Determination of bulk values of twist elasticity coefficient in a chiral smectic C* liquid crystal

A method is described for determination of bulk values of a twist elastic coefficient for smectic c-director in chiral smectic liquid crystals with a helical structure. The method was applied to 4-methylbutyloxy phenyl-4-octyloxy-benzoate (C8) in the chiral smectic C* phase. The measurements were performed using optical detection in a small deformation limit. In contrast to the usual methods, initial deformation of the helix (caused by strong surface interactions) was avoided by using homeotropic aligned thick samples. The critical temperature dependence of the measured coefficient was observed. The relation between the measured parameter and the smectic C order parameter is presented.     

Optical activity measurements in the smectic blue phases

Optical activity measurements have been performed on the smectic blue phases ( BP _Sm), which are a new kind of chiral liquid crystal. BP _Sm exhibit both three-dimensional orientational order, like the classical blue phases, and smectic positional order. Thus, they can be viewed as the three-dimensional counterpart of the twist grain boundary phases. A comparison with the optical activity of the classical blue phases is carried out, and an estimation of the BP _Sm lattice parameter is reported for the first time. Received 25 May 2001     

Photonics Devices Based on Hybrid Approach Combining Liquid Crystals and Sol-Gel Waveguides

In this article, the analysis of both an integrated electro-optic switch and a continuously tunable filter based on a Bragg grating in planar waveguide with a liquid crystal overlayer is reported. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure has been utilized in order to investigate the possibility of realizing an integrated electro-optical switch, whereas the soft-mode of smectic A* liquid crystals, allowing a continuous modulation of extraordinary refractive index, have been utilized in order to design an integrated wavelength filter in the wavelength range of interest for optical communication. Moreover, the performance of an electro-optical router, working at the wavelength of 1.55 μm and based on the multimode interference effects together with the electro-optic effect of smectic A* liquid crystal, is theoretically and numerically discussed.     

Photonics Devices Based on Hybrid Approach Combining Liquid Crystals and Sol-Gel Waveguides

In this article, the analysis of both an integrated electro-optic switch and a continuously tunable filter based on a Bragg grating in planar waveguide with a liquid crystal overlayer is reported. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure has been utilized in order to investigate the possibility of realizing an integrated electro-optical switch, whereas the soft-mode of smectic A* liquid crystals, allowing a continuous modulation of extraordinary refractive index, have been utilized in order to design an integrated wavelength filter in the wavelength range of interest for optical communication. Moreover, the performance of an electro-optical router, working at the wavelength of 1.55 μm and based on the multimode interference effects together with the electro-optic effect of smectic A* liquid crystal, is theoretically and numerically discussed.     

The effects of a laser field on phase transitions in liquid crystals

We investigate theoretically some phase transitions in liquid crystals in the presence of a laser beam. We found, in non-absorbing nematics, a laser-induced one-way transition from a paranematic to a nematic phase. In absorbing nematics we found, in addition to this transition, a one-way transition from a nematic to a paranematic phase with increasing laser intensity. Further, we found a reentrant nematic or a reentrant paranematic via paranematic or nematic phase respectively. In the case of smectic A, laser absorption results in a coupling between the positional and orientational orders. As a result, the smectic A to nematic transition can change from second order to first order and the smectic C to smectic A transition can become first-order in the field of a laser.     

New investigation on the tetragonal liquid-crystalline phase or SmQ

Chiral liquid crystals, for which twist and smectic order strongly compete, can exhibit complex architectures. This is the case of the SmQ phase which shows a high degree of 3D order (tetragonal or hexagonal) as well as a liquid signature. We have combined X-ray scattering and optical observations on a new compound exhibiting a SmQ phase to discriminate between the two models proposed by A.M. Levelut et al. (J. Phys. II 7, 981 (1997)) at the molecular level for the phase with the I4₁22 space group.Received: 9 March 2004, Published online: 10 November 2004PACS: 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures - 61.30.Mp Blue phases and other defect-phases     

Theory of the incommensurate smectic-A phases in dimesogenic liquid crystals

A mechanism assuming a mismatch between the basic smectic periodicity and the amplitude and phase modulations periods of the mass density wave order parameter is proposed, for describing the incommensurate smectic-A structure found in dimesogenic liquid crystals. The different sequences of phases found in this family of compounds are described theoretically.PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 64.70.Md Transitions in liquid crystals     

Insights on some chiral smectic phases

Combining layered positional order as smectic order and chirality can generate complex architectures since twist parallel to the layers is not allowed. This paper will review some new experimental results on different phases resulting from the competition between smectic positional order and twist orientational order. It concerns the TGB_A and the NL^*, that is the liquid line phase as well as the SmQ phase. Chiral effects in the isotropic phase will also be discussed.     

Determination of the bulk rotational viscosity coefficient in a chiral smectic C* liquid crystal

The method of determination of the bulk values of the rotational viscosity coefficient of the smectic c-director in chiral smectic liquid crystals with a helical structure is described. The measurements were performed using optical detection in the small deformation limit. In contrast to the usual methods, the initial deformation of the helix (caused by strong surface interactions) was avoided by using homeotropic aligned thick samples. The coefficient was measured in 4-methylbutyloxy phenyl-4-octyloxy-benzoate (C8) in the presence of laminar flow. The critical temperature dependence of the measured coefficient was found. The relation between the measured coefficient and the smectic C order parameter is shown.     

