1. Thomas
J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States
2. Kavli
Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States
3. Institute
of Optics, Information and Photonics and Graduate School in Advanced
Optical Technologies, Friedrich-Alexander-University Erlangen-Nuremberg, 91054 Erlangen, Germany