New technique monitors ultrafast dynamics behind non-reversible photo-induced phase transitions

In the search for new photonic devices, the use of ultrashort laser pulses as external stimuli is very promising, since it opens the way to ultrafast and contactless control in light-based technologies. This necessitates photo-active materials whose physical properties can be switched very rapidly and permanently using light. In this scope, Prussian Blue Analogues (PBAs) represent very attractive multifunctional compounds as their magnetism, ionic conduction or optical properties can be changed using ultrashort light pulses. This work studied the dynamics associated with the photo-induced phase transition in a Co-doped RbMnFe PBA, where the transformation is persistent at room temperature and is accompanied by symmetry and volume changes [1,2].

Monitoring the ultrafast dynamics associated with this non-reversible photo-transformation presented a challenge. Conventional time-resolved techniques are not applicable to non-reversible phenomena as they usually consist of stroboscopic measurements performed on the same sample, assuming that the sample recovers its initial state between two consecutive measurements. A new technique has thus been developed for time-resolved X-ray diffraction, in close collaboration with the ESRF’s sample environment support service and beamline ID09, where measurements are performed on crystals streaming through a liquid jet (Figure 1). In this experimental configuration, named streaming powder diffraction, each measurement is performed on a new batch of crystals, and X-ray diffraction maps the temporal reorganisation during the non-reversible phase transition.

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Fig. 1: Experimental setup of streaming powder diffraction at ID09. PBA crystals are dispersed in a solution flowing through a liquid jet, where a first laser pump pulse initiates the phase transition, probed by a delayed X-ray pulse. Non-reversible photo-conversion from the initial low temperature (LT) tetragonal to the photo-induced cubic high temperature (HT) phase is directly mapped by X-ray diffraction. Insertion of a cooling device ensures that crystals are in the initial phase for the measurements.

For the studied PBA, the time-resolved measurements on ID09 revealed that the conversion from the tetragonal low temperature (LT) to the photo-induced cubic phase is complete within only 150 ps, which indicates an elastic-driven phase transition (Figure 2). The photo-induced cubic phase, stable at room temperature, appears above a threshold laser fluence due to the strong elastic cooperativity of
PBAs. Indeed, within a crystal, a critical fraction of initially photo-converted sites, giving rise to a large volume strain, is required to destabilise the initial tetragonal phase towards the cubic phase.

Click figure to enlarge

Fig. 2: a) Time-resolved X-ray diffraction map showing the transformation of Bragg peaks from tetragonal (Miller indices in blue) to cubic (Miller indices in red) phases. b) Changes in lattice parameters, phase fractions and tetragonal-to-cubic ferroelastic distortion η, obtained from crystallography analysis. c) LT-to-HT conversion rate as a function of incident laser fluence.

This new technique therefore demonstrates that the present PBA material has promising capabilities for ultrafast photo-switching at room temperature and could, in the future, be implemented in novel, optically driven devices. More generally, it is foreseen that this technique will open new avenues in ultrafast material science by making it possible to access non-reversible dynamics using time-resolved X-ray diffraction.

Principal publication and authors
Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction, M. Hervé (a,b), G. Privault (a,b), E. Trzop (a,b), S. Akagi (c), Y. Watier (d), S. Zerdane (e), I. Chaban (a,b), R.G. Torres Ramírez (a,b), C. Mariette (a,d), A. Volte (d), M. Cammarata (d), M. Levantino (d), H. Tokoro (c,b), S. Ohkoshi (f,b), E. Collet (a,b,g), Nat. Commun. 15, 267 (2024); https://doi.org/10.1038/s41467-023-44440-3
(a) Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Rennes (France)
(b) CNRS, Univ Rennes, DYNACOM (Dynamical Control of Materials Laboratory) - IRL 2015, The University of Tokyo, Tokyo (Japan)
(c) Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Ibaraki (Japan)
(d) ESRF
(e) SwissFEL, Paul Scherrer Institut, Villigen (Switzerland)
(f) Department of Chemistry, School of Science, The University of Tokyo, Tokyo (Japan)
(g) Institut universitaire de France (IUF), Paris (France)

References
[1] G. Azzolina et al., Eur. J. Inorg. Chem. 3142-3147 (2019).
[2] G. Azzolina et al., J. Mat. Chem. C 9, 6773-6780 (2021).