Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

Focusing of light by polymer-dispersed liquid-crystal films with nanosized droplets

An analysis is presented of polarization-independent electrically tunable light focusing by polymerdispersed liquid-crystal films with nanosized liquid-crystal droplets. Polymer-dispersed liquid-crystal films with axially symmetric distributions of liquid-crystal droplet concentration and layers with axially symmetric thickness profiles are considered. The paraxial, Rayleigh, and Rayleigh-Gans approximations, as well as the Foldy-Twersky equation, are used to examine the dependence of focal length on lens geometry, droplet size, concentration of nematic liquid-crystal droplets, and applied field. The tunable focusing ranges are evaluated for both lens types considered in the study. Dependence of the transmittance of polymer-dispersed liquid-crystal film on its characteristics is analyzed. The results obtained are compared with those available from the literature.     

Coherent transmittance quenching effect in thin films of polymer-dispersed ferroelectric liquid crystals

The conditions for vanishing of the coherent component of radiation transmitted through a monolayer of polymer-dispersed ferroelectric liquid-crystal droplets are found. The influence of polydispersity and disorientation of axes of semispherical droplets on the conditions of interference quenching is studied. It is shown that these factors cause a change in the average size and concentration of droplets for which the quenching effect is realized. Data on the contrast ratio and the modulation depth of a monolayer, determined from the coherent component of transmitted radiation, are presented. It is shown that these characteristics can be considerably improved under quenching conditions for a film of polymer-dispersed ferroelectric liquid-crystal droplets.     

Retrieval of order parameters of a monolayer of liquid-crystal droplets with weak anchoring

An optical method is proposed for extracting the order parameters of single-layer polymer-dispersed liquid-crystal film (monolayer) containing bipolar liquid-crystal droplets under weak anchoring conditions. The method is based on an analysis of the coherent transmittance of a monolayer irradiated with a normally incident linearly polarized plane wave. The method is used to retrieve the order parameter of a spherical liquid-crystal droplet and the order parameter of a monolayer consisting of such droplets as functions of the applied electric field. The effect of inaccuracy of the input values of monolayer parameters on the error of retrieval of the order parameters is examined. The method can be used to determine the refractive index of the polymer binder, as well as other parameters of the film. It provides a tool for solving the inverse scattering problem with field-dependent or field-independent droplet order parameter.     

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.     

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.     

Electromagnetic torque and force in axially symmetric liquid-crystal droplets

Circularly polarized light exerts torque on birefringent objects. In the case of axially symmetric particles, however, the moment of radiation force balances the direct optical torque. This explains the observation that radial liquid-crystal droplets, in contrast to planar droplets, do not spin in circularly polarized light. The conclusion is in agreement with considerations based on the angular momentum conservation of light [Phys. Rev. Lett.96, 163905 (2006)].     

Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material

We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.     

Study on the algorithm of computational ghost imaging based on discrete fourier transform measurement matrix

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.     

Study of the Effect of Pulsed-Periodic Electric Field and Linearly Polarized Laser Radiation on the Properties of Liquid-Crystal Waveguide

Integrated-optical waveguides with a nematic liquid-crystal 4-cyano-4’-pentylbiphenyl (5CB) waveguiding layer have been investigated for different polarizations of incident laser radiation and under a pulsed-periodic electric field. A dependence of the damping coefficient of waveguide modes and the sizes of quasi-steady-state irregularities of nematic liquid-crystal layer on the linear polarization of laser radiation and the strength of pulsed-periodic field has been found experimentally. The correlation length is estimated for waveguiding layer irregularities. The waveguide scattering method has provided a resolution in correlation length exceeding the classical resolution limit by approximately an order of magnitude. The observed decrease in the damping coefficient of waveguide modes and irregularity sizes under external field is explained by the decrease in the correlation length of director fluctuations.     

