1. Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, Center for Nanophotonics, FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, Centre National de la Recherche Scientifique, 3 Rue Michel-Ange, 75794 Paris Cedex 16, France, Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain