1. Center for Photonics and Nanoelectronics, Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA
2. Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma 74104, USA
3. State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin 130012, China
4. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, Jilin 130012, China