Abstract-Broadband terahertz generation via the interface inverse Rashba-Edelstein effect

C. Zhou, Y. P. Liu, Z. Wang, S. J. Ma, M. W. Jia, R. Q. Wu, L. Zhou, W. Zhang, M. K. Liu, Y. Z. Wu, and J. Qi, 

https://journals.aps.org/prl/accepted/7f07dY95Df118160072f78c38ad2d3d9134dfaf91

Novel mechanisms for electromagnetic wave emission in the terahertz (THz) frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the Ag/Bi Rashba interface, we demonstrate a controllable THz radiation (\textasciitilde 0.1-5 THz) waveform emitted from metallic Fe/Ag/Bi heterostructures following photo-excitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse Spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects. Terahertz (THz) radiation from 0.1--30 THz accesses a diverse group of low-energy elementary excitations in solid-state systems [1], holding great promises for imaging, sensing and security applications [2]. One major challenge in the next generation THz technology is to search novel mechanism(s) providing efficient and broadband THz radiation with a gapless spectrum [3-5]. To date, most broadband THz emission devices [2-5] are based on the femtosecond laser excitations, taking advantage exclusively of the nonlinear or dynamic properties of the electrons. Recently, the emerging ultrafast spintronics [6-13], however, offers an alternative route to the THz emission with the spin-degree of freedom, by converting spin current bursts into THz pulses. In this way, one can effectively generate, control, and detect the spin currents, as well as utilize such spin-to-charge conversion [6-13] within the sub-picosecond timescale to yield efficient ultra-broadband THz emission. Such ultrafast spin-to-charge conversion process in all previous works is mostly based on the inverse Spin Hall effect (ISHE) (14-15), which happens inside the bulk of a metallic system with a strong spin-orbit coupling (SOC). In contrast, the inverse Rashba-Edelstein effect (IREE) occurring at the interfaces with broken inversion symmetry can also provide efficient spin-to-charge conversion [16-19]. In the IREE, the generated charge current in two-dimensional electron gas can be described by [19]$$j_c\propto {\lambda }_{IREE}{j}_s\times \hat{z}$$where ${\lambda }_{IREE}$ is the IREE coefficient which is proportional to the Rashba parameter ${\alpha }_R$, $\hat{z}$ is the direction of the potential gradient (interfacial electric field) perpendicular to the interface, and $j_s$ is the spin current. Although the IREE has been intensively studied under equilibrium or quasi-equilibrium conditions in magnetoresistance measurements [17], non-local spin valves [18], and ferromagnetic resonance experiments [19], it is still elusive whether the IREE can work in femtosecond timescale, and play a vital role in the THz emission. In this work, we report the observation of THz radiation via the interface IREE in the metallic Fe/Ag/Bi heterostructures, which strongly suggests the effect of the interface IREE on the spin-to-charge conversion in femtosecond timescale. This observation brings us to a novel mechanism of emitting ...