1. International Research and Education Centre for Physics of Nanostructures, ITMO University, St. Petersburg 197101, Russia
2. Alferov Saint Petersburg National Research Academic University of the Russian Academy of Sciences, 8/3A Khlopina Street, St. Petersburg 194021, Russia
3. Institute for Analytical Instrumentation of the Russian Academy of Sciences, 31-33A Ivana Chernykh Street, St. Petersburg 198095, Russia
4. Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QD, U.K.
5. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K.