  Excitation localization involving dynamic nanoscale distortions is a central
aspect of photocatalysis, quantum materials and molecular optoelectronics.
Experimental characterization of such distortions requires techniques sensitive
to the formation of point-defect-like local structural rearrangements in real
time. Here, we visualize excitation-induced strain fields in a prototypical
member of the lead halide perovskites via femtosecond resolution diffuse x-ray
scattering measurements. This enables momentum-resolved phonon spectroscopy of
the locally-distorted structure and reveals radially-expanding nanometer-scale
elastic strain fields associated with the formation and relaxation of polarons
in photoexcited perovskites. Quantitative estimates of the magnitude and the
shape of this polaronic distortion are obtained, providing direct insights into
the debated dynamic structural distortions in these materials. Optical
pump-probe reflection spectroscopy corroborates these results and shows how
these large polaronic distortions transiently modify the carrier effective
mass, providing a unified picture of the coupled structural and electronic
dynamics that underlie the unique optoelectronic functionality of the hybrid
perovskites.
