  The ability to probe symmetry breaking transitions on their natural time
scales is one of the key challenges in nonequilibrium physics. Stripe ordering
represents an intriguing type of broken symmetry, where complex interactions
result in atomic-scale lines of charge and spin density. Although phonon
anomalies and periodic distortions attest the importance of electron-phonon
coupling in the formation of stripe phases, a direct time-domain view of
vibrational symmetry breaking is lacking. We report experiments that track the
transient multi-THz response of the model stripe compound
La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, yielding novel insight into its electronic and
structural dynamics following an ultrafast optical quench. We find that
although electronic carriers are immediately delocalized, the crystal symmetry
remains initially frozen - as witnessed by time-delayed suppression of
zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry.
Longitudinal and transverse vibrations react with different speeds, indicating
a strong directionality and an important role of polar interactions. The hidden
complexity of electronic and structural coupling during stripe melting and
formation, captured here within a single terahertz spectrum, opens new paths to
understanding symmetry breaking dynamics in solids.
