1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. Centre for Disruptive Photonic Technologies; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
3. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater OK 87078 USA
4. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering; Tianjin University; Tianjin 300072 P. R. China