1. State Key Laboratory of Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China
3. Hunan Key Laboratory of Nanophotonics and Devices School of Physics and Electronics Central South University Changsha 410083 China
4. EE Department School of Engineering Tokyo Institute of Technology Yokohama 226–8502 Japan