Transient Reorientation of a Doped Liquid Crystal System under a Short Laser Pulse

The transient optical nonlinearity of a nematic liquid crystal doped with azo-dye DR19 is examined. The optical reorientation threshold of a 25-mu m-thick planar-aligned sample of 5CB using a 50 ns pulse duration 532 nm YAG laser pulse is observed to decrease from 800 mJ/mm(2) to 0.6 mJ/mm(2) after the addition of 1 vol% azo dopant, a reduction of three orders of magnitude. When using a laser pulse duration of 10 ns, no such effect is observed. Experimental results indicate that the azo dopant molecules undergo photoisomerization from trans-isomer to cis-isomer under exposure to light, and this conformation change reorients the 5CB molecules via intermolecular coupling between guest and host. This guest-host coupling also affects the azo photoisomerization process