[1] Univ Munich, Dept Phys, Photon & Optoelect Grp, Amalienstr 54, D-80799 Munich, Germany
[2] Nanosyst Initiat Munich, Schellingstr 4, D-80539 Munich, Germany
[3] Univ Antwerp, EMAT, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
[4] Univ Munich, Dept Chem, Butenandtstr 5-13 E, D-81377 Munich, Germany