  Direct visualization of ultrafast coupling between charge carriers and
lattice degrees of freedom in photo-excited semiconductors has remained a
long-standing challenge and is critical for understanding the light-induced
physical behavior of materials under extreme non-equilibrium conditions. Here,
by monitoring the evolution of the wave-vector resolved ultrafast electron
diffraction intensity following above-bandgap photo-excitation, we obtain a
direct visual of the structural dynamics in monocrystalline 2D perovskites.
Analysis reveals a surprising, light-induced ultrafast lattice ordering
resulting from a strong interaction between hot-carriers and the perovskite
lattice, which induces an in-plane octahedra rotation, towards a more symmetric
phase. Correlated ultrafast spectroscopy performed at the same carrier density
as ultrafast electron diffraction reveals that the creation of a hot and dense
electron-hole plasma triggers lattice ordering at short timescales by
modulating the crystal cohesive energy. Finally, we show that the interaction
between the carrier gas and the lattice can be altered by tailoring the
rigidity of the 2D perovskite by choosing the appropriate organic spacer layer.
