1. Department of Physics, Politecnico di Milano, Milano 20133, Italy
2. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
3. Istituto Italiano di Tecnologia, Genoa 16163, Italy
4. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States
5. Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States
6. Istituto di Fotonica e Nanotecnologie─Consiglio Nazionale delle Ricerche, Piazza Leonardo da Vinci 32, Milano I-20133, Italy
7. Smalley-Curl Institute, Rice University, Houston, Texas 77005, United States