1. Department of Physics, Sapienza University of Rome, Rome 00185, Italy
2. Department of Science, Roma Tre University, Rome 00146, Italy
3. IHP─Leibniz-Institut für Innovative Mikroelektronik, Frankfurt (Oder) 15236, Germany
4. European Synchrotron Radiation Facility, Grenoble Cedex 9 38043, France
5. Institute for Microelectronics and Microsystems IMM-CNR, Catania 95121, Italy
6. Institute for Photonics and Nanotechnologies IFN-CNR, Rome 00133, Italy
7. Department of Physics, University of Pisa, Pisa 56127, Italy
8. Institute of Quantum Electronics, Eidgenössische Technische Hochschule Zürich, Zürich 8093, Switzerland