Phase change of a plane wave propagating through polymer dispersed liquid crystal film with nanosized droplets

The coherent field propagation in a polymer dispersed liquid crystal layer is described using a method based on the Foldy-Twersky integral equation for the vector case. Expressions for a polarization-independent phase shift and the coherent transmission coefficient of such a layer containing nanodimensional nematic liquid crystal droplets are obtained. Theoretical results for the phase shift are compared to the available experimental data.     

Droplet deformation and alignment for high-efficiency polarization-dependent holographic polymer-dispersed liquid-crystal reflection gratings

Droplet deformation and alignment are achieved in holographic polymer-dispersed liquid-crystal reflection gratings by applying an in situ shear during recording. High diffraction efficiency (99%) is obtained for light polarized parallel to the shear, with nearly zero efficiency for perpendicular polarization, and no increase of incoherent scattering. Permanent polarization dependence is related to stress-induced morphology changes of liquid-crystal droplets that are frozen by polymerization. The system is studied by electron microscopy and modeled by anisotropic coupled-wave and scattering theory. The morphology is consistent with the theory of small deformations of liquid droplets in fluid flow. Diffraction efficiency measurements are in agreement with theory incorporating this morphology as well as concomitant orientation and alignment of liquid-crystal molecules.     

Vertical profile of aerosol extinction based on the measurement of O_4 of multi-elevation angles with MAX-DOAS

A method for aerosol extinction profile retrieval using ground-based multi-axis differential optical absorption spectroscopy(MAX-DOAS) is studied, which is based on a look-up table algorithm. The algorithm uses parametric method to represent aerosol extinction profiles and simulate different atmospheric aerosol states through atmospheric radiation transfer model. Based on the method, aerosol extinction profile was obtained during six cloud-free days. The O_4 differential air mass factor(dAMF) measured by MAX-DOAS is compared with the corresponding model results under different atmospheric conditions(R~2= 0.78). The aerosol optical thickness, aerosol weight factor in boundary layer, and the height of the boundary layer are obtained after the process of minimization and look-up table method. The retrieved aerosol extinction in boundary layer is compared with PM2.5 data measured by ground point instrument. The diurnal variation trends of the two methods are in good agreement. The correlation coefficients of the two methods are 0.71 when the aerosol optical thickness is smaller than 0.5. The results show that the look-up table method can obtain the aerosol state of the troposphere and provide validation for other instrument data.     

Tunable face-centered-cubic photonic crystal formed in holographic polymer dispersed liquid crystals

We report on the fabrication and electro-optic measurements of face-centered-cubic (fcc) lattices in holographic polymer dispersed liquid-crystal materials. Four linearly polarized coherent plane waves were interfered to generate a fcc optical lattice that was subsequently and indefinitely recorded as an arrayed pattern of nanometer-sized liquid-crystal droplets (approximately 50 nm) at lattice nodes within a polymer matrix. Observed transmission spectra and Kossel diffraction curves are consistent with fcc crystal structure. A completely reversible 2% wavelength shift of the (+/- 111) stop band was observed on application of an electric field.     

Laser-induced periodic surface structures on different poly-carbonate films

Laser-induced periodic surface structures (LIPSS) were generated on oriented and amorphous thick, as well as on spin-coated thin, poly-carbonate films by polarized ArF excimer laser light. The influence of the film structure and thickness on the LIPSS formation was demonstrated. Below a critical thickness of the spin-coated films the line-shaped structures transformed into droplets. This droplet formation was explained by the laser-induced melting across the whole film thickness and subsequent de-wetting on the substrate. The thickness of the layer melted by laser illumination was computed by a heat-conduction model. Very good agreement with the critical thickness for spin-coated films was found. The original polymer film structure influences the index of refraction of the thin upper layer modified by the laser treatment, as was proven by the dependence of the structure’s period on the angle of incidence both for ‘s’- and ‘p’-polarized beams. The effect of the original surface roughness – grains in thick films or holes in thin films – was studied using atomic force microscopy. It was shown that the oblique incidence of ‘s’-polarized beams results in an intensity confinement in the direction of the forward scattering and in asymmetrical interference pattern formation around these irregularities. A new, two-dimensional grating-like structure was generated on spin-coated films. These gratings might be used as a special kind of mask. Received: 10 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Polarimetric method for the determination of the optical axis tilt angle at boundaries of a liquid-crystal layer

