[1] Univ Potsdam, Inst Phys & Astron, Karl Liebknecht Str 24-25, D-14476 Potsdam, Germany
[2] Humboldt Univ Berlin AG Theoret Opt & Photon, Inst Phys, D-12489 Berlin, Germany
[3] Friedrich Schiller Univ Jena, Inst Condensed Matter Theory & Opt, Max Wien Pl 1, D-07743 Jena, Germany