Ultra-fast and persistent photoswitching at room temperature stabilised by volume and symmetry changes

The ability to control physical properties of materials using light as an external stimulus has led to significant advancements in various scientific and technological fields. For example, optical switching devices, which can facilitate ultrafast data processing or photonic crystals for sensors.

Ultrashort laser pulses make it possible to envisage the triggering of extremely rapid and cooperative changes in the physical properties of materials. For applications such as optically controlled devices, the manufacturing of memories or actuators, the photo-response must comply with demanding specifications. These include simultaneously combining important characteristics, such as switching at room temperature, a wide thermal bistability regime, the existence of photoinduced states that can persist long after the application of stimuli, or the possibility of switching ultrarapidly with a single laser flash.

An international collaboration (IRL Dynacom) involving researchers from the CNRS/University of Rennes (IPR) and the University of Tokyo has developed and studied a material from the family of cyanide-bridged assemblies, designed to present a wide range of bistability at ambient temperature.

These assemblies are very attractive compounds that exhibit intermetallic charge transfer, which show multifunctional properties that can be reversibly controlled by light, such as magnetism, ionic conduction or even optical properties such as the rotation of the polarization of light.

An ultra-brief laser pulse can switch the system from a stable state to a permanent photoinduced state, via an electron transfer between the atoms of the crystal. This increases the volume of the crystal and changes its symmetry.

The team came to the ESRF to use ultra-fast crystallography on beamline ID09 and showed that beyond an excitation threshold, the switching takes place on the extremely fast time scale of 100 picoseconds. The light-induced volume change permanently stabilizes a higher symmetry phase.

“The experiment at the ESRF was challenging because we had to develop a powder sample streaming technique to study the ultra-fast dynamics towards permanent state, responsible for this photoswitching process, by probing a fresh sample for each laser shot excitation”, explains Eric Collet, professor at the University of Rennes and co-corresponding author of the publication. “We have been coming to the ESRF for 25 years now and we have seen the capabilities of the beamline and the machine improving enormously over time, allowing us to carry out experiments today that we couldn’t do a few years ago”, he adds.

“The technique developed in this work paves the way for the study of various persistent ultra-fast phenomena using pump-probe techniques to monitor their electronic and structural dynamics”, explains Marius Hervé, post-doctoral researcher in Collet's team.

For Matteo Levantino, scientist in charge of ID09, "the method developed at ID09 in collaboration with Eric Collet's group is indeed applicable to a wide range of systems. As the sample is a stream of mycrocrystals, it is possible to obtain high photoexcitation yields and also to bring the system back to its initial state by changing the sample temperature in between consecutive shots."

The next step for the team is to study the reverse photoinduced process, and this is the aim of the next experiment at ID09.

Reference:

Hervé, M., Nat Commun 15, 267 (2024). https://doi.org/10.1038/s41467-023-44440-3

Text by Montserrat Capellas Espuny

Top image: A zoom on the ID09 experimental hutch. Credits: S. Candé.