Enhanced Terahertz Radiation by Efficient Spin-to-Charge Conversion in Rashba-Mediated Dirac Surface States

Abstract

The enhancement of terahertz (THz) radiation is of extreme significance for the realization of the THz probe and imaging. However, present THz technologies are far from being enough to realize high-performance and room-temperature THz sources. Fortunately, topological insulators (TIs), with spin-momentum-locked Dirac surface states, are expected to exhibit a high terahertz emission efficiency. In this work, the novel concept of a Rashba-state-enhanced spintronic THz emitter is demonstrated on the basis of ferromagnet/heavy metal/topological insulator (FM/HM/TI) heterostructure. We find that the THz emission intensity changes as a function of HM interlayer thickness, and a 1.98 times higher intensity compared to that of FM/TI can be achieved when a meticulously designed thickness of the HM layer is inserted. The improvement of terahertz radiation is ascribed to the additive effect of Rashba splitting and topological surface states at the HM/TI interface. These results offer new possibilities for realizing spintronic THz emitters in TI-based magnetic heterostructures.

Ultrafast Spin‐Charge Conversion at SnBi2Te4/Co Topological Insulator Interfaces Probed by Terahertz Emission Spectroscopy

“…One observes a quasi perfect sine shape dependence of E THz versus θ, representative of the SCC process; and not from other spurious optical rectification effects on the surface crystal (e.g., second harmonic χ (2) ). Indeed, such a sine angular dependence is generally a signature of ISHE [ 8,27 ] and has also recently been observed in the case of Rashba conversion (IREE) in Bi/Ag [ 24,25 ] and in TSS, [ 20,22 ] thus questioning about the exact SCC origin of SnBi 2 Te 4 THz emission. Moreover, from experiments, one can notice the presence of a small vertical shift of the THz signal from the zero angular baseline which may be explained by an additional non‐magnetic contribution from the 35 nm‐thick InAs buffer [ 38 ] (additional discussions are available in Section , Supporting Information).…”

“…These experiments rely on ultrafast spin injection and their conversion into a charge current at TI material interfaces as explored recently for Bi/Bi 2 Te 3 systems. [22] THz emission TDS has proven to be a relevant experimental method [32][33][34] and is particularly important to probe the spin injection efficiency, the spin relaxation and related IREE mediated by interfacial states of TIs. [22,35] For experimental details about the THz-TDS setup, the readers may refer to Section S1, Supporting Information and ref.…”

“…[22] THz emission TDS has proven to be a relevant experimental method [32][33][34] and is particularly important to probe the spin injection efficiency, the spin relaxation and related IREE mediated by interfacial states of TIs. [22,35] For experimental details about the THz-TDS setup, the readers may refer to Section S1, Supporting Information and ref. [27].…”

“…Indeed, such TIs or Dirac materials are known to provide very efficient SCC owing to the presence of topologically‐protected surface states and spin momentum locking (SML) at their surfaces. When combined with a ferromagnetic material (FM) in bilayer structures, the IREE in TIs has recently shown magnetization reversal by SOT [ 20 ] and THz wave emission, [ 21,22 ] similar to that observed with symmetry breaking Rashba surface states [ 23 ] in Bi/Ag systems. [ 24,25 ] From a fundamental point of view, the issue of a discrimination between surface IREE and bulk ISHE remains very important in that the two effects do not probe the same physical quantities, either the dynamical spin current for ISHE and spin accumulation for IREE.…”

Temperature‐Dependent Terahertz Emission from Co/Mn2Au Spintronic Bilayers

“…A crucial missing is a direct experimental observation of ultrafast spin–charge conversion of AFM within the THz‐frequency range, although it has been widely investigated in paramagnetic heavy metals, [ 31–34 ] 2D MoS 2 , [ 35 ] and topological insulators of Bi 2 Se 3 and Bi 2 Te 3 . [ 36–38 ]…”