1. Department of Electrical and Computer Engineering and Photonics Center, Boston University, 8 Saint Mary's Street, Boston, Massachusetts 02215, USA
2. Department of Materials Science and Engineering, University of Wisconsin–Madison, 1509 University Avenue, Madison, Wisconsin 53706, USA
3. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA