Repetition frequency locking of a terahertz quantum cascade laser emitting at 4.2 THz Wen Guan, Ziping Li, Kang Zhou, Wenjian Wan, Xiaoyu Liao, Yiran Zhao, Sijia Yang, J. C. Cao and Hua Li TST, 13 1 (2020) 32-40 Published online: 31 March 2020 DOI: 10.1051/tst/2020131032