1. Key
Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education,
School of Physics and Electronics, Hunan University, Changsha 410082, China
2. Department
of Electrical and Computer Engineering, University of California, San Diego, 9500 Gilman Drive., La Jolla, California 92093, United States
3. Department
of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore