Pretilt Angle Generation in Photoaligning Materials

It is easy to see that the pretilt angle is not defined when normally incident polarized light is used in photoalignment. The two molecular symmetrical pretilt 

Photoalignment of Liquid Crystalline Materials Physics and Applications V. Chigrinov, V. Kozenkov and H.-S. Kwok 2008 John Wiley Sons, Ltd 

A normal incidence single-exposure system to obtain stable pretilted alignment of NLCs on photoalignment polymer layers was proposed using a photorubbing 

The pretilt appears in such a way that the nematic director is lifted up in the scan direction, similar to the case of conventional cloth rubbing. This photoscan method allows fairly accurate control of the pretilt angle by way of the scan speed and light intensity. The LC pretilt angle was increased to almost 3° at a scan rate of 70 p,m/s. 

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