Photoalignment of Liquid-Crystal Systems.

Liquid-crystal elastomers (LCEs) are rubbers whose constituent molecules are orientationally ordered. Their salient feature is strong coupling between the orientational order and mechanical strain. For example, changing the orientational order gives rise to internal stresses, which lead to strains and change the shape of a sample. Orientational order can be affected by changes in externally applied stimuli such as light. We demonstrate here that by dissolving-rather than covalently bonding-azo dyes into an LCE sample, its mechanical deformation in response to non-uniform illumination by visible light becomes very large (more than 60 degrees bending) and is more than two orders of magnitude faster than previously reported. Rapid light-induced deformations allow LCEs to interact with their environment in new and unexpected ways. When light from above is shone on a dye-doped LCE sample floating on water, the LCE 'swims' away from the light, with an action resembling that of flatfish such as skates or rays. We analyse the propulsion mechanism in terms of momentum transfer.

Fast liquid-crystal elastomer swims into the dark

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Photomodulation of liquid crystal orientations for photonic applications

The history of human civilization is accompanied by the development of novel materials, from the period of natural stone, the bronze and iron age, to the modern times of synthetic materials and promising advanced materials. Liquid crystals (LCs) are one of such kind of materials that greatly infl uence our daily life, and which are not limited to displays such as TVs and personal computers. Being an intermediate phase of matter, LCs show characteristics of both controllable mobility of an isotropic liquid and ordered regularity of a crystalline solid. Combined with their photoresponsive properties, photocontrollable LC actuators have enabled a variety of applications in various fi elds, which include fl at panel displays, photonics, photo-driven devices, and more recently nanotechnology. The LC’s unique features provide soft materials in a liquid-crystalline state with interesting properties such as 1) selfassembly, 2) fl uidity with long-range order, 3) molecular and supramolecular cooperative motion (MCM and SMCM), 4) large birefringence and anisotropy in various physical properties (optical, mechanical, electrical and magnetic), 5) alignment change induced by external fi elds at surfaces and interfaces, and 6) deformation of LC elastomers (LCEs) in response to stimuli. [ 1–4 ]

Photocontrollable liquid-crystalline actuators.

Learning to use new technologies by older adults: Perceived difficulties, experimentation behaviour and usability

Laser-induced reorientation was studied in absorbing nematic films. The optical Freedericksz threshold occured at an intensity level of 50 W/cm2 in contrast to the normally observed few KW/cm2 value for transparent layers. Thermomechanical coupling is considered as a possible explanation of the observed anomaly.

Anomalous optical Freedericksz transition in an absorbing liquid crystal

The optical reorientation process in nematic guest-host mixtures is investigated. It is found that for certain dyes the optical Freedericksz threshold is two orders of magnitude lower as in the pure host. The observations can be interpreted in terms of an enhanced optical torque which increases with the dye concentration.

Low-Power Optical Reorientation in Dyed Nematics

We investigated the temperature dependence of the optical Fr\'eedericksz threshold in nematic liquid crystals doped with anthraquinone dyes. In the presence of these dyes, the Fr\'eedericksz threshold is anomalously low and exhibits an unusual temperature dependence. A microscopic model is proposed, according to which the anisotropic orientational distribution and the rotational dynamics of the excited dye molecules play an important role in the observed phenomena.

Ashley Lloyd

Papers

Learning to use new technologies by older adults: Perceived difficulties, experimentation behaviour and usability

Anomalous optical Freedericksz transition in an absorbing liquid crystal

Low-Power Optical Reorientation in Dyed Nematics

Temperature dependence of the optical Fréedericksz transition in dyed nematic liquid crystals

The saturation limit to picosecond, induced absorption in dyes