MULTIPHOTON EXCITATION AND RELAXATION OF THERMAL GRATINGS IN THE NEMATIC LIQUID-CRYSTAL 5CB

We have observed strong thermal grating excitation in the isotropic and nematic phase of pentylcyanobiphenyl (5CB) in wave-mixing experiments using intense green picosecond excitation pulses at lambda(exc) = 532 nm. It is shown that optical heating is provided by a three-photon absorption process in the commonly called ''highly transparent'' liquid crystal. We have further investigated the relaxation of the induced thermal gratings, which exhibits a critical slowing at the nematic-isotropic phase transition.