Depolarization of light in microstructured fibers filled with liquid crystals

In the paper we analyze microstructured optical fibers filled with typical nematic liquid crystals, i.e., 5CB and 6CHBT under influence of external electric field or temperature. We use the modified Mueller matrix method with an additional depolarization matrix to calculate degree of polarization changes of the light propagating in the liquid-crystal infiltrated microstructured optical fibers. Preliminary experimental results of light depolarization measurements during propagation in these microstructured fibers are also presented.     

Efficiency of second harmonic generation for partially coherent light in anisotropic crystals

This paper analyzes the second harmonic generation (SHG) efficiency of light with partial temporal coherence due to depolarization effects in birefringent media. It discusses relations between SHG efficiency fading, light source spectrum, crystal birefringence, and phase matching conditions. The efficiency of SHG pumped by the partially coherent light beam that may depolarize light in nonlinear birefringent crystal is also analyzed. The basic theory of SHG with its modification for partially coherent light with depolarization and some numerical calculations of the SHG process are described. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 207, May 17–20, 2007, Świnoujście, Poland     

Bichromatic coherent random lasing from dye-doped polymer stabilized blue phase liquid crystals controlled by pump light polarization

In this paper, we investigate the bichromatic coherent random lasing actions from the dye-doped polymer stabilized blue phase liquid crystals. Two groups of lasing peaks, of which the full widith at half maximum is about 0.3 nm, are clearly observed. The shorter- and longer-wavelength modes are associated with the excitation of the single laser dye (DCM) monomers and dimers respectively. The experimental results show that the competition between the two groups of the lasing peaks can be controlled by varying the polarization of the pump light. When the polarization of the pump light is rotated from 0° to 90°, the intensity of the shorter-wavelength lasing peak group reduces while the intensity of the longer-wavelength lasing peak group increases. In addition, a red shift of the longer-wavelength modes is also observed and the physical mechanisms behind the red-shift phenomenon are discussed.     

Bistability in dual-frequency nematic liquid crystals

Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

部分相干光离焦像面散斑照相法研究

本文研究了部分相干光离焦像面散斑的运动规律.激光离焦散斑是其特例.讨论了制约离焦量的因素,并对其实验技术进行了探讨和验证.     

Features of light absorption in chiral photonic crystals with a strong local anisotropy

The interaction of light with a layer of a chiral photonic crystal is considered. Features of the Borrmann effect at a strong anisotropy are studied. The natural polarizations are considered, and it is shown that in the limit δ << 1 (δ = (?₁ ? ?₂)/(?₁ + ?₂), where ?₁ and ?₂ are the principal values of the dielectric permittivity tensor in the plane perpendicular to the medium axis) these polarizations are orthogonal and quasi-circular, while in the limit δ >> 1 they are orthogonal but linearly polarized. It is established that in the general case the chiral photonic crystals show a complete selectivity not to the circular polarizations (as at δ << 1), but to the natural polarizations.     

On the deflection of light by topological defects in nematic liquid crystals

The influence of controllable parameters like temperature and wavelength on the trajectories of light in a nematic liquid crystal with topological defects is studied through a geometric model. The model incorporates phenomenological details as how the refractive indices depend on such parameters. The deflection of light by the topological defect is then shown to be greater at lower temperatures and shorter wavelengths.     

Density functional theory for chiral nematic liquid crystals

ABSTRACT

In this note, the density functional theory for chiral nematic liquid crystals is expressed in terms of rotational invariant expansion coefficients of the pair-excluded area at fixed separation. This modification from the standard approach, without making any additional assumptions, yields a more efficient procedure for minimising the free energy, as well as providing insight into the origins of the effect away from the long-pitch limit.     

Phase transitions in ferroelectric liquid crystals

Molecular chirality plays an important role in the science of liquid crystals, leading to cholesteric liquid crystal, blue phases, ferroelectric and antiferroelectric smectic phases and twist grain boundary phases. In all of these mesogens, chirality is an intrinsic property built into the chemical structure of mesogenic molecules. The study of ferroelectric liquid crystals has seen substantial experimental strides. In theoretical aspects, there has been relatively little basic work on this fascinating class of material. This review will try to present a comprehensive overview of the current status of the phase transitions in ferroelectric smectic liquid crystals. The article begins with a brief introduction about the symmetry and structure of ferroelectric mesophases. An attempt is made to identify a range of problems and related questions associated with the study of phase transitions. In the remaining parts of the article the important experimental and theoretical developments are summarized. Finally, some of the future directions have been identified.     

