Charge Dynamics in Spintronic Terahertz Emitters

We show that to correctly describe the ultrafast currents in spintronic terahertz emitters it is necessary to take charge equilibration into account. It has been extensively described that in a spintronic terahertz emitter a femtosecond laser pulse can induce a perpendicular spin current that is then converted by the inverse spin Hall effect into an ultrafast lateral charge-current pulse, and the underlying physics have been thoroughly analyzed. We show that subsequent to spin-to-charge conversion, this lateral charge current leads to ultrafast charging phenomena at the edge of the illuminated area. The following charge relaxation leads to a current backflow with a delay and a time constant that depend mainly on the conductivity and the dielectric properties of the emitter. On the one hand, only this delayed charge equilibration allows the detection of the primary current pulse via terahertz emission because an instantaneous backflow would cancel any far-field emission. On the other hand, especially for longer light pulses, the backflow can significantly change the emitted spectrum compared with the initial spin-current pulse by suppressing lowfrequency components. For the analysis of spin physics based on the charge-current profile, it is important to understand that the timing of the spin current cannot be inferred from the charge current unless the contribution from the initially induced charge current and the backflow can be deconvoluted.