Ultrafast Dynamics of Spin Current and Electron Temperature in Spintronic Terahertz Emitters

Femtosecond laser-pumped spintronic terahertz (THz) emissions have attracted intense academic interest, thanks to their intrinsic capability to generate ultrafast THz pulses that cover a wide spectral range. To gain deep insight into the interplay between the ultrafast thermal dynamics in ferromagnets and the spin-to-charge conversion is critical for advancing this field, yet up to now it has not been fully explored, partially due to a lack of efficient tools to measure these two processes simultaneously. Here, we experimentally employ optical-pump THz-probe (OPTP) spectroscopy to promote this study in a typical ferromagnet (Ni₈₀Fe₂₀) nano-film. It not only elucidates the electron and lattice temperature dynamics by quantifying the optical-pump-induced THz transmission changes, but also provides time-resolved THz spectral maps to analyze the sub-picosecond changes, which determines a time lag of 63 ± 8 fs between THz emission and laser excitation. This is an intrinsic parameter constraining the upper frequency limit of spintronic THz emission. Our work deepens the fundamental understanding of ultrafast laser excitation mechanisms for spintronic THz emissions and may offer a new perspective to achieve high-performance THz emitters.