1. Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of the Ministry of Education, Department of Physics, Hunan Normal University, Changsha 410081, Hunan, People's Republic of China
2. School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
3. State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, People's Republic of China