Optical trapping in liquid crystals: recent results

We report on our recent results about optical trapping in liquid crystals. Trapping in these materials is due to the optoelastic interaction occurring between a colloidal particle and a "ghost" colloid which results from the local distortion of the liquid crystal induced by a laser beam. This kind of interaction is induced by minimization of the elastic free energy of the system leading to particle trapping by the ghost colloid, as is the case occurring between two real colloidal particles. The effect has been investigated under experimental conditions which prevents the occurrence of conventional optical trapping induced by high gradient optical fields. We show that application of a low frequency electric field to a homeotropic sample of a nematic liquid crystal is able to control the strength and the range of the force acting on the nematic colloid. In case of liquid crystal with positive dielectric anisotropy the existence of a quenching voltage is demonstrated above which no interaction takes place, while in case of negative dielectric anisotropy an extremely large interaction range is demonstrated. Interaction range of few hundred microns has been observed.