Three-dimensional vector holograms in photoreactive polymer dissolved liquid crystal composite

A detailed analysis of three-dimensional vector holograms is presented which were recorded in a photoreactive polymer dissolved liquid crystal. The diffraction properties of resultant periodic anisotropic structures were investigated. Experimental observations confirmed that the diffraction efficiencies of the structures varied with slight changes of the grating pitch, and also that they strongly depended on the incident angle of the probe beam. These diffraction properties were explained by the finite-difference time-domain optical simulations.     

A novel liquid crystalline system for partial alignment of polar organic molecules

A new system for partial alignment of polar organic molecules to measure residual dipolar couplings in NMR consists of a 1:1 or a 2:1 mixture of water and DMSO including 3-13% n-alkylpentaethylene glycol as the surfactant. Temperature and concentration dependence of the alignment system are investigated and, as examples, the 13C,1H residual dipolar couplings for the amino acid methionine 1 and for an alpha-methylene-gamma-butyrolactone 2 have been obtained and are compared with those obtained from the alignment media consisting of n-alkylpentaethylene glycol, n-alkyl alcohol and water.     

The application of time resolved EPR to the study of photoreactive molecules in solid and liquid crystalline environments

The use of transient EPR spectroscopy for the study of photoreactions in solid and liquid crystalline is demonstrated. The method is applied to an intramolecular photoreaction in 2-(4-methylpentyl)-7-nonanoylfluorene (MPNOF) and to an intermolecular photoreaction in 4-cyano-4′-octylbiphenyl (8CB) doped with phenazine. It is shown that both MPNOF and phenazine undergo photoreactions via their first excited triplet state. Orientational ordering in the solid and liquid crystalline phase is studied via the excited triplet powder spectrum. Spectra of precursor and products can be identified and separated by their kinetics. The possible nature of the products of reaction is discussed.     

Formation of positive and negative anisotropic holographic gratings depending on recording energy in photoreactive liquid crystalline copolymer films

Anisotropic diffractive gratings formed by birefringent molecular alignment are fabricated by a holographic technique using a photo-cross-linkable liquid crystalline copolymer (PLCCP). Depending on the exposure energy used for holographic recording, bright or dark regions of the interference fringe can be chosen for the molecular-aligned region, analogous to positive- and negative-type photoresist processing in photolithography. In conjunction with simultaneously generated surface relief gratings, the anisotropic refractive index modulation inherent in the PLCCP film exhibits different diffraction properties depending on the incident beam polarization. The anisotropic refractive index grating profiles are quantitatively presented for two cases determined by the exposure energy, based on the Fourier analysis of diffractive optics. The selection of different grating formations selected by varying only the exposure energy could provide a strategy for the fabrication of functionalized diffractive optical elements.     

In-situ elastomer composite based on fluorocarbon elastomer/liquid crystalline polymer blend

Blends of fluorocarbon elastomer (FKM) and liquid crystalline polymer (LCP) have been prepared by melt mixing technique. Processing studies indicated the decrease in the viscosity and the state of cure with the addition of 10 wt% LCP, and then increased at a higher rate with the addition of more LCP to the blend. The tensile strength values decreased at lower level of LCP. However, the modulus and tear strength values increased with higher increment of LCP content. From the X-ray diffraction measurements it has been observed that the crystalline structure of the FKM is greatly affected by the addition of LCP. The degradation temperatures from thermogravimetric analysis (TGA) suggested improved thermal stability of the fluorocarbon-LCP blends. From the dynamic mechanical analysis (DMA), it has been found that the glass transition temperature (T _g) of the blends increased with increase in LCP content. For the compositions of 10 wt% and 20 wt% LCP blends, enhancement in storage modulus is found above the glass transition of FKM. Under dynamic conditions the increase of LCP content restricts the matrix flow and hence cracks developed at the interface of the LCP fibrils and matrix.     

Mesomorphic properties of a bent-shaped liquid crystalline monomer

We have prepared a new bent-shaped monomer designed for main chain liquid crystalline polymers. The mesomorphic behaviour of the monomer was fully characterized by DSC, polarizing optical microscopy and electro-optical measurements. Based on X-ray analysis of the monomer, its structural properties were established and a possible molecular arrangement in the mesophases is proposed. A direct comparison of the material shows that an introduction of an isolated double bond can slightly modify the mesomorphic properties of the liquid crystalline monomer in comparison with the alkyl-chain analogous bent-shaped compound studied previously.     

