  Layered hybrid perovskites have attracted much attention in recent years due
to their emergent physical properties and exceptional functional performances,
but the coexistence of lattice order and structural disorder severely hinders
our understanding of these materials. One unsolved problem regards how the
lattice dynamics are affected by the dimensional engineering of the inorganic
frameworks and the interaction with the molecular moieties. Here, we address
this question by using a combination of high-resolution spontaneous Raman
scattering, high-field terahertz spectroscopy, and molecular dynamics
simulations. This approach enables us to reveal the structural vibrations and
disorder in and out of equilibrium and provides surprising observables that
differentiate single- and double-layered perovskites. While no distinct
vibrational coherence is observed in double-layer perovskites, we discover that
an off-resonant terahertz pulse can selectively drive a long-lived coherent
phonon mode through a two-photon process in the single-layered system. This
difference highlights the dramatic change in the lattice environment as the
dimension is reduced. The present findings pave the way for the ultrafast
structural engineering of hybrid lattices as well as for developing high-speed
optical modulators based on layered perovskites.
