1. Department of Electrical Engineering and Information Science, Ruhr University Bochum, D-44780 Bochum, Germany
2. School of Physics, Dalian University of Technology, 116024 Dalian, China
3. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Konkoly-Thege Miklós str. 29-33, H-1121 Budapest, Hungary
4. Electrodynamics and Physical Electronics Group, Brandenburg University of Technology Cottbus-Senftenberg, Siemens-Halske-Ring 14, D-03046 Cottbus, Germany