1. Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
2. Department of Microsystems Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany
3. OSRAM Opto Semiconductors GmbH, 93055 Regensburg, Germany
4. Institute of Physics, Technische Universität Chemnitz, 09126 Chemnitz, Germany
5. École Polytechnique, 91128 Palaiseau, France