[1] Institute of Solid State Electronics, Center for Micro and Nanostructures, Vienna University of Technology, A-1040 Vienna, Austria
[2] Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
[3] International Laser Centre, Ilkovicova 3, 841 04 Bratislava, Slovakia
[4] Research Institute for Technical Physics and Material Science, H-1525 Budapest, Hungary
[5] Institute of Electrical Engineering, Slovac Academy of Sciences, 841 04 Bratislava, Slovakia