1. Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
2. Biophysics Program, Stanford University, Stanford, CA 94305, USA.
3. Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA.
4. Department of Physics, University of Alberta, Edmonton AB, T6G 2G7, Canada.
5. National Institute for Nanotechnology, National Research Council of Canada, Edmonton AB, T6G 2M9, Canada.