Scientists create vacuum-ultraviolet attosecond pulses to track ultrafast processes of natural systems

Electrons in atoms interact with each other and with other particles, changing their motion, energies, and other features at incredibly fast timescales, on the order of attoseconds (10^-18 seconds). Capturing these ultrafast changes demands ultrafast light pulses. The pulse’s duration needs to be more or less the same as the effect’s; otherwise, it would be like trying to capture a hummingbird’s wing motion with a slow, long-exposure camera.

At the end of the XIX^th century, physicists thought that only femtosecond pulses (10^-15 seconds) were technically possible. That started to change in the late 1980s, when physicists linked high harmonic generation with attoscience. High harmonic generation (HHG) is a process that upconverts low-frequency photons to higher frequencies, and what these researchers showed was that, when multiple harmonics are emitted, they can combine to form an attosecond pulse of light —something that was finally realized in 2001.

Attosecond science was born by generating and then employing extreme-ultraviolet (XUV) pulses, and as a consequence the methods developed to detect and characterize them focused in this frequency range. More than 20 years later, the creation of attosecond pulses to perform attoscience tasks remains XUV centered. Despite the many advances that XUV attosecond pulses have provided, they also pose a challenge. Most atoms, when hit by such an energetic light source, loose one or more electrons and become positively charged — a process known as ionization. But many worth studying processes in nature occur with non-ionized atoms, which remain in the so-called bound state. Since XUV light does not provide access to the bound states of the natural systems, their study has remained out of reach for attoscience. To address this, a source delivering less energetic attosecond pulses (for instance, in the vacuum-ultraviolet spectral range) and new methods to measure their main features (duration, intensity, etc.) are needed.

This has now been done by an international team of researchers from ELI-ALPS, Guangdong Technion- Israel Institute of Technology, Technische Universität Wien, Université de Bordeaux–CNRS–CEA, Foundation for Research and Technology-Hellas (FORTH)and ICFO researchers, Philipp Stammer, Dr. Javier Rivera-Dean and ICREA Prof. Maciej Lewenstein. For the first time, the team has demonstrated that semiconductors illuminated by strong mid-infrared laser light emit vacuum-ultraviolet (VUV) attosecond pulses, has retrieved the pulses’ temporal shape and has measured their total duration. These unprecedented results, published in Nature Communications, establish the basis of a novel technique for probing the ultrafast changes that occur in most natural systems, preserving their bound state rather than inducing their ionization.

Producing and characterizing VUV attosecond pulses

To confirm the generation of attosecond VUV pulses and understand their properties, researchers designed and implemented a setup that combined semiconductor materials with strong laser fields.

In the experiment, a mid-infrared (mid-IR) light beam illuminated a semiconductor crystal. This generated high harmonics, whose superposition formed VUV attosecond pulses. These VUV pulses were directed to cesium atoms, one of the few species that, in clear contrast with those used in conventional attoscience schemes, can be ionized by VUV light. As expected, in the presence of the mid-IR field, VUV pulses ionized the cesium atoms. By detecting and analyzing the ejected electrons, the researchers could measure the attosecond synchronization of the high harmonics constituting the VUV pulses, which was key to extract information about them.

The experimental results coincided with the theoretical predictions confirming the generation of VUV pulses with duration of 950 attosecond.

Potential for future techniques

Their findings, thus, introduce semiconductors illuminated by strong lasers as new attosecond sources and showcases their potential for studying ultrafast processes in a wider range of materials. “We have developed the tools for tracing ultrafast bound state dynamics —which occur very often in nature— in all states of matter, including atoms, molecules-in gas and liquid phase and solids”, explains Dr. Paraskevas Tzallas, FORTH researcher and corresponding author of the article. 

The authors also highlight that, according to previous studies, HHG radiation coming from atoms can produce quantum light, showing features like entanglement or squeezing. Their research represents a crucial step toward experimentally demonstrating this in semiconductors, which once achieved would enable new applications in several quantum information processing tasks.

