1. Department of Physics, University of California San Diego, La Jolla, California 92093, United States
2. Department of Physics and Astronomy, University of California, Riverside, California 92521, United States
3. Department of Physics, Boston University, Boston, Massachusetts 02215, United States
4. Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, United States
5. Department of Physics, California State University, San Marcos, California 92096, United States
6. Graphene Research Centre and Department of Physics, National University of Singapore, 117542, Singapore
7. Max Planck Institute of Quantum Optics and Center for NanoScience, 85714 Garching, Germany