Peter Raynes

Handbook of liquid crystals

This four-volume reference aims to provide information on the basic principles of both low- and high-molecular weight materials, as well as the synthesis, characterization, modification, and applications (such as in computer displays or as structural materials) of all types of liquid crystals. Volume 1, for example, deals with the basic physical and chemical principles of liquid crystals, including structure-property relationships, nomenclature, phase behaviour, characterization methods, and general synthesis and application strategies.

Short pitch cholesteric electro-optical device based on periodic polymer structures

The helical flexoelectro-optic effect produces a submillisecond, temperature-independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback is that it relies on a texture, the uniform lying helix (ULH), which is intrinsically unstable. We present a method based on the use of periodic polymeric microchannels to create highly ordered and stable ULH structures. Electro-optic measurements performed on a test device show a large contrast ratio between bright and dark states (better then 100:1), fast switching (200 μs), and large optical rotation (>30°).

Uniform Lying Helix Alignment on Periodic Surface Relief Structure Generated via Laser Scanning Lithography

The main drawback in the exploitation of the chiral-flexo-electro-optic effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is unstable when the cholesteric is sandwiched between spatially uniform aligning surfaces (UASs). It has been shown that the ULH can be promoted by periodic (horizontal/vertical) anchoring conditions or by the presence of periodic polymeric walls. Here we show that periodic surface relief structures can also promote the formation of a stable ULH texture. The surface relief structure was created by curing an ultraviolet curable material via a two-photon excitation laser-lithography process. The process allows sub-micron resolution and flexibility in creating the topographic relief.

Alignment of the Uniform Lying Helix Structure in Cholesteric Liquid Crystals

The uniform lying helix (ULH) configuration in cholesteric liquid crystals, where the helical axis of a chiral nematic is aligned uniformly in the plane of two confining substrates is of interest for both electro-optic and photonic applications. However, the formation of the well-aligned ULH structure required is non-trivial. Since the helical axis tends to align at an angle with respect to the surface alignment direction, cells where both substrates have planar alignment layers can cause the formation of a two-domain ULH structure. Here we investigate the orientation and nature of the domains as a function of field application. Further we show that it is possible to obtain mono-domain structures when the alignment directions on each substrate are skewed so as to match the angle that the helix forms with respect to the alignment at the electric field applied. This improves the electro-optic contrast of devices.

Investigations of optical activity of natural products and chiral pharmaceuticals using liquid crystal technologies

In this article we demonstrate the use of liquid crystal technologies in the determination of enantiomeric excess and in the sensing of molecular chirality, particularly at values near to zero. We use this technique to investigate the optical purities of the commercially available chiral natural product, menthol, and in the investigation of the optical activity of racemic ibuprofen.

Effects of molecular shape on the photoluminescence of dyes embedded in a chiral polymer with a photonic band gap

Circularly polarized photoluminescence has been generated by embedding fluorescent guest molecules in a helically aligned polymer host with a selective reflection band, which coincides with the emission wavelength of the embedded dyes. We have investigated the photoluminescence from a range of guest molecules of varying shapes and found that one circular polarization is suppressed within the reflection band of the host independently of the shape of the guest molecules. However, just outside the band edges, there is an enhancement of the same polarization and the relative magnitudes of the enhancement at the two band edges is strongly influenced by the shape of the guest molecules. This effect has been understood from the ordering of the guest molecules in the polymer host.

High-brightness relaxed-bend state in a pi cell stabilized by synchronized polymerization

The authors report a synchronized polymer-stabilization technique which can be used to stabilize the nonpermanent states in liquid crystal devices. In this paper, a relaxed-bend state in a pi cell which has a lifetime of only 80ms is observed and then stabilized by this proposed technique. After stabilizing, the pi cell is immune to undesirable recovery into the splay or twist states, and its optical contrast is much higher than that of the conventional polymer-stabilized pi cell.

Fast Electro-Optical Device Based on Chiral Liquid Crystals Encapsulated in Periodic Polymer Channels

The chiral flexo-electro-optic effect produces a sub-millisecond, temperature independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback in the exploitation of this effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is intrinsically unstable since neither planar nor homeotropic surface conditions are compatible with it. We present a method, based on the use of periodic polymeric micro-channels, to create highly ordered and stable ULH structures. We show that the periodic structure, which is created holographically, naturally aligns the cholesteric helical superstructure along the micro-channels, without requiring any elaborate ad-hoc procedure, even when the size Λ of the micro-channels is much larger than the pitch P (Λ > 20P). Electro-optic measurements performed on the test-device show a large contrast ratio between bright and dark states (better then 100:1), short switching time (200μs) and large optical rotation (>30°).

Investigation of the transient symmetric H state in a pi cell

The so-called symmetric H (Hs) state has been reported to have submillisecond response times, which results from the symmetric profile of the liquid crystal director; however, no direct evidence has been obtained to show the profile symmetry. The difficulty in proving this symmetric structure by direct observation results from the short lifetime of Hs state (typically around a few tens of milliseconds). In the work reported here, the authors utilize a burst driving method along with stroboscopic illumination from blue and red light emitting diodes to capture conoscopic images for the Hs director profile; these showed good agreement with their modeling.

Circularly polarized emission from dyes embedded in a chiral polymer

Due to the 1D photonic bandgap nature of cholesteric liquid crystals, strongly circularly polarized photoluminescence can be generated by embedding fluorescent guest molecules in a cholesteric host whose resonance region coincides with the emission of the chromophores. In this paper, fluorescent guest molecules are embedded in a cholesteric reactive mesogen (RM) host, which is subsequently polymerized by UV-exposure to generate a chiral polymer. We have investigated the dependency of the form-anisotropy of the dye on the emission of circularly polarized photoluminescence by comparing highly anisotropic rod like dyes with a range of dyes of different shapes.