The interconversion between spin and charge current lies at the core of spintronics. Although significant efforts have been made in the study of spin-to-charge conversion, a close observation of the dynamical process of spin-to-charge remains elusive. Here we explored $\mathrm{Ru}{\mathrm{O}}_2$ film thickness-dependent terahertz (THz) emission from the ultrafast charge current converted from $z$ spins, which are generated by incident circularly polarized lasers via optical orientation. The shift trend of the THz peak position with respect to the film thickness reveals that the conversion process occurs simultaneously with the $z$ spin propagation within the film, rather than at the $\mathrm{Ru}{\mathrm{O}}_2$ surface. A qualitative model of spin-to-charge conversion in $\mathrm{Ru}{\mathrm{O}}_2$ was employed to explain the shift trend, fitting the experimental data with numerical calculation. Our work not only helps improve the fundamental knowledge of spin-to-charge conversion, but also advances the development of $z$ spin-based ultrafast spintronic devices.

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• Magnetotransport
• Spintronics
• Heterostructures
• Thin films
• Transition metal oxides
• Terahertz spectroscopy
• Ultrafast femtosecond pump probe