  Engineering novel states of matter with light is at the forefront of
materials research. An intensely studied direction is to realize
broken-symmetry phases that are "hidden" under equilibrium conditions but can
be unleashed by an ultrashort laser pulse. Despite a plethora of experimental
discoveries, the nature of these orders and how they transiently appear remain
unclear. To this end, we investigate a nonequilibrium charge density wave (CDW)
in rare-earth tritellurides, which is suppressed in equilibrium but emerges
after photoexcitation. Using a pump-pump-probe protocol implemented in
ultrafast electron diffraction, we demonstrate that the light-induced CDW
consists solely of order parameter fluctuations, which bear striking
similarities to critical fluctuations in equilibrium despite differences in the
length scale. By calculating the dynamics of CDW fluctuations in a
nonperturbative model, we further show that the strength of the light-induced
order is governed by the amplitude of equilibrium fluctuations. These findings
highlight photoinduced fluctuations as an important ingredient for the
emergence of transient orders out of equilibrium. Our results further suggest
that materials with strong fluctuations in equilibrium are promising platforms
to host "hidden" orders after laser excitation.
