1. Department of Physics and Institute for Optical Sciences, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada
2. Montana Instruments, 151 Evergreen Dr., Bozeman, Montana 59715, USA
3. Department of Physics, Boston College, 140 Commonwealth Ave., Chestnut Hill, Massachusetts 02467, USA
4. Department of Physics and Center for Advanced Nanoscience, University of California, San Diego, La Jolla, California 92093, USA
5. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA