1. Institute for Advanced StudyShenzhen University Shenzhen 518060 P. R. China
2. Institute of Microscale Optoelectronics and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong ProvinceCollege of Optoelectronic EngineeringShenzhen University Shenzhen 518060 P. R. China
3. Department of Electrical Engineering and Computer ScienceThe University of Michigan Ann Arbor MI 48109 USA