[1] Zhongshan Sun Yat Sen Univ, Dept Phys, Guangzhou 510275, Guangdong, Peoples R China
[2] Zhongshan Sun Yat Sen Univ, State Key Lab Optoelect Mat & Technol, Guangzhou 510275, Guangdong, Peoples R China
[3] Fudan Univ, Dept Phys, Shanghai 200433, Peoples R China
[4] Nanjing Univ, Natl Lab Solid State Microstruct, Nanjing 210093, Peoples R China