Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Further Experimental Evidence for the Wall Structure of the Flexoelectric Domains in Symmetrically Weakly-Anchored MBBA Layers

An attempt for measuring of the penetration non-critical length of MBBA flexoelectric surface-induced domains is made. The disappearance of the domains into subcritical regions is related to the strong nonhomogeneity of the electric field around the gap which is able to inhibit the flexoelectric domain formation. The disappearance of the electrohydrodynamic domains generated from the flexoelectric ones, on the other hand, shows their secondary character as well. The existence of the penetration non-critical length of the flexoelectric domains is further confirmed with the observation of the corresponding supercritical influence length of side walls or air bubbles.

The behavior of the flexoelectric domains in an additionally applied magnetic field reveals their wall structure and permits the determination of the value and sign of the two MBBA flexoelectric coefficients of bend e _3x and splay e _1z, when the value and sign of the total flexoelectric coefficient ( e _z, + e _3x) are known.     

Magnetically induced anisotropy of electric permittivity in the PDMS ferromagnetic gel

A polymer ferromagnetic gel is an assembly of ferromagnetic nanoparticles embedded in a gel. The finely distributed magnetic particles of iron oxide (Fe₃O₄) were suspended, due to adhesive forces, in the flexible matrix (poly(dimethyl siloxane) PDMS). The mean diameter of the suspended nanoparticles, equal to 8.9 nm, was determined by means of a vibrating sample magnetometer. The dispersion of the electric permittivity, studied in the frequency range of 400 Hz-5 MHz, revealed the maxima at approximately 92 kHz and 2.5 MHz. The application of the homogenous magnetic field, ranging from 0 mT to 500 mT induced the anisotropy of the electric permittivity for the mutual parallel orientation of the magnetic field, electric field and the axis of the sample. For the magnetic induction B = 500 mT the highest decrease in the electric permittivity and the shift of the maximum from 2.5 toward 2.9 MHz were found.     

Coarsening Nature of Liquid-Ordered Domain in Model Membrane

We present the coarsening dynamics of a liquid-ordered (Lo) domain in a supported lipid membrane under an electric field. In a relatively small nanosmooth region surrounded by geometrical walls of nanocorrugated regions, full coarsening of the Lo domain was produced while in a large nanosmooth region, an intermediate, less-ordered Lo domain was developed so that the diffusion of charged phospholipids in the liquid-disordered (Ld) domain was allowed across the geometrical walls in the presence of an electric field. No appreciable diffusion of the charged lipids into a fully coarsened Lo domain by the electric field implies that the structural ordering of the membrane components plays a significant role on the formation of lipid rafts for biological processes.     

ELECTRIC FIELD-INDUCED DIRECTOR ORIENTATION OF SMECTIC-A DOMAINS IN AN ISOTROPIC PHASE

Abstract

Electric field-induced director orientation of smeetic-A phase in an isotropic phase has been studied with a polarizing microscope using the binary mixture of octyloxycyanobiphenyl and dodecyl alcohol. Electric field is applied to the samples in which spherical smectic-A domains with and without defects in an isotropic phase are observed. Field-induced orientation of smectic-A director is observed for spherical smectic-A domains with defects but not observed for those without defects below electric field strength of 1.0 V/μm, indicating that the presence of defects facilitates the director orientation. The threshold field for the smectic-A director orientation varies from smectic-A domain to domain, suggesting that the threshold field is dependent on the structure of defects.     

PERFORMANCE OF SHORTER PERIODICAL DOMAIN OF NEMATIC LIQUID CRYSTAL INDUCED BY ELASTIC WAVE PROPAGATION

Abstract

The light diffraction properties of a shorter periodical domain, which is caused by an elastic wave propagating along one of two glass plates composing a liquid crystal cell, are described. The shorter periodical domain is controllable for the application of an electric field to the liquid crystal cell. The response time of the disappearance of the shorter periodical domain decreases with increasing the applied voltage. On the other hand, the response time of the domain formation is independent of the applied voltage. These response times agree quantitatively with those of the birefringence electrooptic effect. The domain disappearance by applying the voltage to the liquid crystal indicates the Freedericksz transition.     

Magnetic and Electric Birefringence in the Isotropic Phase of a Nematic of Large Positive Dielectric Anisotropy

Abstract

Magnetic (Δn_H ) and electric birefringence (Δn_E ) in the isotropic phase of strongly positive (Δ 8)trans-p-n-octyloxy α-methyl-p'-cyanophenyl cinnamate (8 OMCPC) have been measured. It is established that they both exhibit a (T - T*)^?1 dependence, T _NI - T* being 1.4 K. Also, the induced birefringence is found to be proportional to the square of the applied field, magnetic or electric.     

High resolution neutron and X‐ray diffraction studies as a function of temperature and electric field of the ferroelectric phase transition of RDP

Neutron and high resolution X‐ray diffraction investigations on perfect single crystals of RbH₂PO₄ (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric ‐ ferroelectric phase transition, support a disorder ‐ order character of [PO₄H₂]^‐‐groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time‐space average of orthorhombic symmetry. According to the group ‐ subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T_C = 147 K. From the high resolution X‐ray data it is concluded that below T_C the ferroelastic strain in the (a,b)‐plane leads to micro‐angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T_C is decreased. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polarization mechanism of LiNbO3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO₃ crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.     

Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

Compact Permanent Magnetic Circuit with Periodic Magnetic Field Designed for Studies of Liquid Crystals

Abstract

We have developed a high-performance compact magnetic circuit to investigate orientational phase behaviors of biological and synthetic macromolecular systems by using spectroscopic methods such as X-ray scattering and light scattering. This magnetic circuit can apply a periodic magnetic field and gradient to samples. The maximum values of the magnetic field strength and gradient are varied from 1.5 tesla to 0.55 tesla and from 2.4 × 10² tesla/m to 7.3 × 10¹ tesla/m. Owing to the high periodicity of the magnetic field strength and gradient macromolecules are subjected to both rotational and translational forces from the magnetic field. We show some prominent features of this magnetic circuit and its first application to the synchrotron radiation small-angle X-ray scattering (SR-SAXS) studies of supermacromolecular liquid crystals.     

The Electron Microscopic Approach to the Structure and Reactivities of Organic Solids

In this paper we continue investigations started in Refs. 1 and 2 studying the effect of a magnetic field on electrohydrodynamics of liquid crystalling complex ethers with dielectric constant anisotropy close to zero. The investigations were made with 100 μm thick layers having initial planar “first” structure of B and C mixtures described in Ref. 2.

Figure 1 shows dependences of optical layer transmittance on the intensity of the magnetic field (H) directed normal to bearing surfaces, and in the absence of an electric field in the absence of an electric field (curves I and II). For some threshold values H = H_th all the samples showed an increased transmission arising due to Fredericks transition.³     

Effect of magnetic field–current annealing on the magnetoimpedance of Co-based ribbons

The roles of applied magnetic field during the current annealing of Co_68.15Fe_4.35Si_12.5B₁₅ soft magnetic amorphous ribbons have been studied. The Joule heating process causes a complex domain structure which is related to the circular field generated by the current. We have shown that magnetic field–current annealing modifies the transverse permeability and under suitable conditions, enhances the GMI response.     

Dynamic Layer Response under Electric Field in Antiferroelectric Liquid Crystal Cells Measured by Synchrotron Microbeam Time Resolved X-Ray Diffraction

Abstract

Direct observation of the local layer response of an antiferroelectric liquid crystal to the step form electric field has been carried out by a time resolved synchrotron X-ray micro diffraction measurement. When an electric field was changed from high voltage to OV, corresponding to the ferroelectric to antiferroelectric phase transition, the local layer transformed from the bookshelf to the quasi-bookshelf structure within 0.3 ms. The horizontal chevron structure was found in both the phases, though the decrease in the horizontal chevron angle was observed during a period of 0.2 ms after turning off the electric field. In the antiferroelectric to ferroelectric phase transition process (from OV to high voltage), the layer structure transformed to the bookshelf within 0.04 ms.     

Switching Behaviour of Pb5Ge3O11 Single Crystals

Single crystals of lead germanate have been grown by Czochralski method. Hysteresis and switching pulses could only be measured after treating the crystal samples at elevated temperature and electrical a.c. field. Hysteresis is strongly frequency dependent. We observed the motion of the domain walls in polarized light simultaneously. The hysteresis loop is distorted at elevated temperature because of electric conductivity of the crystals. Temperature dependence of electric conductivity has been measured.     

OPTICAL TRANSMISSION AND POLARIZATION SWITCHING PROPERTIES OF A FERROELECTRIC LIQUID CRYSTAL CELL WITH A STABLE TWISTED STATE

Abstract

Ferroelectric liquid crystal cells with a stable twisted state were prepared. Dynamic processes of molecular reorientation in a stable twisted state under an applied electric field have been investigated by observing the optical transmittance and polarization switching current with simultaneous observation of the texture change.     

On the Stability of Travelling Waves Arising in Nematic Liquid Crystals

Abstract

The stability of travelling waves which occur when a nematic liquid crystal is subjected to crossed electric and magnetic fields has been studied previously where conditions on a control parameter q for stability to occur have been given. This article is concerned with the behaviour of the stable perturbations as time increases. For each of the three travelling wave solutions we can determine the long-term monotonic or oscillatory behaviour of the perturbations using knowledge of the spectrum of the operator governing the perturbation equation.     

Transformation of the spectrum of TM polaritons of a 1D magnetic photonic crystal under conditions of the quadratic electro-optic effect

Conditions under which quadratic magneto-optical interaction leads to a number of features in the conditions of localization and propagation of collective magnetic TM polaritons are determined by the example of a 1D magnetic superlattice of the easy-axis-antiferromagnet-nonmagnetic-insulator type, in which the easy magnetization axis is orthogonal to the applied external dc electric field.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.     

Dynamic Scattering with Storage in Smectics C and Nematics with Smectic C Ordering

Stored dynamic scattering and stored domain structure were observed in smectic C and in nematic phases of p-n-heptyloxybenzoic acid (HOBA). The sdtored texture can easily be erased with a high frequency electric field. The smallest frequency for erasure decreases with the decreasing temperature. Stored scattering centers and stored domain structure in S₁ solid phase of HOBA were also observed.