For large polarons, Fröhlich theoretically derived an increased effective mass and a decreased energy as compared to free charge carriers. These changes have hitherto eluded direct observation. Here, we report on the direct observation of large polaron formation after photoexcitation in BiOI nanoplatelets, a system known to host spatially separated e-h pairs. We employ time- and momentum-resolved photoemission electron microscopy with 50-fs pulses to monitor the dynamics of the conduction-band (CB) dispersion $E\left(k\right)$. Indeed, we observe a doubling of the effective mass accompanied by a decrease of the CB energy leading to an overall drop of 160 meV.

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• Polarons
• Time-resolved two-photon photoemission spectroscopy