1. Department of Physics, University of Crete, Heraklion, Crete, 71003, Greece, and Institute of Electronic Structure & Laser, Foundation for Research and Technology-Hellas, Heraklion, Crete, 71110 Greece
2. Department of Physics, University of Konstanz, D-78464 Konstanz, Germany
3. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, New Mexico, 87545 United States
4. California Institute of Technology, 1200 E California Blvd, MC 107-81, Pasadena, California 91125, United States