摘要:

It is well known that a continuous wave laser beam impinging on a nematic liquid crystals film can induce a change in the orientation of the molecular director. In the case of homeotropic alignment and for a p-polarized obliquely incident beam, the reorientation process has no threshold and the sample exhibits a giant optical nonlinearity due to the laser-induced change in refractive index of the medium. In the small distortion approximation the phase change is governed by a linear differential equation whose coefficients depend on the ratio between the elastic constants. In this paper we describe an interferometric technique based on high precision two-dimensional spatial fringe analysis Fourier method for measuring the elastic constants of nematic liquid crystals. Since the determination of the elastic anisotropy relies on the measurement of the ratios between the elastic constants, this technique provides a powerful and high accuracy method for characterizing nematic liquid crystals.

ISSN:1058-725X    

DOI:10.1080/10587259808024379

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We describe non-standard techniques for optical studies of electroconvection structures in nematics. The time-resolved textures in the high frequency oscillatory regime are analyzed and compared with results of optical simulations. Conclusions on the relation between director pattern and optical texture are drawn. Using an additional phase plate in the optical set-up, we monitor director twist deformations in the convection structures which are usually neglected. We demonstrate their role in the evolution of the chevron texture.

ISSN:1058-725X    

DOI:10.1080/10587259808024380

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The results of study of structural genesis of the nonoriented nematic MBBA at different temperature pre-history are reported. The temperature dependent transformations of the vibrational Raman spectra and their connection with the change of MBBA structure were investigated in the spectral region 1100–1600 cm−1and in the temperature interval 163–333 K. The method to be used allows to prognosticate the structural evolution of MBBA and to identify its phase states. In particular it is found that the several coexisting fractions of different conformers may be obtained in the temperature region lower the glass transition temperature in dependence on the sample cooling rate.

ISSN:1058-725X    

DOI:10.1080/10587259808024381

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Dynamics of polymorphous transformations in liquid crystals under different temperature conditions is studied by analysis of vibrational Raman band behavior. The correlation between bands parameters and structural especial features of different phases allows to determine the composition proportions of complex phase and detailed diagrams of phase transformations.

ISSN:1058-725X    

DOI:10.1080/10587259808024382

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The negative birefringence optical compensation films can be used to significantly improve the viewing angles of single-domain and multi-domain LCDs. We review the major applications of the applications of negative birefringence compensation to TN, STN, vertically aligned LCDs, and parallel aligned LCDs. Most recently, there are three TFT/LCD panels with improved large viewing cone being commercialized with the use of the negative birefringence optical compensation films. These panels include: (1). US TFT/LCD vendors compensated single domain TFT/TN panels; (2). Sharp compensated 2-domain Super-V TFT/TN panels; and (3). Fujitsu super-wide compensated 4-domain vertical aligned TFT/VA panels.

ISSN:1058-725X    

DOI:10.1080/10587259808024383

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report the first observation of the formation of permanent light-induced annular patterns in a planar cell filled with dye-doped liquid crystal. The sample is placed between one rubbed surface, giving unidirectional planar alignment, and one isotropic surface, providing degenerated planar alignment. The patterns appear under the irradiation of the isotropic surface through the liquid crystal layer. The experimental results are explained in terms of spatial modulation of the light polarization over the isotropic surface, due to conformation nonlinearity of the mixture. The light-induced adsorption of azo-dye on the initially isotropic surface leads to “freezing” of this modulation and the appearance of a twisted director structure.

ISSN:1058-725X    

DOI:10.1080/10587259808024384

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report on the results of optical, X-ray and NMR studies of light-induced phenomena in smectic phases triggered by molecular phototransformations. UV irradiation causes trans-to-cis isomerization of the dopant molecules while irradiation in the visible part of the spectrum facilitates cis-to-trans relaxation. Cis-to-trans isomerization decreases the smectic layers' spacing; as the result, a smectic slab undergoes an undulation instability and/or formation of focal conic domains. These textures are nonuniform and scatter light; this effect can be used for optical data storage. The opposite trans-to-cis isomerization disorders the orientation of molecules and increases the thickness of smectic layers. Two beams of different wavelengths with distinct yields of isomerisation can be used to write and erase optical information.

ISSN:1058-725X    

DOI:10.1080/10587259808024385

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Fast electro-optical switching with planar nematic liquid crystals in multipass and also Fabry-Pérot modulators has been realized. By using short driving pulses of 150V, we achieved microsecond switching times for nematic planar cells (NPC) and even sub micro-second switching in case of nematic Fabry-Pérot modulators (NFPM), which have been applied as active Q-switches in Nd:YAG solid state lasers. Four different resonator configurations have been investigated. Typically, giant pulses of 35–70 ns duration (FWHM) with energies between 10–25 mJ are emitted. A rather plain Q-switch resonator design consists of only two optical components: a cholesteric liquid crystal polarizing mirror and a reflecting nematic liquid crystal modulator.

ISSN:1058-725X    

DOI:10.1080/10587259808024386

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Several homologues of new liquid crystals of nonchiral, banana shaped molecules are investigated by generation of the second harmonic (SHG) of an optical wave. Symmetry properties of the apparent thermodynamic phases are studied and first quantitative values of the nonlinear optical coefficients are given.

ISSN:1058-725X    

DOI:10.1080/10587259808024387

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A new reorientation effect in nematic liquid crystals of discotic molecules has been observed and investigated for the first time, which occurs under the combined influence of DC-electrical and optical fields. It is shown that illumination with intensities as low as 10−3W/cm2is sufficient for a strong photo-induced reorientation process, indicating a rather large nonlinear optical coefficient. The dependence of photoexcitation upon the applied voltage as well as from intensity and wavelength of the light wave has been investigated. Furthermore, a first model will be discussed, which explains the experimental observations to result from photo-induced space charge fields which enhance the electrical field-induced reorientation of the discotic molecules.

ISSN:1058-725X    

DOI:10.1080/10587259808024388

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor