Using the THz emission spectroscopy, we investigate ultrafast spin-to-charge current conversion in epitaxial thin films of the altermagnetic candidate ${\mathrm{RuO}}_2$. We perform a quantitative analysis of competing effects that can contribute to the measured anisotropic THz emission. These include the anisotropic inverse spin splitter and spin Hall effects in ${\mathrm{RuO}}_2$, the anisotropic conductivity of ${\mathrm{RuO}}_2$, and the birefringence of the ${\mathrm{TiO}}_2$ substrate. We observe that the leading contribution to the measured signals comes from the anisotropic inverse spin Hall effect, with an average spin-Hall angle of $2.4\times {10}^{-3}$ at room temperature. In comparison, a possible contribution from the altermagnetic inverse spin-splitter effect is found to be approximately $2–4\times {10}^{-4}$. Our work stresses the importance of carefully disentangling spin-dependent phenomena that can be generated by the unconventional altermagnetic order, from the effects of the relativistic spin-orbit coupling.

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• Inverse spin Hall effect
• Spin current
• Spintronics
• Ultrafast magnetization dynamics
• Terahertz time-domain spectroscopy