Photoalignment of Liquid Crystalline Materials: Physics and Applications

Book Preface

Among all the flat panel display technologies, liquid crystal display (LCD) is the most dominant. The manufacture of LCDs is now very mature and done with  huge glass substrates measuring over 5 m2. The availability of such inexpensive high-resolution displays has accelerated the transformation of our society into a display-centric one.

However, despite its dominance, LCD is still in need of further improvement. Among other things, light utilization efficiency, cost, and optical performance such as response times of LCD are still not optimal. It is no wonder that much research is still being performed and new LCD modes are still being invented with better properties such as response time or viewing angles.

In this book, we concentrate on one aspect of LCD manufacture, namely that of the alignment surface. In particular, we shall present a comprehensive review of photoalignment technologies. Photoalignment has been proposed and studied for a long time [1–12]. In fact, the subject of light–molecule interactions has been a fascinating subject of research for a long time and is still capturing the imagination of many people. Light is responsible for the delivery of energy as well as phase and polarization information to materials systems. In this particular case, the alignment of the molecules takes place due to a partial ordering of the molecular fragments after a topochemical reaction of a photoselection (Weigert’s effect). While the first photo-patterned optical elements, based on polyvinyl-cinnamate films, appeared in 1977 [1], the technology became an LCD one only at the beginning of the 1990s [2–5]. It was soon shown that these materials could provide high-quality alignment of molecules in an LC cell. Over the last 20 years, many improvements and variations have been made for photoalignment. Commercial photoalignment materials are now readily available. Many new applications, in addition to the alignment of LCD, have been proposed and demonstrated. In particular, the application of photoalignment to active optical elements in optical signal processing and communications is currently a hot topic in photonics research.

Photoalignment possesses obvious advantages in comparison with the usually ‘rubbing’ treatment of the substrates of LCD cells. Possible benefits for using this technique include:

1. Elimination of electrostatic charges and impurities as well as mechanical damage of the surface.
2. A controllable pretilt angle and anchoring energy of the LC cell, as well as its high thermal and UV stability and ionic purity.
3. The possibility to produce structures with the required LC director alignment within the selected areas of the cell, thus allowing pixel dividing to enable new special LC device configurations for transflective, multidomain, 3D, and other new display types.
4. A potential increase of manufacturing yield, especially in LCDs with active matrix addressing, where the pixels of a high-resolution LCD screen are driven by thin film transistors on a silicon substrate.
5. New advanced applications of LCs in fiber communications, optical data processing, holography, and other fields, where the traditional rubbing LC alignment is not possible due to the sophisticated geometry of the LC cell and/or high spatial resolution of the processing system.
6. The ability for efficient LC alignment on curved and flexible substrates.
7. Manufacturing of new optical elements for LC technology, such as patterned polarizers and phase retarders, tunable optical filters, polarization non-sensitive optical lenses, with voltage-controllable focal distance etc.