Overall, Dr. Tzallas thinks that “these new tools and methodology could be used for conducting studies in natural systems, investigating their ultrafast dynamics and even possibly using them to engineer novel quantum light states.”

Reference:

Nayak, A., Rajak, D., Farkas, B. et al. Attosecond metrology of vacuum-ultraviolet high-order harmonics generated in semiconductors via laser-dressed photoionization of alkali metals. Nat Commun 16, 1428 (2025).

DOI: https://doi.org/10.1038/s41467-025-56759-0

###

Acknowledgements:

We thank Bálint Kiss, Levente Ábrók and Rajaram Shrestha for their technical support and their efforts on the operation of the mid-IR laser system. We also thank Arnold Péter Farkas, for the methods that he developed for introducing the Cs sample in the interaction chamber. The experiments were carried out at ELI ALPS, and ELI-ALPS is supported by the European Union and co-financed by the European Regional Development Fund (GINOP-2.3.6-15-2015-00001).

Tzallas group at FORTH acknowledges support from: The Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT) under grant agreement CO2toO2 Nr.:015922, the European Union’s HORIZON-MSCA-2023-DN-01 project QU-ATTO under the Marie Skłodowska-Curie grant agreement No 101168628, the LASERLABEUROPE V (H2020-EU.1.4.1.2 grant no.871124), The H2020 Project IMPULSE (GA 871161), and ELI–ALPS.

Lewenstein group at ICFO acknowledges support from: ERC AdG NOQIA; Ministerio de Ciencia y Innovation Agencia Estatal de Investigaciones (PGC2018–097027–B–I00 / 10.13039/ 501100011033, CEX2019–000910–S / 10.13039 / 501100011033, Plan National FIDEUA PID 2019–106901GB–I00, FPI, QUANTERAMAQS PCI 2019–111828–2, QUANTERA DYNAMITE PCI 2022–132919, Proyectos de I+D+I “Retos Colaboración” QUSPIN RTC 2019–007196–7); MICIIN with funding from European Union Next Generation EU (PRTR–C17.I1) and by Generalitat de Catalunya; Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (European Social Fund FEDER and CERCA program, AGAUR Grant No. 2021 SGR 01452, Quantum-CAT U16–011424, co-funded by ERDF Operational Program of Catalonia 2014-2020); Barcelona Supercomputing Center MareNostrum (FI–2022–1–0042); EU Horizon 2020 FET–OPEN OPTOlogic (Grant No 899794); EU Horizon Europe Program (Grant Agreement 101080086—NeQST), National Science Centre, Poland (Symfonia Grant No. 2016/20/W/ST4/00314); ICFO Internal “QuantumGaudi” project; European Union’s Horizon 2020 research and innovation program under the Marie-Skłodowska-Curie grant agreement No 101029393 (STREDCH) and No 847648 (“La Caixa” Junior Leaders fellowships ID100010434 : LCF / BQ / PI19 / 11690013, LCF / BQ / PI20 / 11760031, LCF / BQ / PR20 / 11770012, LCF / BQ / PR21 / 11840013).

Stammer acknowledges funding from: The European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847517. J. Rivera-Dean acknowledges funding from: the Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya, the European Social Fund (L’FSE inverteix en el teu futur)–FEDER, the Government of Spain (Severo Ochoa CEX2019-000910-S and TRANQI), Fundació Cellex, Fundació Mir-Puig, Generalitat de Catalunya (CERCA program) and the ERC AdG CERQUTE. Y. Mairesse acknowledges funding from the Agence Nationale de la Recherche (ANR)– Shotime (ANR-21-CE30-038-01), and thanks Samuel Beaulieu for fruitful discussion. M. F. Ciappina and C. Granados acknowledge financial support from the Guangdong Province Science and Technology Major Project (Future functional materials under extreme conditions - 2021B0301030005) and Guangdong Natural Science Foundation (General Program project No. 2023A1515010871).