部分相干光与相干光在湍流中的传输特性实验研究

相干光经湍流传输引起光强闪烁效应,理论研究表明利用部分相干光可降低该影响,而相关的实验验证鲜有报道。利用水介质的对流湍流池模拟产生了Rytov方差为0.04~0.16的湍流,进行了弱起伏区湍流中部分相干光及相干光的传输实验,由接收光强计算出其光强闪烁指数随湍流Rytov方差的变化关系。实验结果表明：随着湍流强度的增加,部分相干光与相干光的闪烁效应均增强,但部分相干光的闪烁指数小于相干光。将实验数据与Andrews等人建立的闪烁指数理论模型进行比较,得出了较为一致的结论。     

部分相干光与相干光在湍流中的传输特性实验研究

相干光经湍流传输引起光强闪烁效应,理论研究表明利用部分相干光可降低该影响,而相关的实验验证鲜有报道。利用水介质的对流湍流池模拟产生了Rytov方差为0.04~0.16的湍流,进行了弱起伏区湍流中部分相干光及相干光的传输实验,由接收光强计算出其光强闪烁指数随湍流Rytov方差的变化关系。实验结果表明:随着湍流强度的增加,部分相干光与相干光的闪烁效应均增强,但部分相干光的闪烁指数小于相干光。将实验数据与Andrews等人建立的闪烁指数理论模型进行比较,得出了较为一致的结论。     

General properties of lasing effect in chiral liquid crystals

Numerical simulations and experimental studies of the lasing effect in chiral liquid crystals are presented. It is shown that ring-like light emission, which is often observed experimentally, is a true lasing effect. Lasing condition and different modes in thin liquid crystal layers are discussed.     

Determination of the liquid crystals phase transition temperatures using optical rotation effect

Using optical rotation effect, a sensitive, simple optical analytical system is developed for determining the phase transition temperatures of liquid crystals (LCs). When a monochromatic polarized light passes through LCs sample and analyzer, the light intensity changes with temperature. Especially, during the phase transition process, the intensity varies greatly due to optical rotation effect. The variation of light intensity versus variation of temperature curve shows the phase transition temperatures of LCs clearly. The phase transition temperatures of three cholesteric liquid crystals (ChLCs) and a nematic liquid crystals (NLCs) were detected by this method, and compared with those of the differential scanning calorimetry (DSC) and polarized light microscope (PLM) methods.     

Lattice models for liquid crystals

A problem in the theory of liquid crystals is to construct a model system which at low temperatures displays long-range orientational order, but not translational order in all directions. We present five lattice models (two two-dimensional and three three-dimensional) of hard-core particles with attractive interactions and prove (using reflection positivity and the Peierls argument) that they have orientational order at low temperatures; the two-dimensional models have no such ordering if the attractive interaction is not present. We cannot prove that these models do not have complete translational order, but their zero-temperature states are such that we are led to conjecture that complete translational order is always absent.Work of EHL supported by U.S. National Science Foundation Grant MCS 75-21684 A02. Financial assistance from the Danish Natural Science Research Council is also gratefully acknowledged.     

Induced ferrielectricity in antiferroelectric liquid crystals

The behaviour of the antiferroelectric SmC_A liquid crystal phase under applied electric field is discussed theoretically. The phase diagram involving the SmA, SmC_A and SmC _A ^* phases is worked out and shown to exhibit a Lifshitz critical point. The deformation of the bilayer structures induced by the field transforms the SmC_A phases into a ferrielectric phase whose specific configuration is described. Received: 23 October 1997 / Revised: 8 April 1998 / Accepted: 14 July 1998     

Ultrasound diffraction of light beams in gyrotropic crystals

The features of the Bragg diffraction of Gaussian light beams at ultrasound waves in gyrotropic crystals were investigated. The dependence of the diffraction efficiency on the intensity of an ultrasound beam, the cross-section radius of a light beam, and their divergence ratio was found. An asymptotic expression for the transverse distribution of a diffracted beam was derived. The crystal gyrotropy has been found to affect significantly the change in the amplitude profile of the diffracted light at ultrasound frequencies of f ∼ 10²–10³ MHz. The maximum diffraction efficiency in the gyrotropic medium is shown to be reached at a certain ratio between the divergences, ultrasound intensity, and specific gyration of the crystal. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 370–374, May–June, 2000.     

液晶胆甾相的形成机制

采用量子理论的方法研究了胆甾相液晶的形成机制.证明了通常采用的手性作用势确实能够导致胆甾相的形成，并给出螺距的表示式. 关键词： 液晶 量子理论 螺距     

Short range orientational order in nematic liquid crystals

The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.