1. Center for Nanophotonics, FOM Institute AMOLF, Science Park Amsterdam 104, 1098 XG Amsterdam, The Netherlands
2. Department of Physics, Duke University, Durham, North Carolina 27708, United States
3. Institute of Inorganic Methodologies and of Plasmas-CNR, and INSTM, via Orabona, 4-70126 Bari, Italy
4. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, United States
5. Army Aviation & Missile RD&E Center, RDMR-WDS, Redstone Arsenal, Alabama 35898, United States