1. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2. Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic, Chaberska 57, 18251 Prague, Czech Republic
3. Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, 5 Engineering Drive 1, Singapore 117608
4. Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
5. Department of Electronic & Electrical Engineering, University College London, London WC1E 7JE, United Kingdom
6. Optical and Semiconductor Devices Group, Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2BT, United Kingdom