1. MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, and Shaanxi Key Laboratory of Optical Information Technology, School of Science; Northwestern Polytechnical University; Xi'an 710072 China
2. Department of Electronics and Nanoengineering; Aalto University; Espoo FI-00076 Finland
3. QTF Centre of Excellence, Department of Applied Physics; Aalto University; Espoo FI-00076 Finland