Damian J. Gardiner

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 that have unprecedented thermal stability in the range -125 to 125 °C, and that act as both mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.

Paintable band-edge liquid crystal lasers

In this paper we demonstrate photonic band-edge laser emission from emulsion-based polymer dispersed liquid crystals. The lasing medium consists of dye-doped chiral nematic droplets dispersed within a polymer matrix that spontaneously align as the film dries. Such lasers can be easily formed on single substrates with no alignment layers. The system combines the self-organizing periodic structure of chiral nematic liquid crystals with the simplicity of the emulsion procedure so as to produce a material that retains the emission characteristics of band-edge lasers yet can be readily coated. Sequential and stacked layers demonstrate the possibility of achieving simultaneous multi-wavelength laser output from glass, metallic, and flexible substrates.

Polymer stabilized chiral nematic liquid crystals for fast switching and high contrast electro-optic devices

A fast switching electro-optic device, based upon the in-plane addressing of very short pitch polymer stabilized chiral nematic liquid crystals, is presented. Polymer stabilization of the standing helical arrangement is essential to prevent the appearance of defects above the in-plane electrodes. Response times as short as 50 μs are observed at room temperature along with contrast ratios greater than 3000:1 owing to the high optical extinction at visible wavelengths in the “Off” state. The combination of these fast response times with such high contrast ratios is of great importance for next generation electro-optical elements.

Band-edge and random lasing in paintable liquid crystal emulsions

Lasing mechanisms within paintable dye-doped chiral liquid crystal emulsions are investigated. Evidence shows that by variation in liquid crystal droplet size, by simple control of mechanical mixing speeds, a change in the lasing mechanism from band-edge lasing (large droplets) to diffuse nonresonant random lasing (small droplets) can be facilitated. This approach represents a facile technique for the variation in lasing mechanism, within a self-organizing, flexible, and conformable system, and offers the opportunity of developing controllable linewidth laser sources.

Spontaneous induction of the uniform lying helix alignment in bimesogenic liquid crystals for the flexoelectro-optic effect

Using in-plane electric fields, the electrical induction of the uniform lying helix (ULH) alignment in chiralnematic liquid crystals is reported. This process permits spontaneous induction of the ULH alignment to give an in-plane optic axis, without the need for complex processing. Flexoelectro-optic switching is subsequently obtained by holding the in-plane electrodes at a common voltage and addressing via a third, plane-parallel electrode on a second, or upper, substrate to give a field across the device in the viewing direction. For this device, in optimized bimesogenic materials, we demonstrate full intensity modulation and sub-millisecond response times at typical device temperatures.

Ultra-fast-switching flexoelectric liquid-crystal display with high contrast

Abstract— The flexoelectro-optic effect provides a fast-switching mechanism (0.01–0.1 msec), suitable for use in field-sequential-color full-motion-video displays. An in-plane electric field is applied to a short-pitch chiral nematic liquid crystal aligned in the uniform standing helix (or Grandjean) texture. The switching mechanism is experimentally demonstrated in a single-pixel test cell, and the display performance is investigated as a function of device parameters. A contrast ratio of 2000:1 is predicted.

Highly anisotropic conductivity in organosiloxane liquid crystals

In this paper, we present the conductivity and dielectric characterization of three homologous series of smectic A siloxane containing liquid crystals. The materials studied include one monomesogenic series, which consists of a 4-(ω-alkyloxy)-4′-cyanobiphenyl unit terminated by pentamethyldisiloxane, and two bimesogenic series, which consist of twin 4-(ω-alkyloxy)-4′-cyanobiphenyls joined via tetramethyldisiloxane or decamethylpentasiloxane. All of the compounds exhibit wide temperature range enantiotropic smectic A phases; the effect of the siloxane moiety is to suppress nematic morphology even in the short chain homologs. We find that these compounds exhibit a highly anisotropic conductivity: the value perpendicular to the director is to up to 200 times that parallel to the director. For the nonsiloxane analog 4-(ω-octyl)-4′-cyanobiphenyl (8CB), this value is approximately 2. It is also found that the dielectric anisotropy is reduced significantly; a typical value is ∼1 compared to 8.4 for 8CB. We propo...

Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals

Using a chiral nematic liquid crystal with a negative dielectric anisotropy, it is possible to switch between band-edge laser emission and random laser emission with an electric field. At low frequencies ( 5 kHz), where the helix is stabilized due to dielectric coupling. These results demonstrate a method by which the linewidth of the laser source can be readily controlled externally (from 4 nm to 0.5 nm) using electric fields.

Increasing the flexoelastic ratio of liquid crystals using highly fluorinated ester-linked bimesogens

We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.

Electro-optic bistability in organosiloxane bimesogenic liquid crystals

In this paper we report the electro-optic characterization of two homologous series of low molar mass bimesogenic siloxane-containing liquid crystals. The materials used have two alkoxycyanobiphenyl mesogenic units with variable alkyl chain joined by a two- or five-siloxane moiety and all exhibit stable smectic A mesophases over wide temperature ranges (up to 100°C wide). Due to their inherent ruggedness these materials have potential for use in polarizer-free, bistable, scattering display and storage devices. The bistable modes are at low and high frequencies. The low frequency mode (write) is a highly scattering focal conic texture resulting from electrohydrodynamic instabilities while the high frequency mode (erase) is a clear state due to dielectric reorientation of the material. Both modes are preserved upon removal of the applied electric field. We present threshold voltages as a function of temperature, frequency, and cell thickness and response times as a function of voltage for each of the bistab...