[1] Univ New Mexico, Ctr High Technol Mat, Albuquerque, NM 87106 USA
[2] Univ Notre Dame, Dept Elect Engn, Notre Dame, IN 46556 USA
[3] Tech Univ Denmark, Dept Photon Engn, DTU Foton, DK-2800 Lyngby, Denmark
[4] SUNY Buffalo, Dept Elect Engn, Buffalo, NY 14260 USA
[5] Univ Bielefeld, Fak Phys, D-33615 Bielefeld, Germany
[6] CNR, Ist Nanosci, NEST, Pisa 56127, Italy
[7] Scuola Normale Super Pisa, Pisa 56127, Italy