Femtosecond analysis of the spinning motion of liquid crystal molecule in Smectic A phase

Abstract

Molecular orientational dynamics of liquid crystalline (LC) materials is a subject of many researchers and has been studied by a large variety of techniques. More recently, with ultrashort laser pulses, time resolved measurements have been used to probe directly the fast dynamics for many different LC. Most of the works focus on the isotropic phase of nematogenic substances where the LC molecules are assumed to have cylindrical symmetry. Then orientational dynamics arise from molecular tumbling motion. This assumption is not valid if the molecules are biaxial and the molecular spinning motion about the molecular long axis is not negligible. For isotropic liquids, this happens on the picosecond time scale and has been demonstrated, for example, in liquid biphenyl1. The smectic A (Sm-A) phase is most suitable for investigation of the spinning orientational dynamics using for instance the transient optical Kerr effect. When homeotropically aligned, a Sm-A film has its molecules oriented with their long molecular axis parallel to the surface normal. A linearly polarized laser beam propagating along the surface normal will reorient the molecules around the surface normal via the spinning motion while the Fredericksz transition is forbiden thanks to the high elastic constants of the smectic layers. The molecular reorientation can be measured through the optical birefringence induced by the reorientation. This geometry was previously used to investigate the collective spinning dynamics at the Sm-A- Sm-C* transition2. So far, however, no such study in the femtosecond time scale pertaining to individual molecular reorientation has ever been reported. In this paper we present such a study in a number of cyanobiphenyl ( nCB with n denoting the number of carbons in the alkyl chain) Sm-A LC media.