1. School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Department of Physics, The University of Texas at Arlington, Arlington, Texas 76019, and Center for Terahertz waves, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, People’s Republic of China