Hot-electron-induced substrate response in transient absorption spectroscopy of tantalum

We demonstrate that for extreme ultraviolet (XUV) transient absorption spectroscopy measurements of thin-film metals, the substrate can significantly contribute to the observed change in absorption even if it is transparent to the pump pulse's excitation wavelengths and does not itself produce a transient signal. When a thin-film tantalum layer deposited on a silicon nitride substrate is irradiated by a near-infrared femtosecond pulse, a coherent acoustic phonon is excited in both the tantalum and the substrate. The substrate response emerges on sub-picosecond timescales and is driven by excitation from the hot-electron distribution in the metal layer. This study underscores the importance of considering substrate effects in XUV transient absorption spectroscopy, which represent a critical factor for accurate modeling and analysis of ultrafast phenomena in thin films.