[1] Univ Marburg, Fac Phys, D-35032 Marburg, Germany
[2] Univ Marburg, Ctr Mat Sci, D-35032 Marburg, Germany
[3] Univ Marburg, Fac Chem, D-35032 Marburg, Germany
[4] Phys Tech Bundesanstalt, D-38116 Braunschweig, Germany
[5] Univ Lucknow, Dept Phys, Lucknow 226007, Uttar Pradesh, India
[6] TOPTICA Photon AG, D-82166 Munich, Germany