INVESTIGATION OF THE SOFT-MODE RELAXATION IN A FERROELECTRIC LIQUID-CRYSTAL WITH PICOSECOND LASER-INDUCED DYNAMIC GRATINGS

The relaxation of light-induced tilt-angle changes in a ferroelectric smectic-C* liquid crystal is investigated with picosecond laser-induced dynamic gratings. Grating buildup and decay is monitored by diffraction of a cw laser beam. It is shown that reorientation of the liquid-crystal molecules is achieved by rapid optical heating and follows the temperature rise with a characteristic time constant, depending on temperature. The observed rise times correspond to the soft-mode relaxation of the order parameter in a Landau theory describing the smectic-C*-smectic-A phase transition.