Transient laser-induced molecular reorientation and laser heating in a nematic liquid crystal

Transient molecular reorientation induced in a liquid crystal by a giant laser pulse was observed for the first time in a nematic substance 4′-$n$-pentyl-4-cyanobiphenyl (5CB). Even though the response time of molecular reorientation was several orders of magnitude longer than the laser pulsewidth, the effect on the refractive indices was still large enough to be easily detectable by a Mach-Zehnder interferometer. Transient laser heating of the medium also affected the refractive indices, but with proper geometric and polarization arrangement, it could be decoupled from molecular reorientation and measured separately. Theoretical calculations using the Ericksen-Leslie theory for molecular reorientation in liquid crystals and the heat-diffusion theory for the laser heating effect were able to explain the experimental observations quantitatively.