  Symmetry breaking across phase transitions often causes changes in selection
rules and emergence of optical modes which can be detected via spectroscopic
techniques or generated coherently in pump-probe experiments. In second-order
or weakly first-order transitions, fluctuations of the order parameter are
present above the ordering temperature, giving rise to intriguing precursor
phenomena, such as critical opalescence. Here, we demonstrate that in magnetite
(Fe$_3$O$_4$) light excitation couples to the critical fluctuations of the
charge order and coherently generates structural modes of the ordered phase
above the critical temperature of the Verwey transition. Our findings are
obtained by detecting coherent oscillations of the optical constants through
ultrafast broadband spectroscopy and analyzing their dependence on temperature.
To unveil the coupling between the structural modes and the electronic
excitations, at the origin of the Verwey transition, we combine our results
from pump-probe experiments with spontaneous Raman scattering data and
theoretical calculations of both the phonon dispersion curves and the optical
constants. Our methodology represents an effective tool to study the real-time
dynamics of critical fluctuations across phase transitions.
