Specific features of optical diffusion in nematic liquid crystals

Methods of computer simulation are used to study multiple light scattering in the ordered phase of a nematic liquid crystal. The development of the diffusion regime is studied in detail. It is demonstrated that the transient time depends on the external magnetic field and the direction of the incident radiation. The diffusion coefficients along and across the director are calculated. The validity of the diffusion approximation is controlled using the moments of the distribution function. Analysis of the diffusion coefficients versus the external field shows that the diffusion coefficients remain almost unchanged in the experimental range of the magnetic fields. In the presence of relatively strong fields, the diffusion coefficients increase due to a variation in the shape of the scattering indicatrix and the free-path length of photons. The dependence of the diffusion coefficients on the radiation wavelength is also calculated. Comparison of the simulated results and the experimental data shows that the calculated diffusion rate across the director virtually coincides with the experimental rate. The calculated diffusion coefficient along the director is substantially greater than the corresponding experimental rate.     

The holographic investigation of diffusion of azo-dye in liquid crystals host

The diffusion of azo-dye (DR-1) in a planar liquid crystals host (5CB) at various temperatures has been investigated by laser-induced holographic grating relaxation technique. The decay of the diffraction intensity provides information about the diffusion of photoexcited azo-dye molecules. The relaxation time constants can be derived from the time dependence of the diffraction intensity fitted by a single exponential function. Thus, the diffusion coefficients parallel and perpendicular to the director of liquid crystals at various temperatures can be obtained from the plots of the reciprocal of the relaxation time versus the square of the grating vector. From the analysis of the holographic grating relaxation, the diffusion is faster along the molecular director than for the perpendicular case, and the diffusion increases with rising temperature either parallel or perpendicular to the nematic director of liquid crystals. PACS 42.40.Eq; 42.40.Lx     

Self-diffusion coefficient of nematic liquid crystals from diffusion concentration gradient

A dynamic study of the mass transport by optical methods provides a new approach to evaluate the nematic self-diffusion coefficient. This value is obtained by extrapolation to C = 0 in an empirical relation D_C = D₀ exp (-βC), where D_C is the mass-corrected impurity diffusion coefficient and C is the local concentration of the diffusion profile.     

Measurements of depolarization ratios and polarizability anisotropies of gaseous molecules

Values for the depolarization ratio (?^T_V) of Rayleigh scattered light are reported for a large number of gas molecules. When possible, the results are combined with refractive index data to yield the anisotropy in the optical polarizability.     

Photosensitization of ZnO single crystals by means of dyes

Photoconductivity in single crystalline ZnO sensitized by Eosine (p-type) and Rhodamine 6 G (n-type) has been measured in the temperature region 80–300 K. The dependence on wavelength of the exciting light shows in both cases a structure with three peaks. Measurements of the dependence on time tell that rise and decay times are strongly wavelength dependent, and indicate further that at least two processes are involved in the build up and decay of the photocurrent. The activation energies found confirm this. A model with two different kinds of surface traps, including both energy and electron transfer is proposed.     

Positronium diffusion in crystals

The temperature dependence of the diffusion coefficient is analyzed for the two cases of localized and delocalized positronium in a crystal. The diffusion coefficient for the localized positronium is calculated from the Kubo formula, while that for the free positronium is calculated in the relaxation-time approximation. The temperature dependence of the tunneling diffusion coefficient for positronium is specified by analogy with impuritons in a quantum crystal.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 25–28, July, 1981.     

Nonlocal 3D diffusion model of writing thick diffraction gratings in polymer-dispersed liquid crystals

The dynamics of the process of writing of holographic diffraction gratings in polymer-dispersed liquid crystals is considered with allowance for absorption. Diffusion of molecules of not only the monomer, but also of the polymer, as well as the influence of inhomogeneity of illumination inside the sample because of the light absorption are taken into account. Also the role of nonlocality of the response of polymerization process is considered.     

Adsorption of cyanine dyes on AgCl crystals studied by XPS

Adsorption of 3,3'-dimethyl-9-ethylthiacarbocyanine dyes with iodide or bromide counter ions showed two energy states of sulfur in the dye molecule, one is the adsorption state to AgCl substrate and the other occurs when dye molecules aggregated. Though iodide conversion occurred on KI adsorption on AgCl, the counter ion of the dye molecule did not cause halide conversion on adsorption. The counter ion was observed only after the first dye monolayer on AgCl was completed, and increased parallel with adsorbed dye amounts. On Ag-deposited quartz plate monolayer formation occurred only by dye adsorption compared by its counter ion, while only iodine adsorbed in KI adsorption.     

