Probing Spin-injection in the Time Domain in Topological Insulators via THz-TDS Spectroscopy

Spin-to-charge conversion (SCC) in femtosecond laser excited magnetic heterostructures may generate high efficiency and wide-bandwidth terahertz emission with a magnetically controllable polarization state¹. The origin of this THz emission has been assigned to the generation of a spin-polarized current and subsequent conversion of the spin current to a transverse charge current². Two main SCC mechanisms are generally involved: the Inverse Spin Hall Effect (ISHE) and the Inverse Rashba- Edelstein Effect (IREE). The discovery of metallic quantum states at the surface of 3D topological insulators (TIs) has opened exciting new functionalities owing to their time-reversal symmetry property and their spin-momentum locking (SML) properties. The resulting SCC combining strong spin-orbit coupling (SOC) and SML is expected to be enhanced compared to the spin Hall effect (SHE) of heavy metals. SCC has been demonstrated in a range of Bi-based TI compounds, including bismuth selenide Bi₂Se₃, bismuth telluride Bi₂Te₃, Bi₂(Se,Te)₃ or Bi_1-xSb_x (BiSb). To benefit fully from IREE, the charge currents should be confined in the surface states and any current flowing through the bulk states should be avoided.<br/><br/>In this talk, I will report particularly on our detailed investigation of the surface state SML properties of ultrathin (111)-oriented Bi_1-xSb_x epitaxial films. They exhibit a topological phase as recently confirmed by angular-resolved photo-emission spectroscopy (ARPES)³ and mainly in-plane spin texture as shown by SARPES. SCC mediated by the BiSb surface states is probed at the sub-picosecond timescale. Unprecedentedly large SCC is measured with efficiencies beyond the level of carefully optimized Co/Pt systems. I will present our more recent results dealing with the fabrication of sputtered BiSb materials giving rise to very efficient spin-charge conversion in the time domain at the level of up-to-date best bilayers spintronics emitters.<br/> <br/>References<br/>[1]. Seifert T. et al., “<i>Efficient metallic spintronic emitters of ultrabroadband terahertz radiation</i>”, Nature Photon 10, 483–488, 2016.<br/>[2]. Dang T. H. et al., “<i>Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy</i>”, Appl. Phys. Rev. 7, 2020<br/>[3]. L. Baringthon et al “<i>Topological surface states in ultrathin Bi_1-xSb_x layers</i>” Physical Review Materials 6, 074204, 2022<br/>[4] E. Rongione et al., “<i>Spin-Momentum Locking and Ultrafast Spin-Charge Conversion in Ultrathin Epitaxial Bi1 – xSbx Topological Insulator”, </i><i>Adv. Sci. </i>2023, 2301124