1. Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom
2. Department of Electrical and Computer Engineering and Photonics Center, Boston University, 8 Saint Mary's St., Boston, Massachusetts 02215, USA
3. Instituto de Fisica, Universidade Federal do Rio de Janeiro, Rio de Janeiro-RJ 21941-972, Brazil
4. Center for Opto-Electronic Materials and Devices Research, Korea Institute of Science and Technology, Seoul 02792, South Korea
5. Division of Material Science and Engineering, Boston University, 15 Saint Mary's St., Brookline, Massachusetts 02446, USA
6. Department of Physics, Boston University, 590 Commonwealth Ave., Boston, Massachusetts 02215, USA