1. State Key Laboratory of Precision Spectroscopy and Department of Physics, East China Normal University, Shanghai 200062, China
2. Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA
3. Institute of Physical and Engineering Sciences, Faculty of Science, Kunming University of Science and Technology, Kunming 650500, China
4. Department of Physics, University of Colorado Boulder, Boulder, CO 80309, USA