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Photo-induced non-thermal lattice disorder in aluminium thin-film

We investigate the ultrafast dynamics of photo-induced non-thermal lattice disorder in a polycrystalline aluminium thin film to elucidate transient short- and long-range lattice distortions, their thermalization and electron-phonon coupling timescales. Using a high-repetition-rate 95-keV ultrafast electron diffraction setup (UED), we measured the transient dynamics for the differential scattering signal in a momentum transfer, $q$, range longer than in conventional keV UED setups. Analysis of ten Bragg and six diffuse scattering revealed a prompt increase in the mean-square displacement (MSD), indicating rapid energy transfer from the excited electronic system to the lattice. The subsequent relaxation dynamics of the elastic scattering intensities exhibit a pronounced dependence on diffraction order. Lower-order reflections relax more rapidly, whereas higher-order reflections show significantly slower relaxation or near-plateau behaviour, indicating that lattice equilibration proceeds on multiple $q$-dependent timescales. Exponential fits to the MSD dynamics reveal oscillatory residuals, indicative of coherent non-thermal lattice motion. Power spectral density analysis of these residuals uncovers coherent lattice oscillations with a fundamental frequency of $ω_0 = 0.192$ THz, corresponding to the acoustic breathing (A$_{1g}$) mode of aluminium. Higher frequency components are also observed, consistent with coherent phonon oscillations originating from a single zero-wavevector mode populated by multiple coherent phonons. While individual phonon branches are not directly resolved, the observed dependence on the lattice plane of the relaxation behaviour and oscillatory signatures are consistent with a mode-selective lattice response and non-thermal energy redistribution as described by non-thermal lattice models.