Defects in a TGBA phase: A theoretical approach

Following our experimental observations of disclination lines in freely suspended droplets and free-standing films (Yu.A. Nastishin et al., Eur. Phys. J. E 5 353 (2001)), topological defects of the twist grain boundary (TGBA) phase are considered according to two aspects: topological and energetical. There are two classes of line defects, disclinations (as in the cholesteric (N*) phase and the liquid vortices phase (NL*), relating to the directors tripod symmetries) and dispirations (relating to the translation-rotation symmetries); there are no topological point defects. Differences between N*, NL* and TGBA disclinations are physical, not topological. The absence of focal conic domains in the TGBA phase is an immediate consequence of the materialization of the helical axis (along the χ-director); the same feature, coupled to the trend to parallelism of the smectic layers, accounts for the predominance of λ-lines. Finally, the presence of defects akin to developable domains is explained in the frame of the leastcurvaturemodel, that requires the introduction of a third type of defects: the densitiesofedgedislocations of the smectic layers. Received 20 February 2002     

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract

A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.     

Tunable local surface plasmon resonance in liquid-crystal-coated Ag nanoparticles

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.     

Effects induced by 𝛄-irradiation on intraband transitions in Sr2+-doped ammonium zinc chloride crystals

Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr²⁺ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr²⁺ content on optical parameters of (NH₄)₂ZnCl₄: x Sr²⁺ single crystals (x?=?0.00, 0.020, 0.039, 0.087 or 0.144?wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH₄)₂ZnCl₄: x Sr²⁺ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (E _g ) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x?=?0.087 or 0.144?wt.% Sr²⁺. The results can be discussed on the basis of γ-irradiation-induced defects and Sr²⁺ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed.     

Complementary interference method for determining optical parameters of liquid crystals

ABSTRACT

Wedge cells of small apex angle, filled with liquid crystals, were used to determining optical characteristics as functions of temperature for three liquid crystalline mixtures recently produced and a reference nematic. The interference fringes appearing in polarised monochromatic light (of sodium yellow line) normally incident on the cell were exploited to measure the ordinary and extraordinary refractive indices in the reflection mode and birefringence in the transmission mode. The measurements were repeated using Abbe's refractometer for 6CHBT as the reference to verifying the precision. Additionally the order parameter was computed from birefringence as a function of temperature. The results confirm the usefulness of the method and provide the properties of two nematic liquid crystals of small and large birefringence and one smectic liquid crystal of medium birefringence, recently produced. The experimental systems served also to investigating phase transition between the liquid crystals and the isotropic liquid at near-clearing temperature.     

Low-amplitude Screening-photovoltaic Spatial Solitons in Biased Photovoltaic Photorefractive Crystals

The low-amplitude spatial solitons in biased photovoltaic photorefractive crystals are investigated theoretically. The analytical solutions for both the bright and the dark low-amplitude screening-photovoltaic spatial solitons in photorefractive crystals are obtained. The expressions for the width of these solitons are given. The explicit expressions for the spatial deflection and angular deviation of the bright low-amplitude screening-photovoltaic spatial soliton are also presented by taking into account the effect of diffusion.     

一维磁化等离子体光子晶体非互易特性研究

在理想条件下,为了研究等离子体回旋频率、等离子体频率、等离子体层厚度、周期常数和入射角在TM模式下对一维磁化等离子体光子晶体的非互易特性的影响,用利用传输矩阵法计算得到的TM波正向和反向传播的透射率来研究其非互易特性。研究结果表明,增加等离子体回旋频率和入射角度能够改善非互易特性;而一味地增加等离子体频率和等离子体层厚度将会使得非互易传播特性变得恶化;增加周期常数不能明显地改善非互易传播特性,但是通过改变外加磁场的施加方式能够改善其非互易特性。     

Science and technology of stressed liquid crystals: display and non-display applications

ABSTRACT

Stressed liquid crystals (SLCs) have emerged as promising tunable electro-optical materials more than a decade ago. They are optically transparent and are characterized by a giant phase modulation of the incident light (Nπ, N >> 1), fast (millisecond and shorter) electro-optical response, and a relatively low driving voltage (～1 V/μm). Surprisingly, despite their advanced electro-optical performance, these new materials did not receive due attention in the research community. One possible reason of such an inadequate interest in SLCs is the lack of the well-documented procedure describing how to actually produce these materials. This paper is aimed at the development of such a step-by-step practical guide suitable for experimentalist and engineers. The proposed technology is applied to produce and characterize SLCs. In addition, some applications of the materials are briefly discussed and a broader overview of their possible use is outlined.     

使用掺杂液晶的相共轭器件的研究

首次报道了掺偶氮染料液晶盘中相位共轭输出的现象．测量了它的响应时间、衍射效率和阈值功率．讨论了该器件的分辨率．实验表明：它具有ｍｓ级的响应时间和ｍＷ级的阈值功率，是一种有前途的新型相位共轭器件．     

Temperature dependence of photoluminescence and electroluminescence of ZnS:Mn crystals

By means of comprehensive analysis of the temperature dependences of the photoluminescence for ZnS:Mn crystals, we have observed persistent changes in the intensity, shape, and position of the maximum in the emission spectrum associated with changes in the immediate environment of the manganese luminescence centers. We have also observed inflection points on the voltage vs. brightness characteristics of the samples, the position of which depends on the frequency of the exciting voltage and the temperature. The observed dependences are explained using concepts describing the mechanisms of pre-breakdown luminescence. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 631–636, September–October, 2006.