Jiali Zhang

The two-dimensional van der Waals material CuTe₂Cl (CTC) has been theoretically predicted to possess exceptional charge separation, strong visible-light absorption, and robust dynamic stability, making it a promising candidate for high-performance optoelectronic devices. However, its carrier relaxation pathways and the experimental validation of these predicted properties remain underexplored. Here, we investigate the photocarrier dynamics of CTC flakes using ultrafast optical pump-terahertz probe spectroscopy at 1.55 and 3.1 eV. Sub-bandgap excitation (1.55 eV) yields three distinct relaxation components (lifetimes τ₁ ≈ 2-4 ps, τ₂ ≈ 17-34 ps, τ₃ ≈ 418-667 ps), corresponding to ultrafast defect trapping, defect-mediated recombination, and three-carrier Auger recombination, respectively. In contrast, above-bandgap excitation (3.1 eV) suppresses the intermediate channel and accelerates Auger recombination due to enhanced absorption and reduced penetration depth, producing only τ₁ and τ₃. These findings provide critical insight into the carrier relaxation mechanisms of CTC, advancing its potential for next-generation optoelectronic applications.

Spintronic terahertz emitters (STEs) based on ferromagnet/nonmagnet heterostructures present a compelling alternative to conventional THz sources. In this work, we systematically investigate the effects of boron (B) doping and post-deposition annealing on spin transport and THz emission in W/(CoFe)1−xBx/Al trilayers with B concentrations from 0% to 20%. Time-domain THz emission spectroscopy reveals a monotonic increase in THz field amplitude with increasing B content, despite the reduction in saturation magnetization. This enhancement is attributed to more efficient spin current injection, evidenced by increased spin mixing conductance (g↑↓) extracted from time-resolved magneto-optical Kerr effect measurements, and reduced THz reabsorption stemming from lower THz conductivity in B-rich samples. Furthermore, thermal annealing also reveals a modulation effect induced by B doping, where the THz emission varies nonmonotonically with annealing temperature and peaks near 300 °C. Below this critical temperature, B precipitation improves magnetic ordering and boosts THz emission, whereas higher temperatures degrade spin transport due to interlayer diffusion and increase THz absorption, primarily as a result of crystallization-induced conductivity enhancement. Compared to undoped CoFe, B-doped alloy films exhibit both superior emission intensity and enhanced thermal stability, demonstrating great potential for efficient and robust STE applications.

Two-dimensional (2D) magnetic materials, with their unique van der Waals (vdW) layered structure, tunable magnetism, and strong spin-orbit coupling, are promising for spintronic applications like terahertz (THz) emitters. Our study investigates the THz radiation properties of the newly discovered vdW ferromagnet FePd₂Te₂, which features a one-dimensional Fe zigzag chain structure and strong in-plane uniaxial anisotropy. Using ultrafast reflective THz emission spectroscopy (TES), we demonstrate that the paramagnetic FePd₂Te₂ produces weak THz emission at room temperature. However, when capped with a 3 nm Pt layer, the THz emission intensity is significantly enhanced, attributed to spin-to-charge conversion (SCC) via the inverse spin Hall effect (ISHE). In the FePd₂Te₂/Pt heterostructure, the THz emission intensity is closely linked to the spin texture induced by crystal twinning in FePd₂Te₂ and depends on the polarization angle of the pump laser. Two mechanisms for the photoexcited spin current generation are identified: one from spin-polarized hot electrons at the FePd₂Te₂/Pt interface, and the other, which is dominant, from angular momentum transfer through a nonlinear optical effect that induces impulsive magnetization. These findings provide valuable insights into the spin dynamics of 2D materials and open avenues for high-density, low-power THz spintronic device development based on advanced 2D vdW magnets.

Laser-induced magnetization dynamics and terahertz (THz) emission in CoFe/TixW1−x bilayers with varying Ti concentrations are systematically investigated using the time-resolved magneto-optical Kerr effect and time-domain THz emission spectroscopy. The incorporation of Ti into heavy metal W leads to a significant reduction in spin pumping damping, particularly for Ti concentrations below 50%. Similarly, the THz emission peak amplitude decreases with increasing Ti concentration. Both effects are attributed primarily to the reduced spin current transmittance at the CoFe/TixW1−x interface, caused by the substantially decreased electrical conductivity of the TixW1−x layer. Interestingly, while spin pumping damping continues to decrease, the THz emission amplitude starts to increase at x = 63%, where the THz signal approaches zero due to the opposite spin Hall angles of W and Ti. This behavior underscores the distinct yet correlated mechanisms governing spin pumping damping and THz emission, reflecting their specific dependences on spin current reflection, propagation, dissipation, and spin-to-charge conversion in ferromagnet/nonmagnetic metal bilayer systems. These findings enhance our understanding of ultrafast spin dynamics and spin transport properties, offering valuable insights for advancing the development of miniaturized and high-speed spintronic devices.

The 2D magnet Fe3GaTe2 has received considerable attention for its high Curie temperature (TC), robust intrinsic ferromagnetism, and significant perpendicular magnetic anisotropy (PMA). In this study, the dynamic magnetic properties of Fe3GaTe2 are systematically investigated using an all-optical pump–probe technique. We find that the spin precession frequency (f) is as high as 351.2 GHz at T = 10 K under a field of H = 70 kOe. However, it decreases to 242.8 GHz at 300 K, mainly due to the reduced effective PMA field (Hkeff). The Gilbert damping factor (α) is modest, which increases from 0.039 (10 K) to 0.075 (300 K) owing to the enhanced scattering rate. Interestingly, when Fe3GaTe2 is coupled with 2 nm of Co, the Hkeff, f, and α just decrease slightly, highlighting the dominant influence of Fe3GaTe2. These findings substantially deepen our understanding of Fe3GaTe2, promoting the development of spintronic devices based on advanced 2D magnetic materials.