1. MNES, IMNE, FEI, Technical University of Ilmenau, PF 100565, 98684 Ilmenau, Germany, Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, P.O. Box 95, 1113 Sofia, Bulgaria, Polymer Institute, Slovak Academy of Sciences, 9 Dubravska cesta, 842 36 Bratislava, Slovakia, and Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784, Sofia, Bulgaria