1. Department of Electrical Engineering and Computer Science The University of Michigan Ann Arbor Michigan 48109 USA
2. National Laboratory of Solid State Microstructures College of Engineering and Applied Sciences and Collaboration Innovation Center of Advanced Microstructures Nanjing University 22 Hankou Road Nanjing 210093 China
3. National Laboratory for Infrared Physics Shanghai Institute of Technical Physics Chinese Academy of Sciences 500 Yutian Road Shanghai 200083 P. R. China
4. Department of Energy Systems Research Ajou University Suwon 16499 South Korea
5. Department of Electrical and Computer Engineering Ajou University Suwon 18499 South Korea