1. Institut de Ciència de Materials de Barcelona (ICMAB‐CSIC) Campus UAB Bellaterra 08193 Spain
2. Institute of Photonics University of Eastern Finland P.O. Box 111 Joensuu FI‐80101 Finland
3. Institute of Materials Science Technische Universität Darmstadt Alarich‐Weiss‐Str. 2 64287 Darmstadt Germany