Large bandgap oscillations in two-dimensional Dion–Jacobson phase perovskites caused by coherent longitudinal acoustic phonons [Invited]

Two-dimensional Dion–Jacobson (D-J) phase perovskites are prospective photovoltaic and optoelectronic materials. To study their mechanical properties and carrier-lattice interactions, we conduct femtosecond spectroscopic experiments on the films of a D-J perovskite. After optical excitation, a ∼33 meV bandgap oscillation is observed in the film by transient absorption spectroscopy. With the help of transient reflection methods, we reveal that the oscillation originates from the transport of coherent longitudinal acoustic phonons through the film. Large bandgap oscillation indicates a strong coupling between carriers and lattice, and significant bandgap modulation by strains in D-J perovskites.