Photoalignment

Photoalignment is a technique for orienting liquid crystals to desired alignment by exposure to polarized light and a photo reactive alignment chemical.^[1] 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 compound acting as the command surface.^[2] Shortly after publication of Ichimura’s results, the groups from the USA (Gibbons et al.^[3]), Russia/Switzerland (Schadt et al.^[4] and Ukraine (Dyadyusha et al.^[5][6]) 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.^[7][8] Since then several chemical combinations have been demonstrated for photoalignment and applied in production of liquid crystal devices like modern displays.^[1][9]

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 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.^[10]

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.^[1][11]
• 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, polarizer, optical vortex generator, etc.^[12][13]

References

1. Yaroshchuk, Oleg; Reznikov, Yuriy (2012). "Photoalignment of liquid crystals: basics and current trends". J. Mater. Chem. 22 (2): 286–300. doi:10.1039/c1jm13485j. ISSN 0959-9428.
2. Ichimura, Kunihiro; Suzuki, Yasuzo; Seki, Takahiro; Hosoki, Akira; Aoki, Koso (September 1988). "Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer". Langmuir. 4 (5): 1214–1216. doi:10.1021/la00083a030. ISSN 0743-7463.
3. Gibbons, Wayne M.; Shannon, Paul J.; Sun, Shao-Tang; Swetlin, Brian J. (1991). "Surface-mediated alignment of nematic liquid crystals with polarized laser light". Nature. 351 (6321): 49–50. doi:10.1038/351049a0. ISSN 1476-4687. S2CID 4267126.
4. Schadt, Martin; Schmitt, Klaus; Kozinkov, Vladimir; Chigrinov, Vladimir (1992-07-01). "Surface-Induced Parallel Alignment of Liquid Crystals by Linearly Polymerized Photopolymers". Japanese Journal of Applied Physics. 31 (7R): 2155. doi:10.1143/JJAP.31.2155. ISSN 1347-4065. S2CID 123181249.
5. Dyadyusha, A.G., Kozenkov, V.M., Marusiy, T.Y., Reznikov, Y.A., Reshetnyak, V.Y. and Khizhnyak, A.I., 1991. Light-induced planar alignment of nematic liquid-crystal by the anisotropic surface without mechanical texture. Ukrainskii Fizicheskii Zhurnal, 36(7), pp.1059-1062.
6. Dyadyusha, A. G.; Marusii, T. Ya.; Reznikov, Yu. A.; Khizhnyak, A. I.; Reshetnyak, V. Yu. (1992-07-01). "Orientational effect due to a change in the anisotropy of the interaction between a liquid crystal and a bounding surface". Soviet Journal of Experimental and Theoretical Physics Letters. 56: 17. Bibcode:1992JETPL..56...17D. ISSN 0021-3640.
7. EP 0525478, Chigrinov, Vladimir Grigorievich; Kozenkov, Vladimir Marcovich & Novoseletsky, Nicolic Vasilievich et al., "Liquid crystal display cell", published 1993-02-03, assigned to Hoffmann La Roche and Niopic Moscow Research & Production Association 
8. US 5389698, Chigrinov, Vladimir Grigorievich; Kozenkov, Vladimir Marcovich & Novoseletsky, Nicolic Vasilievich et al., "Process for making photopolymers having varying molecular orientation using light to orient and polymerize", published 1995-02-14, assigned to Hoffmann La Roche and Niopic Moscow Research & Production Association 
9. Murata, Mitsuhiro; Yokoyama, Ryoichi; Tanaka, Yoshiki; Hosokawa, Toshihiko; Ogura, Kenji; Yanagihara, Yasuhiro; Kusafuka, Kaoru; Matsumoto, Takuya (May 2018). "81-1: High Transmittance and High Contrast LCD for 3D Head-Up Displays". SID Symposium Digest of Technical Papers. 49 (1): 1088–1091. doi:10.1002/sdtp.12126. ISSN 0097-966X.
10. Seki, Takahiro (2014-08-13). "New strategies and implications for the photoalignment of liquid crystalline polymers". Polymer Journal. 46 (11): 751–768. doi:10.1038/pj.2014.68. ISSN 0032-3896.
11. Chigrinov, Vladimir G.; Kozenkov, Vladimir M.; Kwok, Hoi-Sing (2008-06-06). Photoalignment of Liquid Crystalline Materials. Wiley-SID Series in Display Technology. Chichester, UK: John Wiley & Sons, Ltd. doi:10.1002/9780470751800. ISBN 978-0-470-75180-0.
12. Pan, Su; Ho, Jacob Y.; Chigrinov, Vladimir G.; Kwok, Hoi Sing (2018-02-14). "Novel Photoalignment Method Based on Low-Molecular-Weight Azobenzene Dyes and Its Application for High-Dichroic-Ratio Polarizers". ACS Applied Materials & Interfaces. 10 (10): 9032–9037. doi:10.1021/acsami.8b00104. ISSN 1944-8244. PMID 29442496.
13. Ji, Wei; Wei, Bing-Yan; Chen, Peng; Hu, Wei; Lu, Yan-Qing (2017-02-11). "Optical field control via liquid crystal photoalignment". Molecular Crystals and Liquid Crystals. 644 (1): 3–11. doi:10.1080/15421406.2016.1277314. ISSN 1542-1406. S2CID 100118998.