Voltage-assisted photoaligning effect of an azo dye doped in a liquid crystal with negative dielectric anisotropy

We demonstrate a method of controlling the pretilt angle by field-assisted photoalignment in a vertically-aligned liquid crystal doped with a dichroic azo dye. By dynamically measuring both the pretilt angle and anisotropy in absorbance, we present detailed alignment variations in both the polar and azimuthal directions with the time of laser exposure that induces photoisomerization of the dye molecules. Significant differences in the resulting pretilt as well as optically-absorptive anisotropy are manifested between a regular photoalignment procedure and that in conjunction with a bias voltage during irradiation. It is found that the field-assisted exposure not only speeds up the photoalignment along the polarization direction of the laser beam but also leads to a higher orientational order. (C) 2010 Optical Society of America