Materials exhibiting spin-orbit coupling (SOC) possess spin (S) and orbital (L) angular momentum, leading to spin and orbital currents transverse to an applied electric field. While charge-spin and spin-charge conversions via the spin Hall effect (SHE) and inverse spin Hall effect (ISHE) in heavy metals (e.g., Ta, W) and topological materials have been extensively studied and applied in spintronic devices and THz emitters, the orbital contribution in nonmagnetic materials (NMs) with weak SOC has received less attention. Recent theoretical and experimental work has demonstrated the orbital Hall effect (OHE) in NMs with weak SOC, enabling efficient orbital current generation with an applied electric field. In OHE, charge current (Jc) converts to orbital current (JL), and subsequently to spin current (Js), exerting an orbital torque to switch magnetization in ferromagnetic (FM) layers. However, the inverse process, IOHE (JL ↔ Jc), and efficient orbitronic THz emitters based on IOHE have remained elusive. This study addresses this gap by investigating Ti and Mn, known for their significant OHE, to explore IOHE and the efficiency of orbitronic THz emission stemming from IOHE. The research aims to understand the fundamental physics of IOHE and its potential for developing novel spin-orbitronic devices and high-performance THz emitters.

Extensive research has been conducted on the spin Hall effect (SHE) and inverse spin Hall effect (ISHE) in materials with strong spin-orbit coupling (SOC). These effects, involving the conversion of charge current to spin current and vice-versa, have shown great promise in spintronic applications, particularly in the development of spin-orbit torque devices and terahertz emitters. Heavy metals such as Ta and W, and topological insulators and semimetals, have been the primary focus of these studies. However, the orbital contribution to these phenomena, particularly in materials with weak SOC, has been less explored. Recent theoretical work has predicted and experimental observations have confirmed the existence of the orbital Hall effect (OHE), where charge current is converted into orbital current. The potential of OHE for manipulating magnetization in ferromagnetic layers through orbital torque has been highlighted in several studies, but the inverse process, the inverse orbital Hall effect (IOHE), has not been as extensively investigated. This paper aims to fill this knowledge gap by focusing on materials with weak SOC to study IOHE and its application in the development of orbitronic terahertz emitters.

The researchers designed and fabricated bilayered structures of Co (2 nm)/X (4-60 nm), where X represents Ti or Mn. They also created trilayer structures incorporating a W layer to investigate its impact on THz emission. These samples were deposited on Al2O3 (0001) single crystal substrates using ultrahigh vacuum magnetron sputtering, with a 5 nm MgO capping layer to prevent oxidation. The THz emission was measured using a home-built THz emission spectroscopy setup employing a Ti:sapphire laser oscillator (800 nm center wavelength, 100 fs pulse duration, 2 W average power, 80 MHz repetition rate). The electro-optic sampling technique with a 2 mm thick ZnTe (110) crystal was used for detection. Samples were subjected to a 50 mT in-plane magnetic field during room temperature measurements. The normalized THz signals were obtained by normalizing the original THz signals to the laser absorbance of the FM layer and the THz radiation impedance. This normalization process, detailed in the methods section, accounts for the variations in laser absorption and THz impedance across different samples and allows for a more accurate comparison of the THz emission efficiency. Control samples including Co/W, Co/Pt, Co/MgO, Ti/MgO, Mn/MgO, and MgO were also fabricated and measured to rule out any contributions to the THz emission from the individual layers or the substrate. The researchers employed equations to describe the relationship between THz signal and the efficiency of spin/orbit-charge conversion, enabling estimation of the spin/orbit-charge conversion efficiency of the NM layers.

The study successfully demonstrated the inverse orbital Hall effect (IOHE) in materials with weak spin-orbit coupling, specifically Ti and Mn. Significant orbitronic terahertz (THz) emission was observed in Co/Ti and Co/Mn bilayer structures. The polarity of the THz signals confirmed the J_L → J_c conversion, attributed to IOHE. The introduction of a W layer, a material with strong SOC, between the Co and Ti/Mn layers led to a substantial enhancement in the THz emission amplitude. This enhancement is attributed to the additional conversion of orbital current to charge current facilitated by the W layer. The thickness dependence of the THz emission in Co/Ti structures indicated a long orbital diffusion length in Ti. The study also investigated the interplay between IOHE and ISHE in different sample configurations (Co/Ti/W and Ti/Co/W). In Co/Ti/W structures, the THz emission is a result of the competition between IOHE and ISHE, leading to a complex thickness dependence of the THz signal. In Ti/Co/W structures, the THz emission is enhanced by the combined effect of IOHE from Ti and ISHE from W, resulting in a significantly larger THz emission than that observed in Co/W structure. These results demonstrate that the orbitronic THz emission can be manipulated by controlling the sample structure and utilizing the cooperation or competition between IOHE and ISHE. The long orbital diffusion length observed in Ti suggests its potential for use in orbitronic devices.

The findings of this study significantly advance our understanding of the inverse orbital Hall effect (IOHE) and its potential for developing high-performance spin-orbitronic devices and terahertz (THz) emitters. The successful demonstration of IOHE in Ti and Mn, materials with weak spin-orbit coupling, expands the range of materials suitable for spin-orbitronic applications. The observation of orbitronic THz emission and the enhancement achieved by incorporating a W layer provide valuable insights into the mechanisms governing these phenomena. The ability to manipulate the THz emission through structural design offers promising avenues for creating tunable and efficient THz sources. The long orbital diffusion length observed in Ti highlights its potential as a key material for future orbitronic devices. The interplay between IOHE and ISHE, as demonstrated in the various sample configurations, offers additional possibilities for controlling and optimizing the performance of such devices. This research opens new avenues for exploring the fundamental physics of spin-orbit coupling and its applications in next-generation technologies.

This study successfully demonstrated the inverse orbital Hall effect (IOHE) in materials with weak spin-orbit coupling, specifically Ti and Mn, and observed significant orbitronic terahertz emission. The incorporation of a W layer significantly enhanced this emission. Moreover, the manipulation of the emission was achieved by carefully designing the sample structures to control the interplay between IOHE and ISHE. This research provides a fundamental understanding of IOHE and offers promising pathways towards advanced spin-orbitronic devices and efficient THz emitters. Future work could focus on exploring other materials with weak spin-orbit coupling, optimizing the device structures for higher efficiency, and integrating these components into functional devices.

The study primarily focused on a limited set of materials (Ti, Mn, and W) and specific layer thicknesses. Further investigation is needed to explore the generality of these findings and to determine the optimal parameters for maximizing THz emission. The analysis relied on a model that simplifies the complex interplay between different physical processes; a more sophisticated model might be needed to fully capture the observed phenomena. While the normalization procedure helps to account for variations in laser absorbance and THz impedance, other factors could potentially influence the THz emission efficiency. Further investigations could explore the impact of other material properties and experimental parameters on the observed effects. More detailed investigations of the underlying microscopic mechanisms are needed to fully understand the observed phenomena.