By applying ultrafast light pulses to a semiconductor, physicists at the University of Fribourg in collaboration with an international team of scientists have succeeded for the first time in creating an ephemeral topological metallic phase – a configuration in which electrons can circulate in a highly stable and efficient way. This achievement a theory formulated over a decade ago.

By sending light pulses of just a few femtoseconds – one millionth of a billionth of a second – physicists from the University of Fribourg have succeeded in modifying the behaviour of electrons in a tin telluride (SnTe) crystal, a semiconductor well-known to scientists for its special properties.

Thanks to a state-of-the-art spectroscopy technique which allows the movement of the electrons to be observed at their own timescale, the research group was able to see how light modifies the properties of the material. «We observed that under the effect of the laser, the electronic structure of the tin telluride was temporarily , creating hybrid states known as Floquet-Bloch in which light and matter merge,» explains Claude Monney, professor at the Department of Physics at the University of Fribourg.

Fleeting occurrence of a novel topological state
For the authors of the study, however, the most spectacular result was the occurrence of a Dirac cone – a quantum signature characteristic of a topological phase. This state allows electrons to circulate virtually unobstructed in a material which does not allow such motion in its original state. «Our findings, which were made in the context of basic research and the acquisition of new knowledge, open up the possibility of controlling the properties of semiconductors by modifying them momentarily as needed using light,» says Claude Monney.

What is remarkable here is that this special state of matter exists only as long as the light pulse is applied – around 100 femtoseconds – after which it disappears. «Thanks to the light pulse, we succeeded in temporarily creating a material with completely new properties,» says a delighted Frédéric Chassot, a research scientist at the Department of Physics.

Long-awaited proof
Back in 2011, scientists had theoretically demonstrated that light could be used to transform a material and endow it with topological properties. But no one had succeeded in proving the theory in the laboratory – until now. This study constitutes the first direct observation of this phenomenon.

These findings could eventually lead to the development of ultrafast, laser-driven electronic components, such as new-generation transistors, and new advances in quantum computing.

- Chassot, F., Pulkkinen, A., Kremer, G. et al. Floquet topological state induced by light-driven band inversion in SnTe. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03341-0
- Picture: © Gérald et Maxime Rumo