1. Department of Electrical and Computer Engineering, Clarkson University, Potsdam, New York 13699, USA
2. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Center for Optoelectronic Materials and Devices and Key Laboratory of Optical Field Manipulation of Zhejiang Province, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
4. State Key Laboratory of Information Photonics and Optical Communications and Laboratory of Information Functional Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China