1. Applied Physics Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
2. Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
3. Department of Physics, University of Colorado Boulder, 2000 Colorado Boulevard, Boulder, Colorado 80309, USA
4. Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, Campinas, SP 13083-859, Brazil
5. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, USA