摘要:

We report an accurate analysis of an optical storage effect observed in dyedoped PDLC films. Permanent diffraction gratings are produced using standard holographic techniques with different kind of PDLC samples. For all these samples the polymerisation process is completed before the writing procedure. The analysis of the experimental results brought us to distinguish the role of the PDLC components in the observed phenomenon and propose some hypothesis about the possible physical mechanisms at the basis of the storage effect.

ISSN:1058-725X    

DOI:10.1080/10587259808024399

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The process of optical second harmonic generation (SHG) in a slab of the helically modulated SmC* material is analysed for a case when the SHG frequency is in the vicinity or inside the region of the selective reflection band. The analysis bases on the numerical solutions of the nonlinear wave equation. Various possible phase matched combinations of the optical eigen-modes are discussed and the dependence of the SHG intensity on the optical frequency and sample thickness is given.

ISSN:1058-725X    

DOI:10.1080/10587259808024400

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

There is considerable interest in reconfigurable, transparent switches with low loss for both telecom and computing applications. Liquid crystal devices are strong contenders for such applications because they are a tested technology, they can be spatially multiplexed and they can be manufactured at relatively low cost. The main performance criteria of the switch, which should be maintained over a 25 nm bandwidth around 1.55 μm, are: less than 3 dB overall loss per channel, and less than -25 dB crosstalk between channels. These conditions require a liquid crystal cell which rotates the incident polarisation vector by 90° in the off-state, and by 0° in the on-state. In the present work, the cell is first studied by numerical simulation. It is assembled using thick glass plates with low absorption conductive coatings, and peripheral aluminium foil spacers. The simulated results were verified by experiment.

ISSN:1058-725X    

DOI:10.1080/10587259808024401

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

With photon correlation spectroscopy we have studied dynamical behavior in several systems of nematic liquid crystals embedded in host matrices, i.e., polymer dispersions of nematic droplets, Nuclepore membranes with cylindrical cavities, and porous silica aerogel. In all cases the dynamics of orientational fluctuations is substantially changed compared to the bulk nematic phase and some new phenomena appear. While due to the confinement no long wavelength modes of the director orientational fluctuations are present in the nematic droplets and cylinders, in the nematic liquid crystal in the aerogel a mode with the correlation length of 1–2 orders of magnitude larger than the average pore size was observed. An additional dynamic process with a nearly logarithmic autocorrelation function extending from millisecond to more than ten seconds is more or less pronounced in all three systems and we attribute it to reorientational diffusion in random potential.

ISSN:1058-725X    

DOI:10.1080/10587259808024402

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We present results of the light scattering properties of PDLC films in the Rayleigh-Gans long wavelength régime. The calculations take into account dependent scattering effects using an effective medium theory. The Percus-Yevick hard-sphere approximation is used to introduce droplet correlation effects. We discuss the light scattering properties of radial and bipolar partially ordered droplets, making detailed comparisons with the literature where appropriate.

ISSN:1058-725X    

DOI:10.1080/10587259808024403

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An electro-optic switch based on a planar, ferroelectric liquid crystal waveguide oriented in the bookshelf geometry has been realized and tested. The waveguide device consists of three stages: two glass waveguides and an FLC waveguide in between, permitting coupling and decoupling of the light into and out of the glass waveguides with prisms. The refractive indices of the FLC used were determined with a total internal reflection method. The time characteristics of the ON-OFF switch and the OFF-ON switch have been studied. Our main aim was the experimental demonstration of previous theoretical studies and no attempt was made up to now to optimize the device performance.

ISSN:1058-725X    

DOI:10.1080/10587259808024404

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

On the basis of the amplitude-phase screen model an analytical expression for the coherent transmittance of the polymer-dispersed ferroelectric liquid crystal (PDFLC) monolayer normally illuminated by linearly polarized plane wave has been obtained. Within the framework of the anomalous diffraction approach the conditions for coherent transmission quenching have been determined. It is shown that for the PDFLC monolayer on the basis of bistable smectic liquid crystals it is possible to provide a coherent transmittance equal to zero and unity for two stable states of droplet directors in electric field. The influence of polydispersity for the γ-size distribution of droplets on the quenching conditions of coherent transmission has been investigated.

ISSN:1058-725X    

DOI:10.1080/10587259808024405

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A design of a vertical directional coupler, operating at the wavelength λ= 632.8 nm, is presented. The coupler consists of a layer of the Hoechst ferroelectric liquid crystal mixture FELIX-M4851–025, homogeneously aligned and sandwiched between two single mode ion-exchanged BK7 glass waveguides. The design of the directional coupler was carried out by using the beam propagation method, taking into account also teflon alignment layer and ITO electrode contributions. Given the FLC mixture refractive indices, no=1.611 and no= 1.466 at λ= 632.8 nm and T= 25°C, calculations show that less than 60 μm coupling length, with an extinction ratio higher than 21 dB can be obtained by using single mode waveguides with a refractive index change at the surface Δn= 0.013, an FLC layer 2 μm thick, and cone axis tilted by about 19° from the propagation direction of the lightwave.

ISSN:1058-725X    

DOI:10.1080/10587259808024406

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Optically induced molecular reorientation in nematic liquid crystals is an interesting phenomenon, suitable of many practical applications, due to the large electric and optical anisotropy of such materials in the nematic phase. Therefore great importance has the study of the dynamics of the molecular reorientation. In the usual configuration the cell contains a homeotropically aligned nematic liquid crystal and a linearly polarized laser beam impinges on it. However, the presence of a Freedericksz transition requires the use of some experimental ingenious contrivances: such as the application of a magnetic field or performing the experiment only for a large enough incidence angle. We show here that the molecular reorientation dynamics of a nematic liquid crystal can be easily studied without any magnetic field, even at normal incidence, using a hybrid aligned nematic liquid crystal cell: i.e. a cell with homeotropic alignment on one boundary and planar alignment on the other.

ISSN:1058-725X    

DOI:10.1080/10587259808024407

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Polymer dispersed liquid crystals (PDLC) are dispersions of liquid crystal micro-droplets in a polymeric binder. Droplets appear as optically uniaxial spheres randomly oriented so that the material is optically isotropic. The application of an external electric field results in a reorientation of the liquid crystal and therefore in a switching of the sample from an opaque to a transparent state. Sample transmittance is also a function of the temperature. If the liquid crystal refractive index in the isotropic phase is equal to the one of the polymer, after the Nematic Isotropic transition the material is transparent. Therefore PDLCs show both thermo-optical and electro-optical effects. In this paper, we present a mathematical model able to describe the dependence of the sample transparency on the temperature, as a function of the applied voltage. We show the model accuracy by comparison with new experimental data that are briefly discussed.

ISSN:1058-725X    

DOI:10.1080/10587259808024408

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor