1. Center for Biomedical Optics and Photonics (CBOP) & College of Physics and Optoelectronic Engineering Key Laboratory of Optoelectronic Devices and Systems Shenzhen University Shenzhen 518060 P. R. China
2. State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China