1. Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA
2. Center for Nano-Optics (CeNO), Georgia State University, Atlanta, Georgia 30303, USA
3. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
4. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore