VOLUME 84, NUMBER 9 PHYSICAL REVIEW LETTERS 28 FEBRUARY 2000 Photoalignment of Liquid Crystals by Liquid Crystals Yuriy Reznikov Institute of Physics of National Academy of Science, Prospect Nauki 46, Kyiv 252022, Ukraine Oksana Ostroverkhova and Kenneth D. Singer Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079 Jae-Hoon Kim, 1 Satyendra Kumar, 1 Oleg Lavrentovich, 2 Bin Wang, 2 and John L. West 2 1 Department of Physics, Kent State University, Kent, Ohio 44242 2 Liquid Crystal Institute, Kent State University, Kent, Ohio 44242 (Received 15 July 1999) We observed light-induced alignment of the nematic liquid crystal, 4, 4 0 -n-pentylcyanobiphenyl (5CB), on a fused quartz. Irradiation of the adsorbed layer with polarized ultraviolet light produced homoge- neous alignment in a 5CB-filled liquid crystal cell with the easy orientation axis perpendicular to the polarization direction. The measured anchoring energy increases with illumination up to 10 24 ergcm 2 . Phase retardation and pretilt measurements confirmed near homogeneous alignment of the LC in a cell, while the polarization dependence of second harmonic generation suggested a near normal alignment of the adsorbed layer. We believe that light-induced rearrangement or phototransformation of the adsorbed layer causes the observed phenomenon. PACS numbers: 61.30.Gd Homogeneous alignment of liquid crystals (LC) is usually achieved using anisotropic polymer films de- posited on glass substrates. Anisotropy can be obtained by illumination with polarized UV light. This noncontact technique has been of great interest in the last decade because of its obvious advantages [1–5]. Alignment effects can be achieved by illumination of dye-doped LCs with polarized light [6,7]. Light-induced adsorption of the excited dye molecules onto the initially isotropic polymer surface is postulated to be the origin of this alignment. Previously, we observed planar alignment of 4, 4 0 -n-pentylcyanobiphenyl (5CB) on a fused quartz surface upon illumination of the cell with polarized UV light, indicating light-induced orientational anisotropy of the adsorbed 5CB molecules on the quartz surface [8]. In this paper, we report planar alignment of bulk 5CB on a 5CB layer adsorbed on a fused quartz substrate that was irradiated with polarized UV light, and on the structure and properties of the aligning layer. Fused quartz substrates were cleaned, and 5CB ad- sorbed on the quartz surface by dipping in a dilute solution of 5CB (E.Merck) in isopropyl alcohol (0.1% by weight) for 10–60 min. The substrates were then rinsed in pure isopropyl alcohol for 5–15 s to remove nonadsorbed molecules, and then gently dried. The presence of a LC layer on the substrate was monitored by measuring the UV absorption of the 5CB layer using a Perkin-Elmer 19 spectrometer. The absorption saturated at times longer than 45 min. An adsorption time of 60 min was chosen to assure an equilibrium adsorption layer of 5CB on the substrate. We also found that the adsorbed layer was not stable with time, or at elevated temperatures. We observed a significant reduction in the absorption after maintaining the samples at room temperature for 6–8 h or upon heating to 90 ± C for 5 min perhaps due to evaporation. Quartz substrates with an adsorbed 5CB layer were ir- radiated with polarized UV light from a Xe lamp (Oriel- 6261) for periods as long as 90 min. The average intensity of the light incident on the sample plane was 10 mWcm 2 . We did not observe reproducible changes in the shape, po- sition, and intensity of the absorption spectra following ir- radiation. We also found that the stability of the adsorbed layer increased after the UV irradiation from a few hours to several days. The alignment properties of the 5CB layers were exam- ined in a combined cell, which consisted of a reference and a test substrate, filled with 5CB. The cell thickness L was 20 mm. The reference glass substrate was covered with a rubbed polyimide (PI), which provided strong pla- nar alignment of 5CB. This surface aligned the 5CB in the cell parallel to the rubbing direction. The fused quartz plate with the irradiated 5CB layer was the test surface. The cells were made so that the angle, u 0 , between the rubbing direction of the reference substrate and the direc- tion of polarization of UV light on the test substrate during the irradiation was 0, 90 ± , or 45 ± . The cell was filled with LC either in the isotropic state T  50 ± C or in the mesophase at room temperature (both gave the same results). The alignment texture was observed with a polarizing microscope. Planar alignment with the director d predominantly parallel to the direction of rubbing was observed in the nonirradiated areas of the samples (the alignment was poor and many defects local- ized on the quartz surface were observed). A homoge- neous twist structure was observed in the illuminated areas for u 0  0 and 45 ± . The alignment of the planar texture 1930 0031-9007 00  84(9)  1930(4)$15.00 © 2000 The American Physical Society