1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 639798, Singapore
2. Institute for Solid State Physics, TU Wien, 1040 Vienna, Austria
3. Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan
4. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
5. Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah 84112, United States
6. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States
7. Department of Material Science and NanoEngineering, Rice University, Houston, Texas 77005, United States