1. Department of Physics and Center for Applied Photonics, University of Konstanz, D-78464 Konstanz, Germany, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, United Kingdom, Experimental Physics IIb, University of Dortmund, D-44221 Dortmund, Germany, Belarussian State University, 220080 Minsk, Belarus, and Carl Zeiss NTS GmbH, D-73447 Oberkochen, Germany