  Metallic spintronic THz emitters have become well-established for offering
ultra-broadband, gap-less THz emission in a variety of excitation regimes, in
combination with reliable fabrication and excellent scalability. However, so
far, their potential for high-average-power excitation to reach strong THz
fields at high repetition rates has not been thoroughly investigated. In this
article, we explore the power scaling behavior of tri-layer spintronic emitters
using an Yb-fiber excitation source, delivering an average power of 18.5 W at
400 kHz repetition rate, temporally compressed to a pulse duration of 27 fs. We
confirm that the reflection geometry with back-side cooling is ideally suited
for these emitters in the high-average-power excitation regime. In order to
understand limiting mechanisms, we disentangle the effects on THz power
generation by average power and pulse energy, by varying the repetition rate of
the laser. Our results show that the conversion efficiency remains mostly
dependent on the incident fluence in this high-average-power,
high-repetition-rate excitation regime if the emitters are efficiently cooled.
Using these findings, we optimize the conversion efficiency to reach 5e-6 at
highest excitation powers in the back-cooled reflection geometry. Our findings
provide guidelines for scaling the power of THz radiation emitted by spintronic
emitters to the mW-level by using state-of-the-art femtosecond sources with
multi-hundred-Watt average power to reach ultra-broadband, strong-field THz
sources with high repetition rate.
