In spintronics, FM|HM stacks consisting of a ferromagnetic-metal (FM) and a heavy-metal (HM) layer are model systems for spin transport and spin-charge interconversion. To explore their potential as detectors for ultrabroadband terahertz electromagnetic pulses, we measure the transient optical birefringence the terahertz field induces. Notably, the signal component linear in the FM magnetization agrees excellently with the shape of the incident terahertz electric field at 1 to 13 terahertz and beyond. Analysis indicates that the birefringence arises from the terahertz-field–driven spin accumulation at the FM/HM interface through the spin Rashba-Edelstein effect (SREE). Because of spin-momentum locking, the SREE decays by electron momentum relaxation in <10 femtoseconds, substantially faster than a spin-Hall-effect–induced bulk spin accumulation. Our experiment demonstrates straightforward spintronic sampling of intense ultrabroadband terahertz fields with flat amplitude and phase response. Furthermore, it provides temporal signatures of the SREE and can be viewed as a versatile implementation of interface-specific terahertz time-domain spectroscopy.
