1. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
2. State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China
3. Faculty of Applied Science (IMTEK), University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany
4. Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland
5. Research Center for Integrated Quantum Electronics, Hokkaido University, North 13, West 8, Sapporo 060-8628, Japan
6. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China