A polarimetric method is proposed to determine the tilt angle of the local optical axis of a liquid crystal at layer boundaries. The method is based on the measurement of the azimuthal polarization angles of light waves passed through a liquid-crystal cell or reflected from it. This method requires neither the knowledge of the refractive indices of a liquid crystal nor the homogeneity of the layer. The method can be applied in the whole range of tilt angles. Experimental data on the measurement of the tilt angles in cells assembled by the scheme of a prism liquid-crystal polarizer are presented.     

Polarization of light by a polymer film containing elongated drops of liquid crystal with inhomogeneous interfacial anchoring

An optico-mechanical model describing the coherent (directed) transmittance and the degree of polarization of forward-transmitted light by a polymer film with elongated liquid-crystal (LC) drops has been developed. This model, based on the Foldy–Twersky and anomalous-diffraction approximations, makes it possible to analyze the optical response of a film under extension as a function of the film thickness, refractive index of the polymer, the sizes and anisometry parameters of liquid-crystal drops, their concentration, internal structure, polydispersity, and orientation of optical axes. The model is verified based on the comparison of numerical and experimental data for the inverse modification of interfacial anchoring by an ion-forming surfactant. The internal drop structure is determined by solving the problem of minimizing the volume free energy density. A comparative analysis of the calculated transmittance and degree of polarization for films with uniform homeotropic and modified inhomogeneous interfacial anchoring is performed. The spectral polarization characteristics of a film with elongated LC drops and single-domain internal structure, formed under mechanical extension with the aid of surfactants, are investigated.     

Infrared and microwave radiative properties of metal coated microfibres

Recent experimental investigations have shown that insulations containing metallised glass or polymer fibres are characterised by an extremely high extinction coefficient for infrared radiation and, as a result, by very good heat shielding properties. This may be explained by the unusual optical properties of single fibres. In this paper, the most general theoretical model for the radiation absorption and scattering by metallised fibres is employed. It is based on the rigorous theory of scattering by an arbitrary oriented two-layer cylinder and may be used for calculations in a wide spectral range. The accuracy of some simple models of metallised fibres is analysed. In particular, the range of applicability of the perfectly conducting cylinder approximation is determined. A comparison with published experimental data shows good agreement between the calculated and the experimental values of the equivalent specific extinction coefficients in the spectral range of 2.5 < λ < 27.5 μm. Computational data on the absorption and scattering of microwave radiation by metallised dielectric fibres are presented for the first time. High values of the absorption efficiency factor for single submicron fibres may be of interest for microwave applications.     

动态细水雾的最佳热辐射消光粒径

研究了细水雾遮蔽衰减热辐射过程中,取得最佳遮蔽效果的动态雾滴初始粒径问题。综合考虑雾滴的光学特性和动力学特性,定义了热遮蔽指数作为度量动态雾滴消光能力的指标。在模型构建中,用索特粒径将多分散性的细水雾等效成单分散系;用Planck平均法获取水雾的灰体辐射特性参数;并采用数组调用、线性插值的方法提高大量计算Mie氏消光因子的效率。研究发现,基于遮蔽指数的最佳消光粒径要远大于基于光学特性的最佳消光粒径。     

Light attenuation by disperse layers with a high concentration of oriented anisotropic spherical particles

An analysis is made of the applicability of Beer’s law to the calculation of coefficients of directed (coherent) transmittance of light-scattering media formed by oriented anisotropic spherical liquid-crystal droplets. It is shown on the basis of comparison with experimental data that the application of Beer’s law to a layer with a high concentration of anisotropic particles leads to large errors in the calculation of transmittance. The interference approximation, taking into account the interference of waves scattered by separate particles, leads to better agreement with the experiment than Beer’s law, which assumes a linear relation between the attenuation factor of a disperse medium and the particle concentration.