1. Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, School of Physics and Materials Science, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, P. R. China
2. Institute of Photovoltaics, College of Physics Science
and Technology, Hebei University, Baoding 071002, P. R. China
3. School of Electronic Engineering, Bangor University, Bangor LL57 1UT, U.K.