1. a National Laboratory for Infrared Physics , Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yu Tian Road , Shanghai, 200083, People's Republic of China
2. b Department of Physics , Sogang University , Seoul, 121-742, Republic of Korea
3. c Key Laboratory of Polar Materials and Devices , Ministry of Education, East China Normal University , 200062 Shanghai, People's Republic of China