1. Ioffe Institute, 26 Politekhnicheskaya, 194021 St. Petersburg, Russia
2. Department of Nanophotonics and Metamaterials, ITMO University, 49 Kronverksky Pr., 197101 St. Petersburg, Russia
3. Laboratory for Solid State Physics, ETH Zurich, Otto-Stern-Weg 1, CH-8093 Zurich, Switzerland
4. INFM and L-NESS Dipartimento di Fisica, Politecnico di Milano, Polo Regionale di Como, Via Anzani 52, I-22100 Como, Italy