1. Department of Electronic Engineering; The Chinese University of Hong Kong; Shatin New Territories 999077 Hong Kong
2. Department of Physics; The Chinese University of Hong Kong; Shatin New Territories 999077 Hong Kong
3. Shenzhen Institute of Advanced Technology; Chinese Academy of Science; Shenzhen 518055 P. R. China
4. Department of Physics; The Hong Kong University of Science and Technology; Clear Water Bay Kowloon 999077 Hong Kong
5. Siyuan Laboratory; Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials; Department of Physics; Jinan University; Guangzhou Guangdong 510632 P. R. China