Some recent studies of liquid crystals: A review

The following topics are reviewed briefly: (i) The first experimental determination of the decrease in the smectic A-nematic transition temperature brought about by a twist deformation, an effect which is the analog of the influence of a magnetic field on the superconductor-normal metal transition; (ii) the similarity between the hydrodynamics of cholesteric liquid crystals and superfluid hydrodynamics, and some of its consequences, e.g., the possibility of a temperature wave attending second sound in the cholesteric phase; and (iii) the nematicsmectic A-smectic C multicritical point.     

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.     

Structural investigations on smectic blue phases

Smectic blue phases ( BP(Sm)) are thermotropic liquid crystalline phases which exhibit both three-dimensional orientational order, such as classical blue phases, and smectic positional order. BP(Sm) appear as the three-dimensional counterpart of twist grain boundary phases. X-ray scattering and optical polarizing microscopy provide information on the hexagonal and cubic symmetries of these new phases.     

Measurements of absolute values of electrooptic coefficients in a ferroelectric liquid crystal

Weak external electric field E causes two effects in chiral smectic liquid crystals: linear change of the optic axis direction and modification of the shape of the indicatrix. The relative changes in the light intensity consist of two components. The first component represents the modulation with the fundamental frequency f and the second one, with the doubled frequency 2f (second harmonic). We carried out measurements of the electrooptic modulation at both the first and second harmonics frequency. The applied calibration procedure allowed for expressing the experimental results as angular quantities independent of experimental conditions. Therefore we were able to determine the absolute values of the coefficients describing both effects of the electric field on the optical properties of the sample. We studied the temperature dependence of the first and second order coefficients in the ferroelectric smectic C? and paraelectric smectic A phases. The experiments delivered an information on the structure of tilted smectic liquid crystals and its dependence on the electric field.     

Viscoelastic modes in chiral liquid crystals

Viscoelastic properties of liquid crystals are very important for applications like display technology. However, there are not many direct techniques to study them. In this review, we describe our studies on the viscoelastic modes of some chiral liquid crystals using dynamic light scattering. We discuss viscoelastic modes corresponding to the C director fluctuations in the chiral smectic C phase and the behaviour of the Goldstone-mode near the chiral smectic C-smectic A phase transition. In cholesteric liquid crystals, we consider the director fluctuations in a wavevector range comparable to the inverse pitch of the cholesteric. Here, the study of the scattered light in the vicinity of the Bragg reflection using a novel geometry will be presented.     

Dielectric permittivity in the smectic a phase of liquid crystals

We propose a functional dependence of the dielectric constant with the smectic order parameter of liquid crystals in their smectic A phase. The proposal is verified by comparing with the published values of heptylazoxybenzene.     

Hexagonal arrays in nematic liquid crystals

In this letter observations are reported which were carried out by diffraction of a laser beam, by polar diagrams of the diffracted light and by optical microscopy, being performed on a hexagonal structure which appears in nematic liquid crystals for a twist geometry.     

Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

平行排列丝状液晶在超短脉冲激光激励下发生瞬态扭曲形变的最佳偏振角

利用光学相位延迟法，观察到平行排列丝状液晶在超短脉冲激光（10ns）激励下的瞬态扭曲形变.实验结果表明，虽然脉冲激光的电场强度远远大于液晶发生扭曲形变的阈值，但并不能保证液晶会发生瞬态扭曲形变.这主要是因为脉冲激光的激励时间非常短，而液晶分子发生扭曲形变的响应时间相对较长，从而使其来不及形变.分析发现：激励激光的偏振方向与平行排列丝状液晶分子的最佳夹角是45°.利用此条件，观察到掺微量D型苏氨酸卟啉的平行排列丝状液晶在超短脉冲激光激励后具有记忆效应，这在光存储和记忆显示方面有很大的实用价值.     

二极管泵浦无机液体激光器出光实验研究

 为探索新的高能激光体系,搭建了二极管泵浦的液体激光器。采用激光二极管作为泵浦光源,单侧泵浦掺钕离子的无机激光液体,进行了出光实验。通过测量输出光束的近场分布、脉冲波形和光谱,证实实现了激光输出,输出激光的波长为1 053 nm。输出的单脉冲激光能量达到47 mJ,光-光转换效率达到14%。其光-光转换效率高出闪光灯泵浦液体条件下2个数量级,说明该激光体系具有向高能激光体系发展的前景。     

One nanosecond photon correlation spectroscopy on smectic liquid crystal films

We have extended photon correlation spectroscopy down to a one-nanosecond time scale, and applied it to a study of layer undulations in freestanding smectic-A films of two cyanobiphenyl liquid crystals. Temporal correlations in the intensity of scattered light reveal an interesting combination of under- and overdamped modes. The underdamped mode is accurately described by a recently calculated correlation function of the smectic layer displacement, although its frequency and damping rate exhibit stronger dispersion at large optical wave vectors than expected from current dynamical models for smectic films.