Hao Meng

Ultrafast terahertz (THz) emission from magnetic heterostructures provides a sensitive probe of spin-orbit-coupled angular-momentum transport. Here, we report stacking-direction-controlled THz emission in Ni/W-based heterostructures. Fe/W and W/Fe reference bilayers show opposite THz polarities, consistent with spin-current-driven inverse spin Hall conversion. In contrast, Ni/W and W/Ni exhibit the same effective polarity but strongly different amplitudes. Additional W-based and Ni-based control bilayers further show that this non-reversing response is material selective rather than a generic consequence of either Ni or W alone. W-thickness-dependent measurements reveal a nearly fixed delay in Ni/W but a systematic delay increase in W/Ni, suggesting different angular-momentum propagation and conversion mechanisms. By introducing Fe or Permalloy as an additional ferromagnetic (FM) layer, we show that the THz emission can be selectively enhanced, suppressed, or reversed depending on the FM/Ni coupling. Rather than simply adding an independent FM emission channel, the added layer amplifies or reshapes the intrinsic transport asymmetry between Ni/W and W/Ni. Our results demonstrate that stacking direction provides an effective route for modifying ultrafast THz emission through interfacial angular-momentum transport.

Spintronic terahertz (THz) emission is a powerful probe of ultrafast spin and orbital angular-momentum transport in magnetic heterostructures. Here, we demonstrate interface-selected THz polarity switching in Ni/Permalloy (Py)/Cr trilayers. Ni/Cr and Py/Cr bilayers emit THz pulses with opposite polarity, indicating two distinct Cr-mediated conversion channels. This contrast is attributed to Py-dominated spin-current-induced inverse spin Hall conversion and Ni-mediated orbital-related inverse orbital Hall conversion, which have opposite effective signs in Cr. By constructing Ni/Py/Cr and Py/Ni/Cr trilayers, we show that the THz response cannot be explained by a simple superposition of Ni- and Py-related ferromagnetic (FM) contributions. Instead, the polarity is selected by the local FM/Cr interface. A clear polarity reversal occurs when an ultrathin Ni or Py layer of only 0.5–1 nm is inserted at the Cr interface, revealing the critical role of early-stage interface formation. Our results establish an interface-engineering route for controlling THz polarity through competing spin and orbital conversion channels in Cr-based spintronic emitters.

The utilization of terahertz (THz) emission spectroscopy in femtosecond photoexcited spintronic heterostructures has emerged as a versatile tool for investigating ultrafast spin transport in a noncontact and noninvasive manner. However, the investigation of ultrafast orbital transport is still in the primitive stage. Here, we experimentally demonstrate the orbital-to-charge current conversion in Co/Ru heterostructures and identify signatures that cannot be consistently explained within a conventional spin-current framework. Time-domain measurements reveal delayed and broadened THz waveforms with increasing Ru thickness, suggesting the presence of long-range angular momentum transport associated with orbital contributions. In Co/Pt/Ru trilayers, the THz emission is further enhanced through cooperative effects between the inverse spin Hall effect (ISHE) in Pt and inverse orbital Hall effect (IOHE) in Ru, where collinear charge currents are generated and add constructively. In contrast, reversed stacking configurations exhibit suppressed emission, highlighting the critical role of stacking sequence and interface-controlled transmission. Ferromagnetic resonance (FMR) measurements reveal a strong correlation between damping and THz amplitude, highlighting efficient angular momentum conversion. These results position Co/Ru as a promising orbitronic platform for tunable ultrafast THz emission. Our results not only strengthen the physical mechanism of condensed matter physics but also pave the way for designing promising spin-orbitronic devices and THz emitters.

We study the transport of the Josephson current in one-dimensional SF1F2F3S junctions, where S represents s-wave superconductors and F1, F2, and F3 denote ferromagnets with noncollinear magnetizations. The Josephson current displays distinct characteristics under various magnetization configurations as we vary the exchange fields (h1, h3) and thicknesses (d1, d3) of the F1 and F3 layers. Building on Eschrig's conversion mechanisms [Eschrig, Phys. Today 64(1), 43 (2011) and Eschrig, Rep. Prog. Phys. 78, 104501 (2015)], we rigorously derive the conversion and transport processes of spin-singlet and spin-triplet Cooper pairs as they traverse the ferromagnetic trilayers, which helps clarify the observed variations in the Josephson current. We demonstrate that opposite-spin Cooper pairs acquire an additional phase of Qjdj upon passing through each Fj layer (j = 1, 3). Here, Qj = 2hj/(ℏvF) represents the center-of-mass momentum of the Cooper pairs, where vF is the Fermi velocity. This result aligns with the description of these pairs provided in Eq. (73) of Eschrig [Rep. Prog. Phys. 78, 104501 (2015)]. The contributions of Qjdj to the Josephson current depend on the exchange field of the F2 layer and the magnetization directions of the F1 and F3 layers. When the F2 layer is a ferromagnet, the azimuthal angle difference (χ3 − χ1) between the F1 and F3 layers modulates the current amplitude. This angular difference has been previously discussed in Grein et al. [Phys. Rev. Lett. 102, 227005 (2009)] and Eschrig [Rep. Prog. Phys. 78, 104501 (2015)], and it has more recently been examined in detail in Schulz et al. [Phys. Rev. B 112, 104514 (2025) and Phys. Rev. B 112, 104515 (2025)]. In contrast, when the F2 layer is a half-metal, this angle difference induces a phase shift in the current without affecting its amplitude, consistent with the results presented in Eschrig and Löfwander [Nat. Phys. 4, 138 (2008)]. These findings open new possibilities for controlling long-range Josephson currents in practical applications.

In recent years, the active control of terahertz waves using artificial microstructures has attracted increasing attention, especially toward the ones that have multiple plasmon-induced transparency (PIT) responses. Here, a homogeneous graphene-metal metasurface, exhibiting tunable dual-PIT in its terahertz (THz) spectral response, is investigated numerically and theoretically. Individual and simultaneous control of the two PIT transmission windows and the two slow-light effects are achieved by reconstructing the Fermi energies of the graphene strips. The modulation behavior can be expounded by the classical coupled three-particle model, which is confirmed by the simulation results. Moreover, the electric field distribution is introduced to analyze the dual-PIT active modulation mechanism. This work provides theoretical guidance for versatile applications in multi-function terahertz switches and slow-light devices.