1. Department of Electrical and Computer Engineering, National University of Singapore, 117576 Singapore
2. Department of Materials Science and Engineering, National University of Singapore, 117575 Singapore
3. Integrative Sciences and Engineering Programme, NUS Graduate School, National University of Singapore, 119077, Singapore
4. Department of Physics, National University of Singapore, 117551 Singapore
5. Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, China
6. School of Mechanical and Engineering, Beijing Institute of Technology, Beijing, 100081, China