[1] Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore
[2] Tianjin Univ, Coll Sci, Dept Appl Phys, Tianjin 300072, Peoples R China
[3] Nanyang Technol Univ, Sch Phys & Math Sci, Singapore 639798, Singapore
[4] Bilkent Univ, Dept Elect & Elect Engn, TR-06800 Ankara, Turkey
[5] Bilkent Univ, Dept Phys, TR-06800 Ankara, Turkey
[6] Natl Univ Singapore, Dept Elect & Comp Engn, Singapore 117608, Singapore