1. School of Electronic and Computer Engineering, Peking University, Shenzhen 518055, China
2. Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
3. École Polytechnique Fédérale de Lausanne (EPFL), Photonic Systems Laboratory (PHOSL), STI-IEM, Station 11, Lausanne CH-1015, Switzerland