1. Laboratory of Optics & Spectroscopy, Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., Sofia BG-1784, Bulgaria
2. Laboratory of Liquid Crystals & Biomolecular Layers, Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., Sofia BG-1784, Bulgaria