1. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305
2. Department of Ophthalmology, Stanford University, Stanford, CA 94305
3. Department of Physics, Stanford University, Stanford, CA 94305
4. Department of Electrical Engineering, Stanford University, Stanford, CA 94305
5. Department of Material Science, Stanford University, Stanford, CA 94305
6. Department of Physics, University of Strathclyde, G1 1XQ Glasgow, Scotland, United Kingdom