Spintronic sources of ultrashort terahertz electromagnetic pulses

Abstract

Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.

All-optical non-linear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields

“…From the technological viewpoint, we believe that the reported phenomenon can be exploited to generate THz electric currents via the inverse spin Hall effect, that would emit electromagnetic THz radiation [34]. In addition, our work also suggests an alternative configuration for magnetic resonance, where the magnetization lies in the plane of the circularly polarized radio frequency excitation, on the THz range (0.3 to 30 THz).…”

“…In addition, such response, being linear with the B-field, prevents the possibility to achieve magnetization switching, as final and initial magnetization states are the same. Although several advances have been made towards the generation of electromagnetic signal in the range of THz (0.1 to 30 THz), the associated intensity is still small as compared to the infrared counterpart [33,34].…”

Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics with example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn

“…Laser-induced ultrafast demagnetization [1,2] and the ensuing THz spin transport and electromagnetic (EM) radiation-from a single magnetic layer [3] or in contact [3][4][5][6][7][8][9] with an additional layer [Fig. 1] of a nonmagnetic (NM) material but hosting strong spin-orbit coupling (SOC) effects-are central phenomena in femtomagnetism and THz spintronics.…”

“…For example, magnetization of a single layer of such material can be reversed [13] on subps time scale for digital memory applications, in contrast to standard current-driven switching via spin torque in near-equilibrium magnets which takes much longer ∼ 100 ps time. In addition, driving bilayers by low-cost and low-power femtosecond laser-where ferromagnetic metal (FM), or metallic or insulating antiferromagnet (AF), is attached to a nonmagnetic SO-material-have opened new avenues [4,[7][8][9] for highly efficient tabletop emitters of ultrabroad band 1-30 THz EM radiation. Conversely, other THz solid-state emitters, such as standard ZnTe crystal, rely solely on physics related to electron charge and deliver emission spectra with substantial gaps [7] and with low peak intensity [8] even for ∼ 100 µm * bnikolic@udel.edu thick crystals.…”

“…Despite more than 20 years of intense studies [1], the underlying physics of the laser-induced demagnetization is still under scrutiny [14] with one of the main puzzles being different paths of spin angular momentum transfer and their interplay to produce ultrafast time evolution of magnetization. In addition, it is widely believed that laser-induced magnetization dynamics generates primarily [10] spin current [4,7,8]--which then has to be injected [15] into the nonmagnetic layer to be efficiently converted into transient charge current as the source of THz radiation-via the inverse spin Hall effect [4,7,8] from the bulk SOC of nonmagnetic layer or other mechanisms involving interfacial SOC [16] or interfacial skewscattering [17]. While pumped (term used to signify current generation in the absence of any bias voltage [18][19][20][21]) spin currents from laser-driven magnetic layers, with short attenuation length ∼ 10 nm [22], have been observed experimentally [22,23], microscopic mechanisms behind them and their relation, or even necessity [10], to demagnetization remain heavily debated [10,24].…”