Optical 2nd-harmonic Generation During Switching In A Ferroelectric Liquid-crystal Cell

We report measurements of the temporal dependence of the second-harmonic generation from a 50 μm thick homeotropically aligned cell of ferroelectric liquid crystal (SCE9) (British Drug House, Ltd.) exposed to a symmetric square wave form electric field. The field switches the molecular configuration between the two unwound states with the director on the opposite positions of the tilt cone. The response of the second-harmonic signal measured in the phase matching geometry for one of these equilibrium states shows that a complete switching process takes several seconds. In experimental geometries away from the equilibrium phase matching geometry a strong transient second-harmonic signal is observed during the switching. It corresponds to the transient realization of the phase matching condition related to the intermediate molecular configurations. The study of this effect provides a precise time resolved monitoring of the molecular dynamics during the switching. Our results show that the director reorientation in a hometropic cell goes on within numerous domains of a size much smaller than the optical wavelength. On the basis of this assumption the temporal dependence of the director reorientation within the domains is calculated.