1. Department of Electrical Engineering and Center for Nanophotonics, Arizona Institute of NanoElectronics, Arizona State University, Tempe, Arizona 85287, Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education and Micro-Nanotechnology Research Center, Hunan University, Changsha, 410082, China and School of MSE, Beijing Institute of Technology, Beijing 100081, China