1. Department of Electrical and Computer Engineering, National University of Singapore, 117583 Singapore, Singapore.
2. Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 117602 Singapore, Singapore.
3. Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712, USA.
4. Graduate School for Integrative Sciences and Engineering, National University of Singapore, 117456 Singapore, Singapore.
5. SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, Shenzhen University, Guangdong 518060, China.
6. School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK.