J. Adrián Reyes

ac and dc electro-optical response of planar aligned liquid crystal cells

Interaction of static electric fields with a nematic is complicated because of the presence of charges, which originates from aligning layers or from molecular breakdown, affecting the electro-optic response of a liquid crystal cell. We analyze the aligning layers effect on the electro-optical response and develop a model based on a description of the ion balance on the conducting plates taking into account both drift current and saturation effects due to the ion migration inside the cell, neglecting diffusion current. The model well agrees with data for low voltages, while for higher ones, diffusion current should be taken into account.

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.

Band structure controlled by chiral imprinting

Using the configuration of an imprinted cholesteric elastomer immersed in a racemic solvent, the authors find the solution of the boundary-value problem for the reflection and transmission of incident optical waves due to the elastomer. They show a significant width reduction of the reflection band for certain values of nematic penetration depth, which depends on the volume fraction of molecules from the solvent, whose handedness is preferably absorbed. The appearance of nested band gaps of both handednesses during the sorting mixed chiral process is also obtained. This suggests the design of chemically controlled optical filters and optically monitored chiral pumps.

Light propagation and transmission in hybrid-aligned nematic liquid crystal cells: Geometrical optics calculations

The authors present a geometrical approach to calculate the transmission of light in a hybrid-aligned nematic cell under the influence of an applied electric field. Using the framework of geometrical optics they present results for the ray tracing as well as the transmission of light as a function of the applied low frequency voltage. Dispersion effects are included through a wavelength dependent dielectric function. Their results for the transmittance as a function of the applied voltage show oscillations that are in good qualitative agreement with previously obtained experimental measurements.

Electrically controlled optical bandgap in a twisted photonic liquid crystal

We consider a one-dimensional twisted photonic liquid crystal, which consists of N nematic liquid crystal slabs in a twisted configuration alternated by N isotropic dielectric layers under the action of a dc electric field (Edc) aligned along the periodicity axis. We write and solve numerically the corresponding Euler-Lagrange equations describing the nematic layer configuration. We express Maxwell’s equations in a 4×4 matrix representation, and by using the transfer matrix formalism, we obtain the optical band structures at arbitrary incidence angles and different external electric fields. We have found that there exists a strong dependence of electric field on the transmission and reflection spectra in enhancing and extinguishing bandgaps. The analysis presented here allow us to propose an electrically shiftable universal rejection filter for incident waves of left- and right-circular polarization. It is observed that by increasing the electric field we can highly enhance the cross-polarized reflection ...

Optical band gap in a cholesteric elastomer doped by metallic nanospheres

We analyzed the optical band gaps for axially propagating electromagnetic waves throughout a metallic doped cholesteric elastomer. The composed medium is made of metallic nanospheres (silver) randomly dispersed in a cholesteric elastomer liquid crystal whose dielectric properties can be represented by a resonant effective uniaxial tensor. We found that the band gap properties of the periodic system greatly depend on the volume fraction of nanoparticles in the cholesteric elastomer. In particular, we observed a displacement of the reflection band for quite small fraction volumes whereas for larger values of this fraction there appears a secondary band in the higher frequency region. We also have calculated the transmittance and reflectance spectra for our system. These calculations verify the mentioned band structure and provide additional information about the polarization features of the radiation.

Transition radiation in cholesteric liquid crystal

ABSTRACT We consider a constant velocity charged particle travelling in an arbitrary direction by a cholesteric liquid crystal. We calculate the time-dependent-induced polarisation in the cholesteric by the electric field generated by the charged particle. Thus, we express the radiation field originated by the induced dipole distribution in the cholesteric in terms of the cholesteric susceptibility. To simplify our procedure, we write Maxwell equations and the constitutive non-local equation for the cholesteric, in the Fourier space since in this representation the equations turn to be simple difference equations. We solve these equations iteratively by assuming small values for the cholesteric birefringence to find the first-order electric field produced by the charge particle immersed in the cholesteric. This allows us to obtain the dominant contributions of the radiation field one of which is the usual Cherenkov effect. We focus in the terms occurring for hypoluminic charged particle and calculate the radiated energy as a function of observing angle, frequency, velocity and direction with respect to the cholesteric axis. Graphical Abstract

Band structure for a bend-core liquid crystal fiber

We study the propagation of optical fields in a cylindrical fiber of a banana bend-core liquid crystal formed by uniformly tilted coaxial cylindrical smectic layers, surrounded by vacuum. By solving numerically Maxwell’s equations, we find the band structure in both uniaxial and biaxial limits. For the biaxial case, we show that the backward propagation is restricted to certain narrow frequency band gaps, whereas for the forward one the fiber behaves similarly to a standard waveguide. We also find that the fiber modes are elliptically polarized slowly leaky modes whose attenuation coefficients are damped oscillating functions of frequency.

Wannier-Mott excitons formed by electrons in a quantum wire and holes in a perpendicular quantum layer

We analyze Wannier–Mott excitons in which the electrons are constrained in one-dimensional quantum wires and the holes in two-dimensional quantum layers perpendicular to the wires. The resulting three-dimensional exciton Schr8 odinger equation in the laboratory frame of reference is solved in terms of the common 3D exciton states in the center of mass frame. ? 2002 Elsevier Science B.V. All rights reserved.

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.