Spin–charge conversion is a spin–orbit coupling phenomena where electrical currents can generate transverse spin currents and vice versa. It is one of the central topics in spintronics and widely applied to manipulate the spin and charge degrees of freedom in materials. Previous research on spin–charge conversion was mainly carried out by transport measurements, which lies in the (quasi)equilibrium and DC/low-frequency limit. The recent development of THz emission spectroscopy applied to this field provides additional insights into the dynamics of the spin–charge conversion process, i.e., its ultrafast timescales. Here, the underlying physics and the latest progress of THz studies on spintronics are introduced. The technical details and some features of this technique are summarized, including spin current generation, signal detection, and data analysis. Finally, some possible developments are discussed as well as future research and applications.

Strong second-order optical nonlinearities often require broken material centrosymmetry, thereby limiting the type and quality of materials used for nonlinear optical devices. Here, we report a giant and highly tunable terahertz (THz) emission from thin polycrystalline films of the centrosymmetric Dirac semimetal PtSe ₂ . Our PtSe ₂ THz emission is turned on at oblique incidence and locked to the photon momentum of the incident pump beam. Notably, we find an emitted THz efficiency that is giant: It is two orders of magnitude larger than the standard THz-generating nonlinear crystal ZnTe and has values approaching that of the noncentrosymmetric topological material TaAs. Further, PtSe ₂ THz emission displays THz sign and amplitude that is controlled by the incident pump polarization and helicity state even as optical absorption is only weakly polarization dependent and helicity independent. Our work demonstrates how photon drag can activate pronounced optical nonlinearities that are available even in centrosymmetric Dirac materials.

Here, using the ultrafast optical pump-probe spectroscopy, we have studied the quasiparticle dynamics in the type-II Dirac semimetal ${\mathrm{NiTe}}_2$. Anomalous dynamic optical responses are detected around a critical temperature $T^{*}$ ($\sim 60$ K), which is also evidenced by the transport experiment. In specific, our results reveal a phonon-assisted electron-hole recombination process showing an anomaly at $T^{*}\simeq 60$ K. We discover an unusual phonon renormalization behavior, manifested by the $T$-dependent phonon energy and lifetime exhibiting anomalies at $T^{*}$. We unveil that the observed anomalous $T$-dependencies can be attributed to the sudden shift of the phonon mode involved in the recombination, which originates from the abrupt change of electronic structure near the Fermi surface. These findings can provide deeper insight into the nonequilibrium carrier and lattice properties in the topological materials.

We investigate the quasiparticle dynamics in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ single crystals using ultrafast optical spectroscopy. Our results show that there exist anomalous dynamical optical responses below the antiferromagnetic (AFM) ordering temperature $T_N$. In specific, we reveal that both the initial carrier decay and recombination processes can be modulated via introducing the AFM order in subpicosecond and picosecond timescales, respectively. We also discover a long relaxation process emerging below $T_N$ with a timescale approaching the nanosecond regime, and can be attributed to the $T$-dependent spin-lattice interaction. There also emerges an unusual phonon energy renormalization below $T_N$, which is found to arise from its coupling to the spin degree via the exchange interaction and magnetic anisotropy. Our findings provide key information for understanding the dynamical properties of nonequilibrium carrier, spin, and lattice in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.

In this work, broadband terahertz (THz) wave emissions have been detected from the trigonal layered PtBi₂ on the excitation of the femtosecond laser pulses. Such THz generation is found to arise from the dominated linear photogalvanic effect, which is further discovered to strongly depend on the unique electronic structures of PtBi₂. Furthermore, an effective nonlinear susceptibility of PtBi₂ is also obtained and is nearly two orders of magnitude larger than that of the traditional nonlinear crystal for THz generation.

Since tantalum arsenide (TaAs) has been experimentally verified as a Weyl semimetal, intensive research has been devoted to study of the unique properties of the material. Despite this, the ultrafast dynamics of TaAs is still not very well understood. In this work, we study the relaxation dynamics in TaAs using transient reflection spectroscopy. From the transient reflection measurement, we observe either a single (fast) or a dual (fast and slow) relaxation, depending on the probing wavelength. The additional relaxation channel is attributed to an asymmetric population of photoexcited electrons and holes.