1. School of Physics & Astronomy; University of Birmingham; Birmingham B15 2TT UK
2. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering; Tianjin University; Tianjin 300072 China
3. SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology; Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province; College of Optoelectronic Engineering; Shenzhen University; Shenzhen 518060 China