Ultrafast lattice deformations studied by means of time-resolved electron and x-ray diffraction

TL;DR

This study employs ultrafast x-ray and electron diffraction to directly observe and analyze femtosecond laser-induced lattice deformations and phonon dynamics in metallic crystals, revealing insights into ultrafast heating and elastic wave propagation.

Contribution

It demonstrates the combined use of ultrafast x-ray and electron diffraction to capture transient atomic motions in crystals, and links ultrafast heating to electrical circuit models.

Findings

Lattice compression occurs within a few picoseconds after laser excitation.

Electron and x-ray diffraction distinguish heating dynamics in different film thicknesses.

Numerical simulations with the two-temperature model agree with experimental data.

Abstract

Ultrafast lattice deformation of tens to hundreds of nanometer thick metallic crystals, after femtosecond laser excitation, was measured directly using 8.04 keV subpicosecond x-ray and 59 keV femtosecond electron pulses. Coherent phonons were generated in both single crystal and polycrystalline films. Lattice compression was observed within the first few picoseconds after laser irradiation in single crystal aluminum, which was attributed to the generation of a blast force and the propagation of elastic waves. The different time scale of lattice heating for tens and hundreds nanometer thick films are clearly distinguished by electron and x-ray pulse diffraction. The electron and lattice heating due to ultrafast deposition of photon energy was numerically simulated using the two-temperature model (TTM) and the results agreed with experimental observations. The ultrafast heating described by TTM was also discussed from an electrical circuit perspective, which may provide new insights on the possible connection between thermal and electrical processes. This study demonstrates that the combination of two complimentary ultrafast time-resolved methods, ultrafast x-ray and electron diffraction will provide a panoramic picture of the transient atomic motions and structure in crystals.