V. A. Loiko

Interference quenching of light transmitted through a monolayer film of polymer-dispersed nematic liquid crystal

A theoretical study of light transmission through a polymer with a monolayer ensemble of bipolar nematic droplets under applied electric film is performed. The possibility of interference quenching of monochromatic coherent light directly transmitted through the film is predicted theoretically and realized in the experiment.

Coherent transmission and reflection of a two-dimensional planar photonic crystal

A method for modeling the radial distribution function for particles of a two-dimensional planar photonic crystal in the form of a monolayer of spatially ordered monodisperse spherical particles is proposed. The coherent transmission and reflection coefficients for layers under normal illumination are calculated in the quasi-crystalline approximation of the multiple wave scattering theory. The dependence of the coherent transmission and reflection of the layer on the degree of ordering of the spherical particles is investigated. The influence of the long-range order on the coherent transmission and reflection coefficients for layers with triangular, square, and hexagonal lattices is estimated. Monolayers of weakly absorbing dielectric and strongly absorbing metallic particles are considered.

Coherent transmittance of a polymer dispersed liquid crystal film in a strong field: Effect of correlation and polydispersity of droplets

A method for computing the coherent transmittance of a film of polymer dispersed nematic liquid crystals aligned by an external field in the case of an oblique incidence of light is developed, and a theoretical analysis of this quantity is performed. The effects of close packing are considered in terms of the interference approximation of the theory of multiple wave scattering. The correlation arising in the spatial distribution of polydisperse droplets is taken into account by introducing partial distribution functions. It is shown that an increase in the concentration of liquid crystal droplets is accompanied by an increase in the coherent transmittance of the film.

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal

We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θs ≤ 8°).

Coherent transmission and reflection spectra of ordered structures from spherical alumina particles

We have studied the transmission of light through a multilayer system of plane-parallel monolayers of monodisperse nonabsorbing spherical alumina particles under conditions of normal illumination in the wavelength range of 0.3–1.0 μm. In the quasi-crystalline approximation of the theory of multiple scattering of waves, we have calculated the coefficients of coherent transmission and reflection of partially ordered monolayers of particles and monolayers of particles with a nearly regular packing (planar photonic crystals with a nonideal lattice). Based on these calculations, using the transfer matrix method, we have calculated the coefficients of coherent transmission and reflection of the multilayer system. The influence of the spatial distribution of particles in individual monolayers on the coherent transmission and reflection spectra of the system has been analyzed. Characteristic minima in transmission spectra of layers caused by the interference of waves have been described. One of these minima is determined by the concentration and optical properties of particles. It appears in the spectrum of a monolayer at ratios of wavelengths and particle sizes roughly corresponding to one of the main maxima of the extinction. The second minimum is determined by the regularity of distribution of particles in the plane of the monolayer. It occurs in the spectrum of the monolayer at particle sizes comparable with the wavelength. The third minimum is the photonic band gap. It appears in the spectrum of a multilayer structure at spacings between monolayers comparable with the wavelength and is caused by the periodicity of the structure. The results make it possible to solve problems of monitoring of the degree of ideality of packing of particles by analysis of the spectral dependences of the coefficients of coherent transmission and reflection, problems of creating multilayer selective reflectors and bandpass filters, diffusers, and other elements based on alumina, which is used, e.g., in novel display technologies. The results of calculations agree well with the available experimental data for the position of the photonic band gap.

A method for calculating the light extinction coefficients of a polymer layer with small nematic liquid-crystal droplets

A method is proposed for analyzing radiation extinction coefficients of polymer films containing small nematic liquid-crystal droplets with cylindrical symmetry. This method is based on the Rayleigh-Gans approximation and the effective scattering matrix. The orientational ordering of both liquid-crystal molecules in droplets and the droplets in a layer is described by multilevel order parameters. The extinction of radiation in polymer films is analyzed for the cases of oblique and normal incidence of light. Simple and convenient expressions are derived for calculating and estimating the extinction coefficients for the case of normal incidence of light on a layer containing spherical droplets. The error of the method is estimated.

Propagation of Polarized Light Through the Pdflc Monolayer

Abstract 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.

Light transmission of polymer-dispersed liquid crystal layer composed of droplets with inhomogeneous surface anchoring

We have developed a model and realized an algorithm for the calculation of the coefficient of coherent (direct) transmission of light through a layer of liquid crystal (LC) droplets in a polymer matrix. The model is based on the Hulst anomalous diffraction approximation for describing the scattering by an individual particle and the Foldy-Twersky approximation for a coherent field. It allows one to investigate polymer dispersed LC (PDLC) materials with homogeneous and inhomogeneous interphase surface anchoring on the droplet surface. In order to calculate the configuration of the field of the local director in the droplet, the relaxation method of solving the problem of minimization of the free energy volume density has been used. We have verified the model by comparison with experiment under the inverse regime of the ionic modification of the LC-polymer interphase boundary. The model makes it possible to solve problems of optimization of the optical response of PDLC films in relation to their thickness and optical characteristics of the polymer matrix, sizes, polydispersity, concentration, and anisometry parameters of droplets. Based on this model, we have proposed a technique for estimating the size of LC droplets from the data on the dependence of the transmission coefficient on the applied voltage.

Transmission spectra of tunable dispersion filters of the type of small particles-liquid crystal

We propose a technique for the calculation of characteristics of tunable dispersion filters of infrared radiation of the type of small particles-liquid crystal. The relation between the structural parameters of filters and their transmission spectra is investigated. The dependence of the halfwidth of transmission spectral lines on the thickness of the filter, the concentration of particles, and their size and polydispersity is investigated.

Small-angle light scattering from polymer-dispersed liquid-crystal films

A method is developed for modeling and computing the angular distribution of light scattered forward from a single-layer polymer-dispersed liquid-crystal (PDLC) film. The method is based on effective-medium approximation, anomalous diffraction approximation, and far-field single-scattering approximation. The angular distribution of forward-scattered light is analyzed for PDLC films with droplet size larger than the optical wavelength. The method can be used to study field-and temperature-induced phase transitions in LC droplets with cylindrical symmetry by measuring polarized scattered light intensity.