New liquid crystalline material with a broad temperature range ferroelectric phase

A new series of chiral liquid crystals having cyano group in the mesogenic core has been synthesized and characterized. The synthesized compounds exhibit a broad ferroelectric chiral smectic C mesophase with a high spontaneous polarization.The work was supported by Grants No. 202/93/155 and No. 106/93/0839 from the Grant Agency of the Czech Republic and by Grant No. 19062 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

On the one-electron theory of crystalline solids in a homogeneous electric field

Assuming that the one-electron states of a perfect crystalline solid are known, an approach for the calculation of the one-electron states in the presence of external fields and/or other deviations from the periodic potential of the perfect crystal is suggested. The treatment is based on the Wannier representation and the use of a method for solving some operator non-polynomial differential equations. In the approximation of the one-band Wannier equation an exact solution of the problem for electron states of the crystals in a homogeneous external electric field is given. The results obtained in the tight binding approximation for cubic crystals show that the electron motion along the field is finite and the degree of its finiteness for a given electric field strength is greater, the smaller the width of the initial energy band considered. In the one-band approximation Considered the electron energy spectrum has the character of the Wannier-Stark ladder. It is also shown that an influence of transverse motion on the character of the finite motion along the field appears when a non-additivity in the initial energy band function with respect to the energies of motions parallel and perpendicular to the field direction is present.     

Polarization of light transmitted through a polymer film with nanosized droplets of liquid crystal

A method is developed for analyzing the state of polarization of a plane wave transmitted through a polymer-dispersed liquid-crystal (PDLC) film with nanosized liquid-crystal (LC) droplets. This method is based on the anisotropic-dipole approximation for describing scattering by a separate droplet and on the Foldy-Twersky approximation for describing propagation of light in a film. Equations are obtained that relate the ellipsometric parameters of coherent (direct) light transmitted through a PDLC film to the order parameters that characterize the morphological and structural properties of the film. Elliptic and circular polarizations and the rotation of the plane of polarization of a wave transmitted through a film are investigated under the normal illumination of the PDLC film by a linearly polarized plane wave. The order parameters of the PDLC film are determined as a function of a control field under the transition from a partially ordered structure of optical axes of LC droplets to a homeotropic structure.     

Polarization and the luminescence spectrum of crystalline sodium uranyl acetate

A photoelectric spectrometer with a linear dispersion of 5.2 Å/mm at was used to study the luminescence spectrum of sodium uranyl acetate crystals at 77°K. The resulting spectrum is interpreted on the basis of the crystal structure. The nature of the optical activity of the crystals is discussed.In conclusion, the authors express their sincere thanks to Academician of the Belorussian SSR A. N. Sevchenko for his interest in and attention to the present study.     

Molecular simulation and theory of a liquid crystalline disclination core

Molecular simulations of a nematic liquid crystal confined in cylinder geometry with homeotropic anchoring have been carried out. The core structure of a disclination line defect of strength +1 has been examined, and comparison made with various theoretical treatments, which are presented in a unified way. It is found that excellent fits to the cylindrically symmetrized order tensor profiles may be obtained with appropriate parameter choices; notwithstanding this, on the time scales of the simulation, the cylindrical symmetry of the core is broken and two defects of strength +1 / 2 may be resolved.     

Optical alignment of azo-polymer-stabilized nematic liquid crystals

Optical alignment and strong residual birefringence in a nematic liquid crystal that is stabilized by a polymer network are described. The polymer's main chain contains an azo dye, which is responsible for the repulsion of the axis of the guest-host system from the electric field of the excitation light.     

Fluorescence study of a field-induced director reorientation in a liquid crystalline polyacrylate

The electric field-induced director reorientation is investigated by fluorescence spectroscopy and turbidimetry. The dynamics of this reorientation are studied as a function of temperature, applied voltage, and frequency.     

Polarization properties of nematic liquid crystal cell with tapered optical fiber

In the paper, an extended analysis of the polarization properties of a liquid crystal cell with a biconically tapered single-mode telecommunication optical fiber was presented. These properties are a result of a sample geometry and used LC materials. They were analyzed by using two theoretical models based on the matrix decomposition methods, i.e., polar and singular-value one. By measuring Mueller matrices, information about losses, depolarization, dichroism and birefringence was obtained. In the experiment two types of tested samples filled with well-known 6CHBT and E7 liquid crystals were prepared and all optical parameters were shown as the voltage dependence. The tested samples have dichroic properties and for both models calculated PDL is similar and it increases from 2.6 to 6.6 dB for E7 and from 0.4 to 2.7 dB for 6CHBT with voltage changes within the range of 40 – 190 V. Optical losses simultaneously decrease from 30 dB to 27 dB and from 36 dB to 28 dB, respectively. The birefringence properties cannot be directly comparable due to differences between both applied models but voltage fluctuations of these parameters are not significant. These results confirm expected dichroic properties of designed device and complete knowledge about its working principles. Moreover, presented analysis validates usefulness of the singular-value decomposition model applied to dichroic optical fiber elements.     

Effects of recording wavelength on three-dimensional vector holograms in photoreactive liquid crystal composites

Effects of recording wavelength on the recently proposed (Sasaki, 2008) three-dimensional vector holograms, in which the optical anisotropy is three-dimensionally modulated, are presented experimentally and theoretically. The polarization states of the interference light are three-dimensionally modulated due to both the polarization interference and optical anisotropy in the recording medium. These spatial distributions of the polarization states and the resulting diffraction properties in the three-dimensional vector holograms are strongly dependent on the recording wavelength. Theoretical consideration based on the finite-difference time-domain (FDTD) method reveals the mechanism of the optical characteristics of the three-dimensional vector holograms recorded by various kinds of light sources with different wavelengths.