Carrier Dynamics in Low-Dimensional and Wide-Bandgap Materials Probed by Terahertz Magneto-Optical Spectroscopy

Terahertz (THz) radiation lies between the microwave and infrared (0.1-10 THz, photon energies ~0.4-40 meV), an energy scale matching the low-energy excitations and picosecond (ps) relaxation of free carriers; this makes THz magneto-optical (MO) spectroscopy a powerful, contactless probe of carrier dynamics in electronic materials. In this review, we summarize the progress in using THz time-domain spectroscopy (TDS) in the Faraday geometry to study the MO properties of low-dimensional and wide-bandgap materials. We first introduce the experimental principles and the theoretical framework, centered on the generalized MO Drude-Smith model that accounts for photon-induced electronic backscattering, and outline how the longitudinal conductivity and the left- and right-handed circularly polarized MO conductivities or conductances are extracted. We also describe how key electronic parameters, the carrier density, relaxation time, effective mass, static dielectric constant, and localization factor, are determined, with the effective mass obtained under a non-cyclotron resonance condition. We then review and compare studies on representative systems, including monolayer and bilayer MoS₂, nitrogen-doped carbon nanodots (N-CNDs), nitrogen-doped diamond, and neutron-irradiated hexagonal boron nitride (hBN). Across these systems, common magnetic-field trends emerge: the carrier density and localization factor decrease, while the relaxation time, effective mass, and dielectric constant increase. For nitrogen-doped diamond, the magnetic-field dependences of the Faraday rotation angle and ellipticity reveal a pronounced THz MO response, highlighting its potential as a THz polarization-rotation material. Finally, we discuss the current challenges and future prospects, including broader spectral coverage, higher magnetic fields, spatially and time-resolved measurements, temperature and carrier-density control, material-specific modeling, and THz MO devices. This review highlights THz MO spectroscopy as a versatile tool for probing carrier dynamics in low-dimensional and wide-bandgap materials.