1. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
2. Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow Region 142190, Russia
3. Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom
4. Science Program, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
5. Department
of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States
6. National Laboratory of Solid State Microstructures and School of
Physics, Nanjing University, Nanjing 210093, China