1. Department of Physics, Brown University, Providence, Rhode Island 02912, United States
2. Department of Physics, University of Marburg, 35037 Marburg, Germany
3. Centro de Investigaciones en Optica A. C., Loma del Bosque 115, Lomas del Campestre, Leon, Guanajuato 37150, Mexico
4. Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts 02138, United States
5. Department of Electrical and Computer Engineering, Holonyak Micro and Nanotechnology Laboratory, University of Illinois at Urbana−Champaign, Champaign, Illinois 61820, United States
6. School of Engineering, Brown University, Providence, Rhode Island 02912, United States