When light strikes certain materials, it can flip them wholesale into a new phase, changing electronic state, volume and symmetry at once. How that transformation actually unfolds has been hidden in the first trillionths of a second. Researchers from the University of Tokyo, the University of Tsukuba and the University of Rennes have now traced it end to end, performing the first ultrafast simultaneous monitoring of X-ray absorption and X-ray diffraction during a light-induced phase transition.
Using an X-ray free-electron laser and a flowing sample of a cobalt-substituted rubidium manganese iron Prussian blue analogue, the team resolved a precise sequence: a reverse Jahn-Teller distortion at manganese sites within 50 femtoseconds, charge transfer from iron to manganese at 190 femtoseconds, and formation of a charge-transfer polaron, a coupled electron and lattice distortion, by 2.1 picoseconds, after which the new phase nucleates and grows.
The polaron, the researchers conclude, acts as a source of internal pressure: elastic strain spreads outwards through the crystal, triggering successive charge transfers that self-amplify until the entire crystal has switched phase. Published in Nature Materials, this staged mechanism, from electronic excitation through local distortion to nucleation and growth, offers concrete design principles for optically writable memory, light-controlled switching devices and future photonic and quantum technologies.