1. Department of Physics and Center for Applied Photonics, University of Konstanz 1 , D-78457 Konstanz, Germany
2. CREOL, College of Optics and Photonics, University of Central Florida 2 , Orlando, Florida 32816, USA
3. BAE Systems Inc., MER15-1813 3 , P.O. Box 868, Nashua, New Hampshire 03061-0868, USA