Novel mechanisms for electromagnetic wave emission in the terahertz frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the $\mathrm{Ag}/\mathrm{Bi}$ Rashba interface, we demonstrate a controllable THz radiation ($\sim 0.1–5\mathrm{THz}$) waveform emitted from metallic $\mathrm{Fe}/\mathrm{Ag}/\mathrm{Bi}$ heterostructures following photoexcitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects.

Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. Here, we discover strong coherent terahertz emission from Weyl semimetal TaAs, which is demonstrated as a unique broadband source of the chiral terahertz wave. The polarization control of the THz emission is achieved by tuning photoexcitation of ultrafast photocurrents via the photogalvanic effect. In the near-infrared regime, the photon-energy dependent nonthermal current due to the predominant circular photogalvanic effect can be attributed to the radical change of the band velocities when the chiral Weyl fermions are excited during selective optical transitions between the tilted anisotropic Weyl cones and the massive bulk bands. Our findings provide a design concept for creating chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics using Weyl physics.

Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.

We investigate the quasiparticle dynamics in the prototypical heavy fermion ${\mathrm{CeCoIn}}_5$ using ultrafast optical pump-probe spectroscopy. Our results indicate that this material system undergoes hybridization fluctuations before the establishment of heavy electron coherence, as the temperature decreases from $\sim 120\mathrm{K}$ ($T^{†}$) to $\sim 55\mathrm{K}$ ($T^{*}$). We reveal that the anomalous coherent phonon softening and damping reduction below $T^{*}$ are directly associated with the emergence of collective hybridization. We also discover a distinct collective mode with an energy of $\sim 8\mathrm{meV}$, which may be experimental evidence of the predicted unconventional density wave. Our findings provide important information for understanding the hybridization dynamics in heavy fermion systems.

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.