Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Liquid Crystal Elastomers for Biological Applications.

The term liquid crystal elastomer (LCE) describes a class of materials that combine the elastic entropy behaviour associated with conventional elastomers with the stimuli responsive properties of anisotropic liquid crystals. LCEs consequently exhibit attributes of both elastomers and liquid crystals, but additionally have unique properties not found in either. Recent developments in LCE synthesis, as well as the understanding of the behaviour of liquid crystal elastomers-namely their mechanical, optical and responsive properties-is of significant relevance to biology and biomedicine. LCEs are abundant in nature, highlighting the potential use of LCEs in biomimetics. Their exceptional tensile properties and biocompatibility have led to research exploring their applications in artificial tissue, biological sensors and cell scaffolds by exploiting their actuation and shock absorption properties. There has also been significant recent interest in using LCEs as a model for morphogenesis. This review provides an overview of some aspects of LCEs which are of relevance in different branches of biology and biomedicine, as well as discussing how recent LCE advances could impact future applications.

Efficiency improvements in a dichroic dye-doped liquid crystal Fresnel lens.

A dichroic dye-doped liquid crystal Fresnel lens was fabricated and investigated to observe the combination of phase and amplitude modulation based focusing. An anthraquinone dichroic dye was doped into a liquid crystal host, which when in the Fresnel lens configuration, generates a Fresnel zone plate with alternating "transparent" and "opaque" zones. The zones were induced by using photo-alignment of a light-sensitive alignment layer to generate the alternating pattern. The voltage dependency of efficiency for the dye-doped and pure liquid crystal Fresnel devices were investigated. Incorporation of dyes into the device yielded a significant 4% improvement in relative efficiency in the lens, giving a maximum of 37% achieved in the device, much closer to the theoretical 41% limit when compared with the non-dye-doped device. The input polarization dependence of efficiency was also investigated, showing very small fluctuations (±1.5%), allowing further insight into the effect of fabrication method on these liquid crystal Fresnel devices.

All-optical responsive azo-doped liquid crystal laser protection filter.

An all-optical switchable twisted nematic liquid crystal system has been designed for use as a laser protection filter, which takes advantage of light-induced modification of liquid crystal order. The filter employs photochromic azo-doped liquid crystal mixtures that have been optically characterized and incorporated into a laser filter device. The ability to switch between transmission and blocking modes is shown to occur, even for incredibly low intensity (0.5 mW) irradiation with a continuous 405 nm laser. The blocking-state extinction is defined only by the polarizer extinction ratio, and sub-second switching is demonstrated for these low laser intensities. The response is sufficiently fast to provide protection for CCD cameras against laser damage. The optical switching time is shown to depend on both temperature and laser power. This automatic photo-switchable device offers an exciting approach for passive laser protection.

Tuneable and switchable liquid crystal laser protection system.

The use of a liquid crystal Lyot filter as a simple and compact switchable laser protection system is demonstrated. The system OFF state exhibits a wavelength-independent transmission and switches to an ON state, which rejects a selected wavelength. The response time of the switchable system is <110 ms, depending on the rejected wavelength, with the ability for faster switching of <5 ms when using a lower-order rejection band. A rejection tuning range between 480 and 640 nm is demonstrated, and the potential to operate outside of the visible spectrum is discussed. In the ON state, the transmission at the rejected wavelength was found to be effectively limited by the polarizer extinction ratio, while transmission at other wavelengths allows for partial observations through the system even when in protection mode.