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Abstract An important vision of modern magnetic research is to use antiferromagnets (AFMs) as controllable and active ultrafast components in spintronic devices. Hematite (α‐Fe2O3) is a promising model material in this respect because its pronounced Dzyaloshinskii‐Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, femtosecond laser pulses are used to drive terahertz (THz) spin currents from α‐Fe2O3 into an adjacent Pt layer. Two contributions to the generation of the spin current with distinctly different dynamics are found: the impulsive stimulated Raman scatting that relies on the AFM order and the ultrafast spin Seebeck effect that relies on the net magnetization. The total THz spin current dynamics can be manipulated by a medium‐strength magnetic field below 1 T. The control of the THz spin current achieved in α‐Fe2O3 opens the pathway toward tailoring the exact spin current dynamics from ultrafast AFM spin sources.

Abstract Transition‐metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin‐based electronics. Here, terahertz emission spectroscopy is used to study spin‐to‐charge current conversion (S2C) in the TMDC NbSe2 in ultra‐high‐vacuum‐grown F|NbSe2 thin‐film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in‐plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe2 thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in‐plane‐polarized spin current into NbSe2. Modeling of the spin‐current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe2 with the opposite, negative sign of the spin Hall angle as compared to Pt. By a quantitative comparison of the emitted THz radiation from F|NbSe2 to F|Pt reference samples and the results of ab initio calculations, it is estimated that the spin Hall angle of NbSe2 for an in‐plane polarized spin current lies between ‐0.2% and ‐1.1%, while the THz spin‐current relaxation length is of the order of a few nanometers.

External stimuli can induce significant changes in the magnetisation of a material; however, these changes can occur very rapidly, making measurements difficult. Herein the authors demonstrate a method of ultrafast magnetometry, enabling the detection of the rapid magnetization changes.

Probing spin pumping in the terahertz regime allows one to reveal its initial elementary steps. Here, the authors show that the formation of the spin Seebeck current in YIG/Pt critically relies on hot thermalized metal electrons because they impinge on the metal-insulator interface with maximum noise.

Low-frequency structural dynamics in liquids and their underlying interactions are complex and challenging to resolve. Here, Sajadiet al. use intense terahertz fields to directly interrogate intermolecular modes in polar liquids by coupling the fields to their permanent molecular dipole moments.

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e., spin-dependent electron scattering), whereas intrinsic (i.e., scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni_{81}Fe_{19}, and Ni_{50}Fe_{50} over the frequency range from dc to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni_{81}Fe_{19}, and Ni_{50}Fe_{50} samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the dc AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magnetotransport phenomena of standard magnetic thin films on ultrafast timescales.

Surface currents in topological insulators can be controlled by light, but the underlying mechanisms are not well understood. Here, Braun et al. report an ultrafast shift photocurrent at the surface of Ca-doped Bi2Se3, whereas injection currents are much smaller than expected from asymmetric depopulation of the Dirac cone.

We use slow-light photonic crystals to enhance optical switching and modulation in silicon. By using dispersion-engineered designs, a switch as short as 5 ¿m was achieved, in which we have demonstrated rerouting of optical pulses on a 3-ps time scale through the absorption of a femtosecond pulse. We additionally demonstrate a modulator with a Mach-Zehnder interferometer (MZI) configuration with flat-band slow-light photonic crystal phase shifters that is designed to give a large group-index-bandwidth product. An extinction ratio in excess of 15 dB is obtained over the entire 11-nm bandwidth of the modulator.

Nature Communications 8: Article number: 14963 (2017); Published: 10 April 2017; Updated: 25 May 2017 This Article contains typographical errors in six instances in which the convolution symbol was mistakenly omitted from six equations during the production stage. Each instance is described below. The penultimate sentence of the first paragraph of the Results subsection titled ‘Model’ should read:

Table-top sources of intense multi-terahertz (THz) pulses have opened the door to studies of extreme nonlinearities in the previously elusive mid- to far-infrared spectral regime. We discuss two concepts of fully coherent coupling of phase-locked THz pulses with condensed matter. The first approach demonstrates two-dimensional multi-THz spectroscopy of the semiconductor material InSb. By phase- and amplitude-sensitive detection of the nonlinear optical response, we are able to separate incoherent pump–probe signals from coherent four-wave mixing and reveal extremely non-perturbative nonlinearities. While this class of interactions is mediated by the electric field component of the THz pulse, the second approach is complementary, as it demonstrates that, alternatively, the magnetic THz field may be exploited to selectively control the spin degree of freedom in antiferromagnetic NiO.