1. Centro
de Física de Materiales, Centro Mixto CSIC-UPV/EHU and Donostia International Physics Center (DIPC), 20018 Donostia-San
Sebastián, Spain
2. Institut
des Sciences Moléculaires d’Orsay, UMR 8214, CNRS-Université Paris-Sud, 91405 Orsay, France
3. Department
of Electrical and Computer Engineering, MS378, Laboratory of Nanophotonics, Rice University, Houston, Texas 77005, United States
4. Joint
Quantum Institute and Quantum Measurement Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States