1. College of Electrical and Information Engineering
2. Hunan University
3. China
4. Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education
5. School of Physics and Electronics
6. Changsha 410082
7. P. R. China
8. Pen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University
9. Xiamen