1. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education and the Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China
2. Institute of Information and Communication Technologies, Electronics and Applied Mathematics (ICTEAM), Université Catholique de Louvain, 1348 Ottignies-Louvain-la-Neuve, Belgium