As a newly emergent type-II Dirac semimetal, platinum telluride (${\mathrm{PtTe}}_2$) stands out from other two dimensional noble-transition-metal dichalcogenides for the unique band structure and novel physical properties, and has been studied extensively. However, the ultrafast response of low energy quasiparticle excitation in terahertz frequency remains nearly unexplored yet. Herein, we employ optical pump-terahertz probe (OPTP) spectroscopy to systematically study the photocarrier dynamics of ${\mathrm{PtTe}}_2$ thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the terahertz photoconductivity (PC) of ${\mathrm{PtTe}}_2$ films shows abrupt increase initially, while the terahertz PC changes into negative value in a subpicosecond timescale, followed by a prolonged recovery process that lasted a few nanoseconds. The magnitude of both positive and negative terahertz PC response shows strongly pump fluence dependence. We assign the unusual negative terahertz PC to the formation of small polaron due to the strong electron-phonon ($e\text{−}\text{ph}$) coupling, which is further substantiated by temperature and film thickness dependent measurements. Moreover, our investigations give a subpicosecond timescale of simultaneous carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is of crucial importance for designing ${\mathrm{PtTe}}_2$-based optoelectronic devices.

By employing optical pump Terahertz (THz) probe spectroscopy, a three dimensional (3D) Dirac semimetal, Cd₃As₂ film, was investigated systematically at room temperature. After photoexcitation at 400/800 nm, the rise time of photoenhanced THz photoconductivity (PC) is about ∼1.0 ps, increasing slightly with the pump fluence, in which time scale, photoexcited electrons and holes establish separate Fermi distribution with electrons in the conduction band and holes in the valence band via fast carrier-carrier scattering and carrier-phonon coupling. The subsequent THz PC relaxation shows single exponential decay with a time constant of ∼6.0 ps that is independent of pump fluence. The relaxation process is dominated by the electron-hole recombination via a radiative and nonradiative way, which is mediated by the phonon-phonon scattering. The optically induced THz complex PC can be well fitted with the Drude-Smith model. Our experimental results shed light on understanding the photocarrier dynamics of the 3D Dirac semimetal materials at THz frequency.

Rare earth orthoferrites (RFeO₃) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO₃ single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61 and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (⁶H₃) of Tm³⁺ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields.

We observe terahertz (THz) emission in Mn₃Sn, Mn₃Sn/Pt, and Mn₃Sn/Co films excited by a femtosecond laser pulse. In the Mn₃Sn film and Mn₃Sn/Pt heterostructures, the THz emission originates from both magnetic-dipole and superdiffusive transient spin current with different proportions. Our results unambiguously demonstrate that THz emission can be controlled by the spin structure of Mn₃Sn. The (0001)-orientated Mn₃Sn produces stronger THz emission than the ($11\overline{2}0$ )-orientated counterpart because for the latter one, only half of the kagome planes of Mn₃Sn are parallel to the field, which can be controlled by the external magnetic field. In the Mn₃Sn/Co heterostructure, the Mn₃Sn layer serves as a spin-to-charge converter. The ($11\overline{2}0$ )-orientated Mn₃Sn emits larger THz signals than (0001)-orientated Mn₃Sn due to the anisotropic inverse spin Hall effect, determined by the relative relation between spin, charge current, and the kagome plane of Mn₃Sn. The spin structure dependent THz radiations in noncollinear antiferromagnetic metal Mn₃Sn provide versatility for both spintronics and THz optics.