A. Corella-Madueño

Twist defect in an imprinted cholesteric elastomer

We have found that a chiral twist defect inserted in a cholesteric elastomer gives rise to circularly polarized localized modes of both handedness. This defect enhances the resonant mode amplitude whose handedness is opposite to that of the cholesteric helix for high cross-linked density, whereas for low cross-linked density, the same mode is decoupled with the defect and thus the resonant mode disappears. Finally, the resonant mode of the same handedness as the elastomer helix is maintained for both high and low cross-linked density.

Electrically controlled reflection bands in a cholesteric liquid crystals slab

We considered a cholesteric slab whose axis is normal to their borders, when a uniform electric field is perpendicularly applied to the helix axis. We found the solution of the boundary value problem for the reflection and transmission of normally incident waves due to the slab. We showed that a left-circularly polarized thin reflection band is immersed in a wider right-circularly polarized band reflection, when the cholesteric is subjected to low amplitude fields. For larger field values, both reflection bands redshift simultaneously as the field increases. A device based on this phenomenon can be used as a electrically tunable universal filter.

Thermically tuned nematic guide

We consider a cylindrical fiber with a liquid crystal core satisfying homeotropic weak anchoring boundary conditions. We find the different textures of the nematic inside the cylinder obtained by changing the temperature. We calculate exactly the spatial distribution of the transverse magnetic modes in the guide as a function of temperature of the system by using a numerical scheme.

Band structure and reflectance for a nonlinear one-dimensional photonic crystal

Abstract We consider a model for a one-dimensional photonic crystal formed by a succession of nonlinear Kerr-type equidistant spaceless interfaces immersed in a linear medium. We calculate analytically the band structure of this system as a function of the incident wave intensity, and find two main tendencies: the appearance of prohibited bands, and the separation and narrowing of these bands. We consider as well a finite version of this photonic crystal for a limited number of alternating linear and non linear set of stacks for which we calculate reflectance as a function of the electromagnetic wave intensity, band index and number of periods. A system with these features can be constructed by alternating very thin slabs of a nonlinear soft matter material with thicker solid films, which can be used to design a device to control light propagation for specific wavelength intervals and light intensities of the same propagating signal.

Electrically tuned optical reflection band for an artificial helicoidal structure

ABSTRACT We analysed the control of optical band gaps for axially propagating electromagnetic waves throughout a nanocomposite structurally chiral medium under the influence of a low-frequency (dc) electric field aligned along the same axis as the periodic structure. This medium is made of metallic nanoballs (silver) randomly dispersed in a structurally chiral material whose dielectric properties can be represented by a resonant effective uniaxial tensor. Structurally chiral material is taken to possess locally a point group symmetry and the Pockels effect is assumed. By establishing the Maxwell equations in a matrix representation, we have computed the eigenvalues and eigenvectors of the corresponding matrix in the system rotating along with the helical structure as function of the filling factor and the electric field. We found that the band gap properties of the periodic system depend strongly on the applied low-frequency electric field which is able to increase the bandwidths of the two sub-band gaps generated by the presence of the metallic inclusions obtained above a given filling factor. The applied electric field is even able to open the mentioned band gaps when they are initially closed. We note that by increasing the filling factor, also by keeping the inclination angle and the electric field fixed, the bands are opened and closed. Then when changing the angle of inclination and the electric field the bands shift, they break and new sub-band gaps appear.

Saturation and stability of nonlinear photonic crystals

We consider a one-dimensional photonic crystal made by an infinite set of nonlinear nematic films immersed in a linear dielectric medium. The thickness of each equidistant film is negligible and its refraction index depends continuously on the electric field intensity, giving rise to all the involved nonlinear terms, which joints from a starting linear index for negligible amplitudes to a final saturation index for extremely large field intensities. We show that the nonlinear exact solutions of this system form an intensity-dependent band structure which we calculate and analyze. Next, we ponder a finite version of this system; that is, we take a finite array of linear dielectric stacks of the same size separated by the same nonlinear extremely thin nematic slabs and find the reflection coefficients for this arrangement and obtain the dependence on the wave number and intensity of the incident wave. As a final step we analyze the stability of the analytical solutions of the nonlinear crystal by following the evolution of an additive amplitude to the analytical nonlinear solution we have found here. We discuss our results and state our conclusions.

Influence of Electric Fields and Boundary Conditions on the Flow Properties of Nematic-Filled Cells and Capillaries

From a rheological point of view, nematic liquid crystals are interesting because they exhibit unique flow properties. Although some of these properties have been known for a long time, they continue to attract the attention and interest of the scientists. As a result, a large amount of theoretical, numerical, and experimental work has been produced in recent years. In particular, a number of publications treat the behavior of nematic liquid crystals in shear and Poiseuille flow fields (Denniston, Orlandini, and Yeomans 2001, Vicente Alonso, Wheeler, and Sluckin 2003, Marenduzzo, Orlandini, and Yeomans 2003, Marenduzzo, Orlandini, and Yeomans 2004, Guillen and Mendoza 2007, Medina and Mendoza 2008, Mendoza, Corella-Madueno, and Reyes 2008, Reyes, Corella-Madueno, and Mendoza 2008, Zakharov and Vakulenko, 2010).

Hydrodynamical and anchoring effects for optical propagation in a nematic wave guide

We consider a cylindrical waveguide with a nematic liquid crystal (NLC) core, having initially the radial escaped configuration and subjected to a pressure gradient applied along the guide. By assuming arbitrary anchoring quasi-homeotropic boundary conditions, we calculate the equilibrium textures for NLC and the velocity profiles, parameterised by the pressure gradient and the strength of its interaction with the cylindrical wall. By performing exact numerical calculations, we calculate the spatial distribution of the transverse magnetic modes in the guide, and we show that the propagation of the optical fields is significantly altered by the induced distortion, the pressure and the realistic anchoring conditions.