1. Institute of Integrated Photonics, RWTH Aachen University, Campus Blvd 73, 52074 Aachen, Germany
2. College of Information Science & Electronic Engineering, Zhejiang University, Hangzhou, China
3. Now at NINT Department, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany
4. College of Optics and Photonics, University of Central Florida, 4304 Scorpius Street, Orlando, Florida 32816-2700, United States
5. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States