Charge dynamics in spintronic THz emitters

TL;DR

This paper demonstrates that accurately modeling ultrafast charge currents in spintronic THz emitters requires accounting for charge equilibration, which influences the emitted THz spectrum and the interpretation of spin current dynamics.

Contribution

It introduces the importance of charge equilibration and backflow effects in the analysis of ultrafast charge currents in spintronic THz emitters, highlighting their impact on THz emission and spin physics interpretation.

Findings

Delayed charge equilibration enables THz detection of primary currents.

Backflow suppresses low-frequency components in emitted spectra.

Timing of spin current cannot be directly inferred from charge current without deconvolution.

Abstract

We show that to correctly describe the ultrafast currents in spintronic THz emitters it is necessary to take charge equilibration into account. The charge current which is locally induced by a fs laser pulse and the inverse spin-Hall effect (ISHE) leads to ultrafast charging phenomena at the edge of the illuminated area. Subsequent discharging leads to a current backflow with a delay and a time constant that mainly depends on the conductivity of the emitter. On the one hand, only this delayed charge equilibration allows the detection of the primary current pulse via THz 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 to the initial spin-current pulse by suppressing low frequency 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 by the ISHE and the backflow can be deconvoluted.