1. Institute of Experimental Physics, Warsaw University, Hoża 69, Warsaw, Poland
2. Grenoble High Magnetic Field Laboratory, MPI/FKF and CNRS, Grenoble, France
3. Institute for Microstructural Sciences, NRC, Ottawa, K1A 0R6, Canada
4. Laboratoire de Photonique et de Nanostructures, CNRS, 91460 Marcoussis, France