[1] Univ Elect Sci & Technol China, Sch Phys Elect, Chengdu 610054, Peoples R China
[2] Hunan Univ, Dept Appl Phys, Changsha 410082, Hunan, Peoples R China
[3] Northumbria Univ, Fac Engn & Environm, Dept Phys & Elect Engn, Newcastle Upon Tyne NE1 8ST, Tyne & Wear, England