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Description
The structural transformations a metal undergoes under intense excitation depend on both the heating rate and the deposited fluence. Femtosecond optical excitation reaches heating rates far beyond those of conventional methods, and how the lattice responds spans a continuum from gentle vibrations to complete structural reordering as the fluence increases. Capturing this full range in a single material requires probing atomic-scale dynamics across (sub)picosecond-to-nanosecond timescales. Here we report a time-resolved diffraction study of thin polycrystalline iron films under femtosecond optical excitation, combining optical-pump / X-ray-probe measurements at a synchrotron and at free-electron-laser sources to follow the structural response as a function of absorbed fluence.
At low intensities, probed at the synchrotron, the excitation launches coherent acoustic deformations: femtosecond heating drives a strain wave that modulates the interplanar spacing, seen as a periodic shift of the Fe Bragg reflections without loss of long-range order. At higher fluences, accessed at the FEL sources, the response crosses into genuine solid–solid transformation, including the formation of a metastable body-centred-tetragonal (bct) phase as a non-equilibrium intermediate [1]. At the highest fluences melting occurs, followed on the nanosecond timescale by crystallisation.
Crucially, femtosecond laser heating drives the lattice along a structural pathway distinct from the equilibrium phase diagram. For example the intermediate bcc→fcc (α→γ) transition in Fe is kinetically hindered on these timescales and is bypassed entirely, so the system evolves from the bcc phase to the melt and back to bcc-Fe without passing through the close-packed fcc structure. Combining the complementary time and momentum resolution of synchrotron and FEL sources, we assemble a coherent picture of this non-equilibrium excitation ladder in a model metal - Fe, from reversible acoustic strain, through a metastable bct phase, to melting and recrystallisation along a route inaccessible under equilibrium conditions.
Acknowledgements
This work was supported by the grant of the Polish Ministry of Education and Science - decision no. 2022/WK/13 and by the National Science Centre, Poland, grant agreement No 2017/27/B/ST3/02860.
References
[1] Liubchenko O, Antonowicz J, Sokolowski-Tinten K, et al (2025) Laser-induced ultrafast structural transformations in thin Fe layer revealed by time-resolved X-ray diffraction. https://doi.org/10.48550/arXiv.2506.18730
