1. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA.
2. Department of Bioengineering, University of California, Los Angeles, CA 90095, USA.
3. JILA and Department of Physics, University of Colorado and National Institute of Standards and Technology (NIST), Boulder, CO 80309, USA.
4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Department of Physics, University of Central Florida, Orlando, FL 32816, USA.