1. Department of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany
2. Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd. NE, Albuquerque, New Mexico 87131, USA
3. CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA