We investigate the ultrafast structural dynamics of ${\mathrm{LaAlO}}_3$ thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved x-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechanism. Time-resolved optical birefringence measurements confirm that the amplitude of this acoustic mode scales linearly with the pump fluence, in agreement with theory. Furthermore, time-resolved x-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a nonthermal increase in structural symmetry originating from preexisting defects. These findings highlight the potential of a multimodal approach—combining time-resolved x-ray and optical measurements and first-principles theory—to elucidate and control structural dynamics in nanoscale materials.

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• First-principles calculations
• Light-matter interaction
• Phonons
• Solid-solid transformations
• X-ray diffraction
• X-ray diffuse scattering