Photoalignment is a technique for orienting
liquid crystals
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. Th ...
to desired alignment by exposure to
polarized light and a
photo reactive alignment chemical.
It is usually performed by exposing the alignment chemical ('command surface') to polarized light with desired orientation which then aligns the liquid crystal cells or domains to the exposed orientation. The advantages of photoalignment technique over conventional methods are non-contact high quality alignment, reversible alignment and micro-patterning of liquid crystal phases.
History
Photoalignment was first demonstrated in 1988 by K. Ichimura on Quartz substrates with an
azobenzene
Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of simi ...
compound acting as the command surface. Shortly after publication of Ichimura’s results, the groups from the USA (Gibbons et al.), Russia/Switzerland (Schadt et al. and Ukraine (Dyadyusha et al.) almost simultaneously demonstrated LC photoalignment in an azimuthal plane of the aligning substrate. The latter results have been particularly important because they provided a real alternative to the rubbing technology. Since then several chemical combinations have been demonstrated for photoalignment and applied in production of liquid crystal devices like modern displays.
Advantages
Traditionally, liquid crystals are aligned by rubbing electrodes on polymer covered glass substrates. Rubbing techniques are widely used in mass production of
liquid crystal displays
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but i ...
and small laboratories as well. Due to the mechanical contact during rubbing, often debris are formed resulting in impurities and damaged products. Also, static charge is generated by rubbing which can damage sensitive and increasingly miniature electronics in displays.
Many of these problems can be addressed by photoalignment.
* Photoalignment is by definition a non-contact process. This allows alignment of liquid crystals even in mechanically inaccessible areas. This has immense implications in use of liquid crystals in telecommunications and organic electronics.
* By optical imaging, very small domains can be aligned which results in extremely high quality alignments.
* By varying the orientation of liquid crystal alignment on a microscopic scale, thin film optical devices can be created like
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
,
polarizer
A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well ...
,
optical vortex
An optical vortex (also known as a photonic quantum vortex, screw dislocation or phase singularity) is a zero of an optical field; a point of zero Intensity (physics), intensity. The term is also used to describe a beam of light that has such a ze ...
generator, etc.
[{{Cite journal, last=Ji, first=Wei, last2=Wei, first2=Bing-Yan, last3=Chen, first3=Peng, last4=Hu, first4=Wei, last5=Lu, first5=Yan-Qing, date=2017-02-11, title=Optical field control via liquid crystal photoalignment, journal=Molecular Crystals and Liquid Crystals, language=en, volume=644, issue=1, pages=3–11, doi=10.1080/15421406.2016.1277314, issn=1542-1406]
References
Liquid crystals
Optical materials
Cleanroom technology