1. Department of Physics and Institute for Soft Matter Synthesis & Metrology, Georgetown University, 3700 O St NW, Washington, D.C. 20007, United States
2. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
3. Department of Physics & Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
4. Department of Materials Science and Engineering, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213, United States
5. Department of Physics, University of California Santa Barbara, Santa Barbara, California 93106, United States