1. a Laboratory of Liquid Crystals, Institute of Solid State Physics , Bulgarian Academy of Sciences , 72 ‘Tzarigradsko Chaussee’ blvd., 1784 Sofia, Bulgaria
2. b Laboratory of Biomolecular Layers, Institute of Solid State Physics , Bulgarian Academy of Sciences , 72 ‘Tzarigradsko Chaussee’ blvd., 1784 Sofia, Bulgaria