CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, which underlie information about localization in real space. In particular, we detect a sudden shift and broadening of the flat bands, while after one picosecond only a broadening survives. We ascribe both these effects to an ultrafast disruption of electron localization, which renormalizes the effect of electron-electron interaction and affects the flat band dispersion. Since both variations are in the order of a few meV for an excitation fluence of a few hundreds of $\mu \mathrm{J}/{\mathrm{cm}}^2$, our measurements suggest that the flat bands are resilient to near-infrared photoexcitation in a moderate fluence reégime.

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• Flat bands
• Kagome lattice
• Angle-resolved photoemission spectroscopy
• Density functional theory
• Time & angle resolved photoemission spectroscopy
• Ultrafast femtosecond pump probe