Broadband fivefold reduction of vacuum fluctuations probed by dyes in photonic crystals

We observed for the first time a strong angle-independent modification of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. Comparison with spectra from such crystals with much smaller lattice spacing, for which emission is in the long wavelength limit, reveals inhibition of emission up to a factor approximately 5 over a large bandwidth of 13% of the first order Bragg resonance frequency. The center frequency and bandwidth of the inhibition agree with calculated total density of states, while the measured inhibition of vacuum fluctuations is much larger. Because of the specific location of the dye molecules, we likely probe the strongly modulated local photonic density of states.     

Detection of phase biaxiality in liquid crystals by use of the quadrupole shift in 131Xe NMR spectra

An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in 131Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the 131Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the 131Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases.     

Energy transfer between fluorescent dyes in photonic crystals

Energy transfer from fluorescein (Fl) to Rhodamine B (RhB) in the opal photonic crystals has been investigated by photoluminescence. The results show that the energy transfer can be enhanced effectively by photonic bandgaps. When the fluorescence emission wavelength of donor Fl overlaps the photonic bandgap the fluorescence intensity of the donor is suppressed, while the fluorescence intensity of acceptor RhB is obviously enhanced. This enhancement can be attributed to the inhibition of radiative emission of the donor in the photonic crystals.     

Chromium tracer diffusion in NiO crystals

Diffusion of ⁵¹Cr in NiO single crystals in air has been studied by the tracer-sectioning technique. In the temperature range 1192–1642°C, the diffusion coefficient can be expressed by the Arrhenius expression D=D_oexp(-Q/RT), with D_o=(8·6±1·2)×10^?3 cm²/sec and Q=67·4±1·1 kcal/mole. The use of a high specific-activity tracer and a special configuration for the diffusion anneal prevented the self-dopling effect found by Seltzer and the evaporation of chromium from the sample surface. The present results, in conjunction with published results on nickel self-diffusion in NiO and interdiffusion in the NiO?Cr₂O₃ system, are used to determine a chromium ion-vacancy binding energy of about 5 kcal/mole in pure NiO.     

Temperature-induced magnetic phase transitions in crystals with competing single-ion and interionic magnetic anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.     

Measurements of magnetic susceptibilities,their anisotropies and crystal field investigations of monoclinic single crystals of Ho2GeO5

Monoclinic single crystals of Ho₂GeO₅ were grown by a flux technique. The molar principal susceptibilities (χ₁ and χ₃) have been measured in the temperature range 300–13.5 K. The principal magnetic anisotropies (χ₁–χ_2,χ₂–χ₃ and χ₁–χ₃) are obtained from the measured anisotropies in different modes of suspensions (b-axis vertical, c-axis vertical and (1 0 0) plane horizontal) of the crystal. The molar principal susceptibility (χ₂) is obtained from the observed values of the principal anisotropies. The observed principal susceptibilities together with their anisotropies are explained by invoking the crystal field interaction of the Ho³⁺ ion with its diamagnetic neighbors in non-axial symmetry. The molar susceptibilities are correlated to the ionic values by finding out the proper orientation of the principal molar susceptibilities, i.e., crystal susceptibilities with respect to that of the ionic susceptibilities (K_x, K_y and K_z). The electronic heat capacities are calculated from the crystal field analysis showing Schottky type anomalies.     

Calculation of the diffusion coefficient in crystals

The diffusion coefficient for vacancies is calculated on the basis of the Kubo formula. Here no assumptions are made about the character of the migration of the atom into the vacant site, and the calculation is carried out on the basis of a Hamiltonian proposed earlier by the author. A general expression is obtained for the temperature dependence of the diffusion coefficient that is valid in a wide temperature range. The case T » T_D (T_D is the Debye temperature) is especially analyzed, where the diffusion process is in the main connected with over-barrier jumping. The corresponding temperature dependence is determined by the expression T^3/2exp (–U/_BT, where U is the height of the potential barrier that separates neighboring sites. The dependence of the diffusion coefficient on the mass of the diffusing atom is also investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 60–65, June, 1985.     

Interface dynamics in liquid crystals

We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out. Received 5 August 1999 and Received in final form 13 September 1999