[1] Univ Duisburg Essen, Fac Engn, Gen & Theoret Elect Engn ATE, D-47048 Duisburg, Germany
[2] CENIDE Ctr Nanointegrat Duisburg Essen, D-47048 Duisburg, Germany
[3] Ruhr Univ Bochum, Photon & Terahertz Technol, Fac Elect Engn & Informat Technol, D-44780 Bochum, Germany