1. Department of Physics and Biophysics, University of Michigan, 450 Church St., Ann Arbor, Michigan 48109, USA
2. Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, Michigan 48109-2136, USA
3. Laboratoire d’Optique et Biosciences, Ecole Polytechnique, CNRS, INSERM, Institut Polytechnique de Paris, 91128 Palaiseau, France