1. a Institute of Applied Physics, University of Electronic Science and Technology of China 610054, Chengdu, China
2. b Institute of Applied Physics, University of Electronic Science and Technology of China 610054, Chengdu, China
3. c Institute of Applied Physics, University of Electronic Science and Technology of China 610054, Chengdu, China, School of Physics and Electronic Information, China-West Normal University, 637002, Nanchong, China
4. d Institute for Infocomm Research, 20 Science Park Road, #02-21/25, TeleTech Park, Singapore 117674