[1] Natl Univ Singapore, Dept Phys, Singapore 117542, Singapore
[2] Oklahoma State Univ, Sch Elect & Comp Engn, Stillwater, OK 74078 USA
[3] Tianjin Univ, Ctr Terahertz Waves, Tianjin, Peoples R China
[4] Tianjin Univ, Coll Precis Instrument & Optoelect Engn, Tianjin, Peoples R China