1. National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base), and Institute of Photonics and Photon-Technology, Northwest University, Xi'an 710069, China
2. Department of Physics, Northwest University, Xi'an 710069, China
3. Department of Micro- and Nanosciences, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland