1. School of Physical and Astronomy, Yunnan University, Kunming 650091, People's Republic of China
2. Kunming Institute of Physics, Kunming 650223, People's Republic of China
3. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
4. Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Science, Suzhou 215123, People's Republic of China