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.

The spin-to-charge conversion of the $\mathrm{Ag}/\mathrm{Bi}$ interface is studied in a device in which a spin current can be injected from either side selectively. The charge voltages generated by the two counterpropagating spin currents show opposite signs, that is consistent with the inverse spin Hall effect rather than the well-accepted inverse Rashba-Eldestein effect in the $\mathrm{Ag}/\mathrm{Bi}$ bilayer. Femtosecond laser is further employed to generate the spin-current-induced terahertz signal in a $\mathrm{Ag}/\mathrm{Bi}$ bilayer, which shows no evidence for the inverse Rashba-Eldestein effect, either. This work provides a clear-cut method to identify the spin-to-charge mechanism in a Rashba electronic state and delivers new understanding for the relevant spin-transport phenomena.

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 non-contact and non-invasive 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/Zr/Al₂O₃ heterostructures. Our experimental results indicate a photoinduced orbital current (J_L) from Co propagating through Zr layer with a long-distance ballistic transport and a velocity of ~\(0.27\)± \(0.02 \, {{\rm{nm}}} \, {{{\rm{fs}}}}^{-1}\). On the one hand, we demonstrate a critical pump fluence required to overcome the collisions in orbital transport, enabling a swifter flow of J_L. On the other hand, a critical temperature is observed, below which the orbital transport is impeded. Finally, we observe a nearly 2.95-fold enhancement in the THz emission due to an additional conversion of the spin-to-orbital current conversion from a 1 nm thick W-insertion layer between Co and Zr layers. Our results pave the way for designing promising opto-spin-orbitronic devices and THz emitters.

We present the experimental study of the spin Hall angle (SHA) and spin diffusion length of Pd with the spin pumping and microwave photoresistance effects. The Py/Pd bilayer stripes are excited with an out-of-plane microwave magnetic field. The pure spin current is thus pumped and transforms into charge current via the inverse spin Hall effect (ISHE) in Pd layer, yielding an ISHE voltage. The ISHE voltage can be distinguished from the unwanted signal caused by the anisotropic magnetoresistance according to their different symmetries. Together with Pd thickness dependent measurements of in and out-of-plane precessing angles and effective spin mixing conductance, the SHA and spin-diffusion length of Pd are quantified as 0.0056 $\pm \hspace{0.17em}0.0007$ and 7.3 $\pm \hspace{0.17em}0.7$ nm, respectively.

Despite intensive research, the mechanism determining the terahertz (THz) emission of the ferromagnetic (FM) metallic monolayers remains elusive. Here, we report on the results of a systematic investigation on the THz emission generated by pumping Ni80Fe20 monolayers on Al2O3 substrates with a femtosecond laser. We found solid evidence that the THz emission is dominated by the anomalous Nernst effect (ANE), in which a transient spin-polarized charge current can be induced by an ultrafast electron temperature gradient on the picosecond timescale, outputting THz emission. We found a polarity reversal of the THz waveform after the introduction of a SiO2 buffer layer to the sample and found that, based on ultrafast temperature simulation, it was a consequence of direction reversal of temperature gradient. Comparing the THz emission of different FM monolayers further confirms that the THz polarity also strongly depends on the sign of the ANE coefficient. These phenomena unambiguously indicate that the ANE plays a decisive role in the process of THz emission. The present work shows the importance of ultrafast spin caloritronics for a spintronic THz emitter. The principle demonstrated here can be applied to other FM metallic materials.