1. College of Physics, Sichuan University, Chengdu 610064, China
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
3. Analytical & Testing Center, Sichuan University, Chengdu 610064, China
4. Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States