  Lead halide perovskite nanocrystals demonstrate efficient up-conversion,
although the precise mechanism remains a subject of active research. This study
utilizes steady-state and time-resolved spectroscopy methods to unravel the
mechanism driving the up-conversion process in CsPbBr3 nanocrystals. Employing
above- and below-gap photoluminescence measurements, we extract a distinct
phonon mode with an energy of ~7 meV and identify the Pb-Br-Pb bending mode as
the phonon involved in the up-conversion process. This result was corroborated
by Raman spectroscopy. We confirm an up-conversion efficiency reaching up to
75%. Transient absorption measurements under conditions of sub-gap excitation
also unexpectedly reveal coherent phonons for the subset of nanocrystals
undergoing up-conversion. This coherence implies that the up-conversion and
subsequent relaxation is accompanied by a synchronized and phased lattice
motion. This study reveals that efficient up-conversion in CsPbBr3 nanocrystals
is powered by a unique interplay between the soft lattice structure, phonons,
and excited states dynamics.
