Shortest pulse of soft X-ray light in 19.2-attoseconds, opening a new window on electron dynamics

Researchers at ICFO have generated and measured a 19.2-attosecond soft X-ray pulse, the shortest flash of light ever reported. The breakthrough establishes a new global benchmark in ultrafast science and allows scientists to observe in detail how electrons move within atoms, molecules, and materials.

Electrons determine everything: how chemical reactions unfold, how materials conduct electricity, how biological molecules transfer energy, and how quantum technologies operate. But electron motion happens on attosecond timescales—far too fast for conventional measurement tools. The new 19.2-attosecond soft X-ray pulse effectively creates a camera capable of capturing these elusive dynamics in real time with unprecedented detail, enabling researchers to observe processes never before observed.

Flashes of light in the soft X-ray spectral range provide fingerprinting identification, allowing scientists to track how electrons reorganize around specific atoms during reactions or phase transitions. Generating an isolated pulse this short required innovations in high-harmonic generation, advanced laser engineering, and advances in attosecond metrology. Together, these developments allow researchers to observe electron dynamics exactly where they occur: using the core-level orbitals that define material properties.

The journey towards this milestone, however, began in 2015, when Prof. Jens Biegert’s team pioneered the generation of soft X-rays attosecond pulses in the regime by successfully isolating attosecond bursts of this light. These pulses demonstrated their breakthrough utility by resolving the interaction of electrons with the crystal lattice in a solid [1] and by elucidating how and when a molecular ring opens [2] as a precursor to processes such as polymerization. But, at that time, the method for pinpointing the duration had limitations that could now be overcome, leading to the demonstration of the shortest pulse ever measured.

“When I came to the group and saw the streaking traces, I had to look into this with a new method,” shares enthusiastically the first author, Dr. Fernando Ardana-Lamas. “Finally, we can say that, to the best of our knowledge, we have confirmed the shortest pulse of light in the world!”

“This new capability paves the way for breakthroughs in physics, chemistry, biology, and quantum science, enabling direct observation of processes that drive photovoltaics, catalysis, correlated materials, and emerging quantum devices”  explains Prof. Biegert, reflecting on the future of attosecond soft X-ray pulses below the atomic unit of time. As he puts it, now that the foundations are laid, “the sky is the limit.”

Some recent results using our attosecond methodology in solids and molecules:

[1] T.P.H. Sidiropoulos, N. Di Palo, D.E. Rivas, S. Severino, M. Reduzzi, B. Nandy, B. Bauerhenne, S. Krylow, T. Vasileiadis, T. Danz, P. Elliott, S. Sharma, K. Dewhurst, C. Ropers, Y. Joly, K. M. E. Garcia, M. Wolf, R. Ernstorfer, J. Biegert, “Probing the Energy Conversion Pathways between Light, Carriers, and Lattice in Real Time with Attosecond Core-Level Spectroscopy”, Phys. Rev. X. 11, 041060 (2021).

[2] S. Severino, K.M. Ziems, M. Reduzzi, A. Summers, H.-W. Sun, Y.-H. Chien, S. Gräfe, J. Biegert, “Attosecond core-level absorption spectroscopy reveals the electronic and nuclear dynamics of molecular ring opening”, Nature Photon. 18, 731-737 (2024).