1. Institute of Physics, University of Freiburg 1 , 79104 Freiburg, Germany
2. Department of Physics, Lund University 2 , SE-221 00 Lund, Sweden
3. CNR-Institute for Photonics and Nanotechnologies (CNR-IFN) 3 , 35131 Padova, Italy
4. Max-Planck-Institute for Nuclear Physics 4 , 67119 Heidelberg, Germany