Ultrafast Optical Control of the Crystal Lattice in a Strained LaAlO₃ Film

Driving the lattice structure with resonant phononic excitations presents a novel way for controlling quantum materials. We present the results of an ultrafast diffraction experiment using THz resonant excitation to drive an IR mode in a thin, strained LaAlO₃ epitaxial film. Our results explore predicted and measured ultrafast, IR light induced structural processes in the film. We do not observe the prediction that nonlinear phonon coupling excites a giant ultrafast response in the lowest frequency Raman phonon. The ultrafast structural phase transition associated with this Raman phonon is not observed.  However, our results show that the IR pump induces the growth of structural domains on the timescale of several picoseconds. Moreover, we find that a longitudinal acoustic phonon forms in the films, the period of which is several picoseconds and is determined by the film thickness. Finally, we observe that the film begins to expand on the time scale of less than 1 picosecond. These results pave the way for better understanding of how strain impacts the dynamics of IR phonon induced ultrafast structural changes in LaAlO3, which is of particular interest as it is often used as a substrate for growing thin films. Our results represent the next step in strain engineering, studying ultrafast lattice dynamics in a strained film.