Observation of interband Berry phase in laser-driven crystals

Scientists from the Materials Science Institute of Madrid (ICMM-CSIC) are participating in an international research endeavor that has successfully measured the paths electrons take when guided by light in crystals. The work has just been published in the journal Nature and opens up a new branch in the study of the dynamic and geometric phases of materials, which in turn enables a new perspective on light-driven topological phenomena and leads to new developments in ultrafast condensed matter physics, a field that studies matter in its solid or liquid phase.

Driven by light, electrons in a material accumulate information (encoded in 'phases') about the paths they have followed within the material. These paths, resembling mountain ranges with valleys and peaks, known as bands, determine the properties of the material and can be modified by light. This work allows for understanding the phase electrons accumulate as they move within the material and paves the way for measuring changes in material properties when induced by light a trillion times faster than a second.

"It is important to know how the laser modifies the band structure to understand what new properties can be generated in the material and how to modify them," says Álvaro Jiménez, a researcher at ICMM-CSIC and one of the authors of the study. But to know this, it is necessary to have a meter that operates at the same speed at which these changes occur, which are taking place on the attosecond scale (10^-18 seconds). To achieve this, they have used a new technique based on studies that earned the 2023 Nobel Prize in Physics for Anne L'Huillier, Pierre Agostini, and Ferenc Krausz.

This is the first time that this type of phase of electronic motion in light-guided crystals has been measured: "We send light to a material and that material emits other light that gives us information about the path the electrons have taken through the different bands and, therefore, what the material is like and its properties," describes Rui Silva, also a scientist at ICMM-CSIC and author of the study. These beams of light have a very strong effect on electrons, which end up exploring a greater number of bands and energy ranges, something inaccessible to other spectroscopic techniques.

The role of these two researchers in this work has been based on theoretical and numerical simulation, with their own computational code, to explain what happens in the experiment. "The experimental part measures the light emitted based on laser parameters, and a priori, the physical phenomena explaining what happens are not very well known. That's what our calculations are for," Silva exemplifies.

"We simulate with controlled conditions the experiments to see that what is observed in them is due to changes in electronic dynamics," Jiménez continues. The operations were carried out on the computational cluster of the Max-Born-Institut in Berlin, and each of the calculations required the equivalent of more than 10,000 hours of conventional computing.

"This technique serves a very broad range of materials," Jiménez adds. Silva, on the other hand, highlights that controlling properties with laser light "opens up infinite possibilities at these timescales, especially for controlling quantum properties of materials, but also for creating new properties."

Reference: A. J. Uzan-Narovlansky, L. Faeyrman, G. G. Brown, S. Shames, V. Narovlansky, J. Xaio, T. Arusi-Parpar, O. Kneller, B. D. Bruner, O. Smirnova, R. E.F. Silva, B. Yan, A. Jiménez-Galán, M. Ivanov, and N. Dudovichv: Observation of interband Berry phase in laser driven crystals, Nature, DOI: s41586-023-06828-5