Fernando Rodriguez Diaz

arXiv2026

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 $\omega_0 = 0.192$ 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.

cond-mat.mtrl-sci↗

arXiv2024

High repetition rate ultrafast electron diffraction with direct electron detection

Ultrafast electron diffraction (UED) instruments typically operate at kHz or lower repetition rates and rely on indirect detection of electrons. However, these experiments encounter limitations because they are required to use electron beams containing a relatively large number of electrons (>>100 electrons/pulse), leading to severe space-charge effects. Consequently, electron pulses with long durations and large transverse diameters are used to interrogate the sample. Here, we introduce a novel UED instrument operating at a high repetition rate and employing direct electron detection. We operate significantly below the severe space-charge regime by using electron beams containing 1 to 140 electrons per pulse at 30-kHz. We demonstrate the ability to detect time-resolved signals from thin film solid samples with a difference contrast signal, ${\Delta}I/I_0$, and an instrument response function as low as $10^{-5}$ and 184-fs (FWHM), respectively, without temporal compression. Overall, our findings underscore the importance of increasing the repetition rate of UED experiments and adopting a direct electron detection scheme, which will be particularly impactful for gas-phase UED. Our newly developed scheme enables more efficient and sensitive investigations of ultrafast dynamics in photoexcited samples using ultrashort electron beams.

physics.chem-ph↗