1. Electrical and Computer Engineering Department, University of California Los Angeles, Los Angeles, California 90095, USA
2. Department of Physics, University of California Los Angeles, Los Angeles, California 90095, USA
3. Department of Engineering Physics, Tsinghua University, Beijing 100084, China
4. Department of Physics, National Central University, Jhongli 32001, Taiwan
5. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
6. Department of Physics, National Taiwan University, Taipei 10617, Taiwan