1,721,000 research outputs found
Magnetic field induced director reorientation in the nematic cell with time-dependent anchoring due to adsorption/desorption of LC molecules RID B-6722-2008
No full textThe time evolution of director orientation in the cell subject to the external magnetic field in the case of time-dependent boundary conditions is investigated. The case when one substrate has strong anchoring and at other substrate director anchoring is determined by process of adsorption/desorption of LC molecules on it is considered. The problem is solved in one constant approximation and Rapini-type potential for interaction between LC molecules in the cell and adsorbed LC molecules is proposed. During the time evolution the orientational distribution function of molecules adsorbed on the cell substrate possesses two maximums. Time evolution of the director orientation, anchoring energy and easy axis at the substrate is investigated numerically
The Frederiks effect and related phenomena in ferronematic materials
No full textUsing continuum and statistical mechanical theories, we study the switching properties of a ferronematic in a nematic liquid crystal cell subject to homeotropic boundary conditions at the cell and particle walls. An external magnetic field normal to the cell plane is also imposed. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the anchoring interaction between the director and the ferroparticle orientation. For low anchoring strengths, there is an inverse Frederiks effect, and the nematic reorientation reduces and then disappears continuously at a critical magnetic field. At intermediate fields, the degree of reorientation reduces at high fields but remains finite. For high fields, however, the director switching saturates. The dimensionless temperature scale in the problem involves the temperature, the mean nematic elastic constant, the colloidal density, and the cell dimension. If this quantity is sufficiently low, then high magnetic fields can cause magnetic segregation. The segregation order parameter is coupled to the director distortion, and this can change the inverse Frederiks transition into a first order transition, leading to bistability in an intermediate field regime. These features are perturbed but not changed structurally by the effect of a small bias magnetic field (<10 Oe) normal to the unperturbed director. Subject to suitable choice of parameters, the theory is also quantitatively consistent with the results of the classic experiment of Chen and Amer in 1983. <br/
Effective-medium theory of polymer dispersed liquid crystal droplets: I: Spherical droplets
No full textWe introduce an effective-medium theory for the local field and mean dielectric function of a polymer dispersed liquid crystal (PDLC) film consisting of identical spherical liquid crystal droplets dispersed in a polymer matrix. The theory takes account of the tensor nature of the dielectric response of the liquid crystals and the subsequent re-orientation of the liquid crystal tensor inside the PDLC droplets, as well as the presence of other droplets. We present explicit numerical results for droplets with radial and bipolar director boundary conditions as a function of external voltage
Cloaking by shells with radially inhomogeneous anisotropic permittivity
No full textWe model electromagnetic cloaking of a spherical or cylindrical nanoparticle enclosed by an optically anisotropic and optically inhomogeneous symmetric shell, by examining its electric response in a quasi-static uniform electric field. When the components of the shell permittivity are radially anisotropic and power-law dependent (ε~r(m)) whereris distance to the shell center, and m a positive or negative exponent which can be varied), the problem is analytically tractable. Formulas are calculated for the degree of cloaking in the general case, allowing the determination of a dielectric condition for the shells to be used as an invisibility cloak. Ideal cloaking is known to require that homogeneous shells exhibit an infinite ratio of tangential and radial components of the shell permittivity, but for radially inhomogeneous shells ideal cloaking can occur even for finite values of this ratio.</p
Photoaligned Tunable Liquid Crystal Lenses with Parabolic Phase Profile
Full text linkWe present a theoretical model of a cylindrical tunable liquid crystal lens based on the modulation of anchoring energy. The latter can be easily obtained using photoalignment techniques. The liquid crystal cell we propose exhibits strong anchoring at the top substrate and anchoring energy with a parabolic profile at the bottom substrate. The model describes the dependence of the focal length on the applied voltage and presents a theoretical study of the lens aberrations. The results obtained are of general relevance and can be used to optimize the performances of every type of liquid crystal lens with a parabolic profile
Nematic director slippage: role of the angular momentum of light
No full textWe propose a theoretical model of the light-induced director slippage effect. In this effect the bulk director reorientation contributes to the surface director reorientation. It is found that the director and ellipticity profiles, obtained in the geometric optics approximation, are dependent on the ellipticity of the incident light wave. The director distribution is spatially modulated in linearly polarized light but grows monotonically in circularly polarized light. The surface director deviation has been examined, and comparison made with existing experimental data, which then permits the magnitude of the orientational nonlinearity coefficient to be calculated
Theory of Dielectric and Optical Properties of PDLC films. Molecular Crystals and Liquid Crystals
No full textTheoretical model of an electrically tunable liquid-crystal-based contact lens
Full text linkMilton, et al. [Opt. Express 22, 8035 (2014) [CrossRef] ] have constructed a model electronic liquid crystal contact lens for the correction of presbyopia. This paper constructs a theoretical model for this lens. Good agreement between theory and experimental data is achieved, although the indications are that the precise parameters of the lens differ slightly from those prescribed by the designers of the lens. We discuss the temperature dependence of the optical power, the sensitivity of the device to manufacturing process and the properties of such lenses containing a number of different known liquid crystals. The model can be used for engineering optimization of the existing prototype
Effective medium theory for anisotropic media with plasmonic core-shell nanoparticle inclusions
No full textCore-shell building blocks have been found useful in recent years as inclusions, in the search for metamaterials with tailored properties. Either the core or the shell of these composite inclusions may be metallic, and the dielectric component may be both radially anisotropic and radially inhomogeneous. In tunable anisotropic metamaterials, the tuning may then be achieved through the host, the core, or some combination thereof. However a theoretical picture is harder to build. Here we propose an approach to an effective medium theory for such materials, valid in the quasi-static limit. The method proceeds first by homogenising the interior of complex particle, and then uses standard anisotropic effective medium methods to provide bulk effective homogenized parameters. By varying the degree of inhomogeneity in the core, shell and dielectric-metal material volume fractions, the technique can be used as a tool for the design of metamaterials with specifically engineered properties. We find that metamaterial properties can be readily tuned by reorienting the optical axis of the host (e.g. liquid crystal). In particular, there is a possibility of switching between hyperbolic and conventional anisotropic metamaterial properties bychanging inclusion shell properties
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