1. Laboratory of Semiconductor Materials, Department of Materials and Nano Physics, KTH—Royal Institute of Technology, Electrum 229, 164 40 Kista, Sweden
2. CNR-Institute for Photonics and Nanotechnologies, P. Leonardo da Vinci, 32 20133 Milano, Italy
3. Department of Physics Politecnico di Milano, P. Leonardo da Vinci, 32 20133 Milano, Italy