Nematic-isotropic phase transitions in thin slabs of liquid crystals with topological defect arrays.

We study the nematic-to-isotropic phase transitions in thin slabs of nematic liquid crystals with photopatterned director fields of topological defect arrays at constant heating rates and show that the transition kinetics is significantly impacted by both the heating rate and the topological strengths of these defects. Specifically, with ±1/2 defect arrays, the isotropic domains emerge from the defect cores when the heating rate is high, while from random places when the heating rate is low. With ±1 defect arrays, the isotropic domains always emerge from the defect cores regardless of the heating rate. Furthermore, the isotropic domains show significant movements at slow heating rates, and the total area of the isotropic domains grows with the temperature T following a simple power law (T - T')γ, where the exponent γ is approximately 1 in most cases and is 2/3 for the ±1 defect arrays at low heating rates when the isotropic domains are pinned on the defect cores. We attribute this phenomenon to an interplay between the surface tension and bulk free energy.

Controlled Placement of Microparticles at the Water-Liquid Crystal Elastomer Interface.

Controlled placement of microparticles is of prime importance in production of microscale superstructures. In this work, we demonstrate the remote control of microparticle placement using a photoactivated surface profile of a liquid crystal elastomer (LCE) coating. We employ light-responsive LCEs with preimposed patterns of molecular orientation (director) in the plane of coating. Upon UV illumination, these in-plane director distortions translate into deterministic topographic change of the LCE coating. Microparticles placed at the interface between the LCE coating and water, guided by gravity, gather at the bottom of photoinduced troughs. The effect is reversible: when the substrates are irradiated with visible light, the coatings become flat and the microparticle arrays disorganize again. The proposed noncontact manipulation of particles by photoactivated LCEs may be useful in development of drug delivery or tissue engineering applications.

Vector photochromism in polarization-sensitive materials.

The phenomenon of vector photochromism was observed in some high-efficient polarization-sensitive materials depending on the radiant exposure of the inducing linearly polarized actinic light. The phenomenon has the purely vector nature because the absorption of the irradiated and unirradiated areas of the material is practically identical when we use unpolarized probing light. However, an essential change in the absorption spectrum was observed under probing the sample by linearly polarized nonactinic light when it passes through an analyzer, and this change depends on the value of radiant exposure. The kinetics of the photoanisotropy induced by linearly polarized actinic light at 457 nm was studied in case of wavelengths of 532 and 635 nm of the probing beam. The noticeable difference in absorbance was observed with increase in radiant exposure from 60 J/cm² up to 250 J/cm² for the used wavelengths of the probing beam. The experimental results obtained in polarization-sensitive material based on the ammonium salt of the azodye Mordant pure yellow in a gelatin matrix are presented. The dependence of the effective anisotropy on the material thickness has been investigated. The mechanism of the phenomenon is discussed. The observed effect can be used for creating dynamic polarization spectral filters controlled by light and the spectrally selective dynamic polarization holographic gratings.

Dynamic polarization holography: 2. Dynamic polarization-holographic gratings and their application.

Dynamic polarization-holographic gratings with a different profile of anisotropy in the band are presented. Polarization-sensitive recording materials of two types are used: material possessing high dark relaxation and highly reversible material without dark relaxation in which the recorded grating is erased by a circularly polarized beam. For a grating recorded by two orthogonally circularly polarized beams a diffraction efficiency of 20% has been obtained at 3.5 W/cm2 power density during 1 ms recording/erasing time. An all-optical cross commutator based on the matrix of dynamic reprogrammable polarization-holographic microholograms is considered. The amplification of the weak beam at two-wave mixing in polarization-sensitive materials has been shown, with an obtained amplification coefficient of 4.95.

Dynamic polarization holography. 1. Dynamic polarization-sensitive materials on the basis of azo-dye-containing polymers.

The theoretical and experimental investigation of the kinetics of photoanisotropy induction in polarization-sensitive media with significant dark relaxation on the basis of azo dyes in polymer matrices is considered. For improving dynamic characteristics of polarization-sensitive materials, new azo dye N,N-dimethyl-4((4-nitrophenyl)diazenyl)aniline has been specially synthesized, and the kinetics of photoanisotropy induction and relaxation in materials on the basis of this dye has been compared with materials on the basis of Dimethyl Yellow. It is shown that spectral sensitivity of the material with new azo dye is in the range of 470-520 nm, the working range is both in the visible and in the infrared range (from 520 nm at least up to 3400 nm), the maximum meaning of effective anisotropy is 38% (Deltan approximately 0.022), and the time of photoanisotropy inducing and erasing is approximately 18 ms. It is shown that, if we confine ourselves to meanings of effective anisotropy of approximately 1-2% (Deltan approximately 0.007), the time of inducing/erasing will be of the order of microseconds. The use of such dynamic polarization-sensitive materials gives the possibility of a new direction--dynamic polarization holography to be developed that combines the possibilities of both dynamic and polarization holography and that opens new prospects for applications.