may be attributed to noncritical phase matching. Also appreciable forth harmonic generation in the ultraviolet occurred upon the thermal enhancement of THG.

We report the recent results of research focused on a new kind of soft matter - the liquid-crystal nanocomposites with controllable mechanical and nonlinear optical properties. These are promising media for implementation of ultra-compact photonic devices and efficient sources of coherent radiation in a wide spectral range. We overview the technology of preparation of nematic-liquid-crystal media saturated with disclination defects. The defects were formed in different ways: by embedding nanoparticles and molecular objects, by exposure to alpha-particle flux. The defect locations were controlled by applying an electric field. We also present and discuss the recently discovered features of nematic-liquid-crystal media: a thermal orientation effect leading to the fifth-order optical nonlinearity, enormous second-order susceptibility revealed by measurements, and structural changes upon exposure to laser radiation. We report on efficient generation of harmonics, sum and difference optical frequencies in nematic-liquid-crystal media. In addition, transformation of laser radiation spectra to spectral supercontinua, and filamentation of laser beams were also observed in nematic-liquid-crystal media. We conclude that most nonlinear optical effects result from changes of the orientational order in the examined nematic liquid crystals. These changes lead to the symmetry breaking and disclination appearances.

The paper describes preliminary investigations of a new effect in conducting anisotropic liquids, which leads to thermoelectric conversion of energy. Nematic liquid crystals with semiconducting dopes are used. A thermoelectric figure of merit ZT = 0.2 is obtained in experiments. The effect can be explained by assuming that the thermocurrent in semiconducting nematics, in contrast to the Seebeck effect, is a nonlinear function of the temperature gradient and of the temperature itself. Though the discovered effect has to be further investigated, the data obtained suggest that it can be effectively used in alternative energy engineering.

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.

The article discusses some of the characteristics of materials used in the THz range. First registered thermo orientation effect in centrally symmetric liquids, leading to structural changes that violate the original symmetry of nematic liquid crystals under the action of heat flow.