The terahertz (THz) frequency range is uniquely suited to probing the fundamental collective excitations of condensed matter systems, from the cyclotron motion of charge carriers to spin waves and superconducting condensates. The FIR/THz molecular gas laser FIRL-100 generates sharp linearly polarized laser lines ranging from 0.25 to 7.5 THz which can be converted into circular polarization using a broadband tunable THz retarder [1], enabling rapid switching between right-handed (RHCP) and left-handed (LHCP) circular polarization. In my talk, I will discuss three experiments performed in the Laboratory of Far-InfraRed Magnetospectroscopy lab (FIRM, part of MGML) in Faraday geometry in magnetic fields up to 10 Tesla.

[1]R. Tesař, M. Šindler, J. Koláček, L. Skrbek Terahertz wire-grid circular polarizer tuned by lock-in detection method Review of Scientific Instruments 89, 083114 (2018).

https://doi.org/10.1063/1.5025427

[2] J. Dzian, et al., Antiferromagnetic resonance in α-MnTe, Physical Review B 112, 024433 (2025).

https://doi.org/10.1103/bb6r-2nwz

[3] R. Tesař, at al., Mass of Abrikosov vortex in high-temperature superconductor YBa2Cu3O7-δ Scientific Reports 11,21708 (2021).

https://doi.org/10.1038/s41598-021-00846-x

[4] N. B.Kopnin, and  V. M. Vinokur, Dynamic vortex mass in clean Fermi superfluids and superconductors, Physical Review Letters 81, 3952 (1998).

https://doi.org/10.1103/PhysRevLett.81.3952