Free-electron laser demonstrates attosecond hard X-ray pulses

Researchers at the Linac Coherent Light Source have experimentally demonstrated attosecond hard X-ray pulses produced by a free-electron laser. Measurements of nonlinear X-ray fluorescence indicated pulse durations of 100–400 attoseconds, with shot-to-shot fluctuations across that range. For the shortest pulses, the inferred time–bandwidth product was approximately 2 femtoseconds-electronvolts, suggesting that a fraction were nearly transform-limited.

The result is based on amplified spontaneous emission from 3d transition metals, a nonlinear optical phenomenon whose yield provides information about the temporal structure of ultrashort X-ray pulses. By analyzing this fluorescence, the team identified the generation of attosecond pulses and measured variations in their duration from shot to shot. The experiment was conducted at the Linac Coherent Light Source using ultrashort free-electron-laser pulses.

One test involved nanofocused XFEL pulses exciting a 20-micrometre-thick copper foil. The resulting collective X-ray fluorescence was used as a diagnostic of the incoming pulse. The researchers examined the yield of the amplified spontaneous emission and inferred both the presence of attosecond-scale pulses and the range of durations produced during the experiment.

Across the measurements, inferred pulse durations extended from 100 as to 400 as. The result therefore demonstrates not only the production of attosecond hard X-rays but also substantial variation between individual pulses. Hard X-ray pulses combine short temporal duration with high spatial resolution.

The authors state that attosecond hard X-ray pulses can be used to probe matter on the timescales of electronic phenomena and with atomic spatial resolution. The researchers also identify a prospective route to single-shot, damage-free X-ray measurements. If an attosecond pulse can complete a measurement before the fastest radiation-damage processes develop, it may be possible to obtain information from an X-ray interaction before damage becomes consequential.

Technically, the central advance is the conversion of a nonlinear fluorescence measurement into evidence for attosecond pulse generation in the hard X-ray regime. Amplified spontaneous emission was chosen as the diagnostic because its collective X-ray-fluorescence yield responds nonlinearly to the ultrashort excitation. Analysis of that yield allowed the researchers to extract attosecond-scale information and characterize changes in pulse duration between shots.

The experiment therefore provides an experimentally verified source of hard X-ray pulses with durations measured in the attosecond range. It also exposes an important practical characteristic: the pulses are not uniform. Durations varied from 100 to 400 as, yet the time–bandwidth analysis indicates that some of the shortest pulses were nearly transform-limited.

Subject of Research: Experimental generation and characterization of attosecond hard X-ray pulses using nonlinear X-ray fluorescence from transition metals

Article Title: Experimental demonstration of attosecond hard X-ray pulses

Article References: Inoue, I., Robles, R., Halavanau, A., Guo, V., Linker, T. M., Benediktovitch, A., Chuchurka, S., Seaberg, M. H., Sun, Y., Zhu, D., Cesar, D., Ding, Y., Esposito, V., Franz, P., Sudar, N. S., Zhang, Z., Osaka, T., Yamaguchi, G., Sano, Y., … Marinelli, A. (2026). Experimental demonstration of attosecond hard X-ray pulses. Nature Photonics. https://doi.org/10.1038/s41566-026-02026-7

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02026-7

Keywords: attosecond pulses, hard X-rays, free-electron laser, Linac Coherent Light Source, amplified spontaneous emission, transition metals, pulse characterization, nonlinear fluorescence, ultrafast science, radiation damage, Experimental, demonstration