Optimizing spin-based terahertz emission from magnetic heterostructures

Terahertz radiation pulses can be generated efficiently through femtosecond laser excitation of a ferromagnetic/nonmagnetic heterostructure, wherein an ultrafast laser-induced spin current results in an electromagnetic THz pulse due to spin-charge conversion. It is, however, still poorly understood how the THz emission amplitude and its bandwidth in the frequency regime can be optimized. Here, we perform a systematic analysis of the THz emission from various magnetic heterostructures. The dynamics of the spin current is described by the semiclassical, superdiffusive spin-transport model, and the energy dependence of the spin Hall effect of hot electrons is taken into account, leading to emission profiles for Co(2 nm)/Pt(4 nm) bilayer in good agreement with experiment. To identify the optimal conditions for THz emission, we study the properties of the emitted THz wave profile by systematically varying the layer thicknesses of metallic bilayers, their interfacial spin-current transmission properties, their materials’ dependence, and the influence of the pump laser-pulse width, allowing us to give optimization guidelines. We find that thin nonmagnetic layer thicknesses of 5–6 nm provide the largest bandwidth in the case of Co/Pt and that the peak frequency of the THz emission depends only on the geometry of the emitter and not on the laser pulse width. The THz bandwidth is conversely found to depend on several factors such as exciting laser pulse width, layers’ thicknesses, and interface transmission-reflection properties, with the limitation that an increase in the bandwidth by tuning the interface properties comes with a trade-off in the energy efficiency of the emitter. Finally, we propose a double pulse excitation protocol of a trilayer system that could provide broadband THz emission with a large bandwidth. Our results contribute to establishing guidelines for optimizing spintronic THz generation.