Secondary Spin Current Driven Efficient THz Spintronic Emitters

Abstract

Femtosecond laser‐induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures has attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of an ultrafast spin current, which is primarily attributed to the direct photoexcitation of the FM layer. However, during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. These results emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin‐transport mechanism.

Modeling terahertz wave propagation and production in out-of-equilibrium complex heterostructure devices

“…The reason is that in the second case, the produced THz radiation has to traverse the quartz substrate, which absorbs in that frequency range. This reproduces experimental findings on quartz substrates [135]. The situation is reversed in the case of a sapphire substrate (material properties taken from Ref.…”

“…This enhancement will become large when the HM layer is thick [135]. We stress that the above contributions, even if ignored in most cases, can be described by our model.…”

“…[138] and at the THz range are taken as the experimentally measured dielectric constants in Ref. [135]. We take the Pt and Co spin diffusion length to be 1.1 nm and 1nm, respectively, [1,2,41,139] as a test case.…”

Reconfigurable Chiral Spintronic THz Emitters

“…[36] Here, we investigate spintronic heterostructure where a pair of FM, NiFe (4 nm), and CoFe (10 nm) is coupled through RKKY exchange interaction across a Ru interlayer, as shown in Figure 1a. Upon femtosecond photoexcitation, the FMs generate antiparallel spin currents, j s1 and −j s2 , which undergoes a superdiffusive spin transport mechanism [37] (subscripts 1 and 2 refer to FM1: NiFe and FM2: CoFe, respectively). As a result, j s1 and −j s2 experience a simultaneous spin-to-charge conversion at Ru and FM/Ru interface [38] to emit THz electric fields transverse to spin current.…”