1. Center for Terahertz waves and College of Precision Instrument and Optoelectronics Engineering; Tianjin University
2. Key Laboratory of Optoelectronics Information and Technology (Ministry of Education); Tianjin 300072 China
3. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater Oklahoma 74078 USA
4. Center for Disruptive Photonic Technologies; Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371