Multilayer Spintronic THz Emitters for On-Chip Ultrafast Current Generation

Spintronic terahertz (THz) emitters based on ultrafast spin-to-charge conversion in heavy metal | ferromagnet heterostructures are compact, broadband THz sources with bandwidths exceeding conventional semiconductor-based emitters. However, achievable current amplitudes per unit area are fundamentally limited by spin-current transport across a single heavy metal | ferromagnet interface. Here, we demonstrate a multilayer spintronic THz emitter architecture comprising repeated heavy metal | ferromagnet bilayers separated by ultrathin MgO barriers, enabling constructive addition of generated charge currents. Using GHz-bandwidth electrical sampling and THz-bandwidth electro-optic sampling, we observe monotonic amplitude enhancement with increasing layer number. This scaling behavior is quantitatively captured by a model incorporating optical absorption and impedance matching effects. Finally, by integrating multilayer emitters into coplanar waveguide structures, we demonstrate on-chip THz current generation and propagation. These results establish multilayer spintronic THz emitters as a scalable platform for ultrafast current generation and integrated on-chip THz electronics.