1. Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, United States
2. Department of Electrical and Computer Engineering and Peterson Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
3. Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, School of Physics, Nankai University, Tianjin 300071, China
4. Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States