1. Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Jena, Germany
2. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
3. School of Engineering and Information Technology, University of New South Wales at Canberra, Canberra, ACT, Australia
4. Graduate School of Engineering, Osaka University, Osaka, Japan
5. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA