RELAXATION-TIME OF MOLECULAR ROTATION ABOUT ITS LONG AXIS IN THE SMECTIC-A PHASE OF FERROELECTRIC AND ANTIFERROELECTRIC LIQUID-CRYSTALS AS OBSERVED BY PICOSECOND OPTICAL KERR-EFFECT

We have studied the molecular rotational motion about its long axis in ferroelectric chiral smectic-C (Sm-C) and antiferroelectric chiral s mectic-C-a (sm-C-A) liquid crystals and observed a few tens of picose cond relaxation time. Previously reported results using degenerate fou r-wave mixing with 25-ps pump pulses [Lalanne et al., Phys. Rev. A 44, 6632 (1991)] claimed to show a critical slowing down in the temperatu re region more than 0.1 degrees C above the Sm-A-Sm-C phase transitio n. Our measurements of the optical Kerr effect with 130-fs pump pulses do not show any critical slowing down in the corresponding temperatur e region above the phase transition from Sm-A to Sm-C or Sm-C-A*. The result may indicate that the laser-induced molecular reorientation wi th tau similar to 10(-11) s scarcely couples with the tilt angle and t he polarization. A possibility has been discussed that a slower rotati onal motion with tau greater than or equal to 10(-10) s plays a primar y role for the emergence of ferroelectricity and/or antiferroelectrici ty.