Picosecond expansion in LaAlO3 resonantly driven by infrared-active phonons

TL;DR

This study demonstrates ultrafast lattice expansion in LaAlO3 driven by resonant infrared-active phonons, revealing a mechanism for controlling nanoscale structural dynamics through combined experimental and theoretical approaches.

Contribution

It introduces a multimodal method combining time-resolved X-ray, optical measurements, and first-principles theory to understand and manipulate ultrafast structural dynamics in LaAlO3.

Findings

Immediate lattice expansion observed upon THz excitation

Acoustic breathing mode scales linearly with pump fluence

THz excitation enhances crystallinity and structural symmetry

Abstract

We investigate the ultrafast structural dynamics of LaAlO3 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, which agrees with the theory. Furthermore, time-resolved X-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a non-thermal 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.