arXiv subjects

F. Castles

Publications and source records attributed to F. Castles.

9 recordsLinked to original sources

arXiv2023

Lower (negative) bounds on the static electric susceptibility of non-equilibrium cubic crystals

We use a classical, microscopic model of point-like dipolarizable entities (a model that is standard in the case of positive polarizability) and investigate its behavior for simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) crystals with one entity per primitive cell when the static polarizability of the entities is negative and the mutual interaction between the entities is taken into account. We find that the static electric susceptibility is bounded below due to an instability towards self-polarization but the lower permissible bound is negative definite in each case, i.e., the concept of negative static electric susceptibility remains robust, according to the model, when mutual interactions are taken into account. The usual Clausius-Mossotti relation between the static polarizability and the static electric susceptibility remains valid in the case of negative parameters, but only down to the lower permissible bound; the value of the bound depends on the crystal structure and is always unrelated to the asymptote of the Clausius-Mossotti curve. The lower permissible bounds of the static electric susceptibility are found to be -0.906 for sc, and -1.00 for bcc and fcc. These results confirm that, although the magnitude of the static electric susceptibility does not diverge in the negative case (as it can in the positive case), the magnitudes attainable in the negative case for condensed media may, nevertheless, be many orders of magnitude greater than those predicted previously for inverted vapors and gases. This is a promising result in relation to the development of potential new technologies that exploit the phenomenon.

cond-mat.mtrl-sci↗

arXiv2017

Active metamaterials with negative static electric susceptibility

Well-established textbook arguments suggest that static electric susceptibility must be positive in "all bodies" [1]. However, it has been pointed out that media that are not in thermodynamic equilibrium are not necessarily subject to this restriction; negative static electric susceptibility has been predicted theoretically in systems with inverted populations of atomic and molecular energy levels [2,3], though this has never been confirmed experimentally. Here we exploit the design freedom afforded by metamaterials to fabricate active structures that exhibit the first experimental evidence of negative static electric susceptibility. Unlike the systems envisioned previously---which were expected to require reduced temperature and pressure---negative values are readily achieved at room temperature and pressure. Further, values are readily tuneable throughout the negative range of stability -1<\chi^{(0)}<0, resulting in magnitudes that are over one thousand times greater than predicted previously [4]. This opens the door to new technological capabilities such as stable electrostatic levitation.

physics.app-ph↗

arXiv2015

A model for the Pockels effect in distorted liquid crystal blue phases

Recent experiments have found that a mechanically distorted blue phase can exhibit a primary linear electro-optic (Pockels) effect [F. Castles \textit{et al}. Nature Mater. \textbf{13}, 817 (2014)]. Here it is shown that flexoelectricity can account for the experimental results and a model, which is based on continuum theory but takes account of the sub-unit-cell structure, is proposed. The model provides a quantitative description of the effect accurate to the nearest order of magnitude and predicts that the Pockels coefficient(s) in an optimally-distorted blue phase may be two orders of magnitude larger than in lithium niobate.

cond-mat.soft↗

arXiv2015

Linear electro-optic effects due to high order spatial dispersion

Two new types of electro-optic effect that are linear in the applied electric field strength are theoretically predicted to exist in transparent dielectric crystals due to high order spatial dispersion. The first effect, which is quadratic in the wave vector of light, is possible in materials belonging to all noncentrosymmetric crystal classes. The second, which is cubic in the wave vector, is possible in all crystals. In the ${\bm O}(432)$ and $\bm{O_h}(m\bar 3m)$ crystal classes, for which the primary and secondary linear electro-optic effects and linear electrogyration are simultaneously absent, these effects lead, respectively, to qualitatively new behavior and constitute the dominant bulk electro-optic effect in the limit of small fields. Thus, bulk linear electro-optic effects are predicted to exist in a wide range of materials---including many of considerable technological importance, such as silicon---where they were previously considered impossible.

physics.optics↗

arXiv2014

Non-polar Flexoelectrooptic Effect in Blue Phase Liquid Crystals

Blue phase liquid crystals are not usually considered to exhibit a flexoelectrooptic effect, due to the polar nature of flexoelectric switching and the cubic or amorphous structure of blue phases. Here, we derive the form of the flexoelectric contribution to the Kerr constant of blue phases, and experimentally demonstrate and measure the separate contributions to the Kerr constant arising from flexoelectric and dielectric effects. Hence, a non-polar flexoelectrooptic effect is demonstrated in blue phase liquid crystals, which will have consequences for the engineering of novel blue-phase electrooptic technology.

cond-mat.soft↗

arXiv2013

Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications

A promising approach to the fabrication of materials with nanoscale features is the transfer of liquid-crystalline structure to polymers. However, this has not been achieved in systems with full three-dimensional periodicity. Here we demonstrate the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I, a chiral liquid crystal with cubic symmetry. Blue phase I was photopolymerized and the remaining liquid crystal removed to create a porous free-standing cast which retains the chiral three-dimensional structure of the blue phase, yet contains no chiral additive molecules. The cast may in turn be used as a hard template for the fabrication of new materials. By refilling the cast with an achiral nematic liquid crystal, we created templated blue phases which have unprecedented thermal stability in the range -125 to 125 [degrees symbol]C, and that act both as mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.

cond-mat.soft↗

arXiv2012

Stretchable liquid crystal blue phase gels

Liquid crystalline polymers are materials of considerable scientific interest and technological value to society [1-3]. An important subset of such materials exhibit rubber-like elasticity; these can combine the remarkable optical properties of liquid crystals with the favourable mechanical properties of rubber and, further, exhibit behaviour not seen in either type of material independently [2]. Many of their properties depend crucially on the particular mesophase employed. Stretchable liquid crystalline polymers have previously been demonstrated in the nematic, chiral nematic, and smectic mesophases [2,4]. Here were report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that may have its optical properties manipulated by an applied strain and, further, remains electro-optically switchable under a moderate applied voltage. We find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and new possibilities for low-voltage electro-optic devices.

cond-mat.soft↗

arXiv2011

The limits of flexoelectricity in liquid crystals

The flexoelectric conversion of mechanical to electrical energy in nematic liquid crystals is investigated using continuum theory. Since the electrical energy produced cannot exceed the mechanical energy supplied, and vice-versa, upper bounds are imposed on the magnitudes of the flexoelectric coefficients in terms of the elastic and dielectric coefficients. For conventional values of the elastic and dielectric coefficients, it is shown that the flexoelectric coefficients may not be larger than a few tens of pC/m. This has important consequences for the future use of such flexoelectric materials in devices and the related energetics of distorted equilibrium structures.

cond-mat.soft↗

arXiv2011

Thermodynamically Stable Blue Phases

We show theoretically that flexoelectricity stabilizes blue phases in chiral liquid crystals. Induced internal polarization reduces the elastic energy cost of splay and bend deformations surrounding singular lines in the director field. The energy of regions of double twist is unchanged. This in turn reduces the free energy of the blue phase with respect to that of the chiral nematic phase, leading to stability over a wider temperature range. The theory explains the discovery of large temperature range blue phases in highly flexoelectric "bimesogenic" and "bent-core" materials, and predicts how this range may be increased further.

cond-mat.soft↗