Flow-alignment and inertial effects in picosecond laser-induced reorientation phenomena of nematic liquid crystals

Collective molecular-reorientation phenomena in the nematic liquid crystal 5CB are investigated in dynamic-grating experiments using picosecond excitation pulses. Grating buildup and decay is monitored by diffraction of a cw laser beam. It is shown that molecular reorientation still increases after the excitation pulse but relaxes exponentially later. The observed dynamics can be explained by a flow-alignment theory considering photoelastic stresses leading to additional forces besides the optical torques, which are well known from previous experiments.