1. Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States
2. Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Departments of Electrical and Computer Engineering and Photonics Center, Boston University, Boston, Massachusetts 02215, United States
4. Institute of Bioengineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland