1. Institute of Photonics and Photon Technology, Northwest University, Xi'an 710069, China
2. Department of Electronics and Nanoengineering, Aalto University, Espoo 02150, Finland
3. Department of Applied Physics, Aalto University, Espoo 02150, Finland
4. QTF Centre of Excellence, Department of Applied Physics, Aalto University, Espoo 00076, Finland