1. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
2. Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore.
3. School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore.
4. Center for Nanoscale Systems, Harvard University, Cambridge, MA 02138, USA.
5. Institute of Modern Optics, Key Laboratory of Optical Information Science and Technology, Ministry of Education of China, Nankai University, Tianjin 300071, China.