1. Department of Electrical Engineering and Information Science, Ruhr University Bochum, Bochum, Germany
2. Department of Physics, West Virginia University, Morgantown, West Virginia 26506, USA
3. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Konkoly Thege Miklós str. 29-33, 1121 Budapest, Hungary