1. Departamentul de Fizica, Universitatea “Politehnica” din Bucuresti, Splaiul Independentei 313, Ro-060042 Bucharest, Romania, Facultatea de Fizica, Universitatea din Bucuresti, P. O. Box MG-11, Ro-077125 Bucharest, Romania, and LiCryl-INFM (Liquid Crystal Regional Laboratory), Department of Physics, University of Calabria, Via P. Bucci Cubo 31/c, I-87036 Rende (CS), Italy