While the technology of microwave and infrared sources is quite mature and has been widely used in our daily life for decades, sources that can work well across the terahertz (THz) range are still lagging behind, which is often referred to as the “THz gap.” As one of the most pioneering THz setups, terahertz time-domain spectroscopy has been a vital tool to explore the properties of materials as well as their underlying physics. The mechanism is to use an ultrafast infrared pump pulse for exciting rapidly decaying currents inside either a nonlinear or a photoconducting medium, known as a THz emitter, which produces free-space coherent THz radiation. Most recently, a novel THz emitter emerges and rises, which is based on the spin-related effects in magnetic/nonmagnetic nanofilms and can cover the full range of the THz band, named as spintronic THz emitter (STE). This perspective aims to elucidate the unique features and advantages of STE as well as its capability and potential to develop novel applications. We summarize the multidisciplinary efforts that have been made to improve the performance and function of STE, including but not limited to spintronics, optics, and electromagnetics. Distinct THz setups based on STE are reviewed, which may inspire various “real world” applications in the near future.

Polarization control via metamaterials boosts the design of polarimetric devices in the realm of terahertz technology for sensitive detection, bio-imaging, and wireless communication. Here, we propose all-dielectric metamaterials composed of silicon brick arrays that operate as terahertz quarter- and half-waveplates with close to unity transmission. Spherical multipole decomposition calculation indicates that the silicon brick can support multiple Mie-type resonances, such as electric dipole, electric quadrupole, magnetic dipole, and magnetic quadrupole modes. By tailoring the multipolar interference among these resonances, near unity transmission can be obtained with over π phase delay. We experimentally realize dielectric terahertz metamaterials that function as a quarter-wave plate at 0.79 THz and a half-wave plate at 0.91 THz with insertion losses of 0.54 and 1.25 dB, respectively. Such anisotropic dielectric metamaterials promise an exotic approach to engineer the interference among multipolar resonances and reveal the feasibility to realize functional, efficient, and compact terahertz meta-devices.

We demonstrate that a metal–dielectric heterodimer structure can satisfy a nearly ideal first Kerker condition at a wavelength close to the resonance peak of the dimer, yielding efficient unidirectional forward scattering with a high forward-to-backward scattering ratio (≈48 dB) and remarkable enhancement of the forward scattering intensity (∼2.68 times compared to a single dielectric nanoparticle). Using a rigorous analytical dipole–dipole interaction model, the underlying mechanism is revealed, in which the originally weak electric dipole moment of the dimer is significantly enhanced owing to the strong resonant interference between the localized surface plasmon resonance of the metal and the Mie resonances of the dielectric material, which could up-match the magnetic dipole moment of the dimer at a wavelength close to the resonance peak, boosting the forward scattering efficiency. To achieve the optimal conditions, the sizes of the metal and dielectric constituents as well as the gap distance of the dimer have to be physically and delicately tuned to ensure a perfect match in both the amplitudes and phases of the electric and magnetic dipole moments of the dimer. On top of that, the loss of the heterodimer can be effectively suppressed to a level well below that of a pure metal nanoparticle, which further benefits the forward scattering efficiency. The flexibility in designing the dimer geometry and choosing metal–dielectric material combinations enables efficient unidirectional forward scattering in a broadband spectrum (UV to visible) with an intermediate gap distance (10–20 nm), greatly expanding the application scope. The proposed hybrid dimer could serve as a powerful and versatile building block in many emergent fields such as metasurfaces, nanoantennae, etc.

Reduction of the reverse leakage current is critical to AlGaN/GaN heterostructures in high power and high frequency applications. Taking AlGaN/GaN Schottky barrier diodes (SBDs) as an example, we demonstrate both theoretically and experimentally that low-fluence neutron irradiation can be a promising way to reduce the reverse leakage current while maintaining other electronic properties almost unchanged. A clear physical picture is given to elucidate the mechanism, which includes three main scenarios: (i) in pre-irradiated AlGaN/GaN heterostructures grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates, the configuration of threading dislocations (DLs) is the mixture of pure DLs and DLs decorated by group-III vacancies (V_III-DLs); (ii) neutron scattered group-III interstitials are mobile and prone to passivate V_III-DLs, changing the configuration of DLs to monomorphic pure DLs; (iii) after the saturation of the passivation, neutron scattered group-III interstitials begin to escape from the system. The physical analysis is consistent with the trends in the experimental data. Our work provides a new post-processing treatment for reducing the reverse leakage current of AlGaN/GaN heterostructures grown by MOCVD on sapphire substrates.