1. The MOE key laboratory of weak-Light Nonlinear Photonics, School of Physics, Nankai University, Tianjin 300071, China
2. Institute of Photo-electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China