1. Institute of Solid State Physics, Technische Universität Berlin, Berlin 10623, Germany
2. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
3. Maryland NanoCenter, University of Maryland, College Park, Maryland 20899, United States
4. Center for Optoelectronic Convergence Systems, Korea Institute of Science and Technology, Seoul 02792 South Korea
5. Joint Quantum Institute, NIST/University of Maryland, College Park, Maryland 20899, United States