Synchronising ultrashort X-ray pulses

At the ATHOS beamline of SwissFEL, PSI researchers demonstrated a technique known as mode-locking, which allows fully coherent, ultrashort X-ray pulses to be produced. In the photo, several undulator modules are visible (blue); between each pair are magnetic chicanes used to delay the electrons.
Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer

Scientists at the Paul Scherrer Institute PSI have, for the first time, demonstrated a technique that synchronises ultrashort X-ray pulses at the X-ray free-electron laser SwissFEL. This achievement opens new possibilities for observing ultrafast atomic and molecular processes with attosecond precision.

Scrutinising fast atomic and molecular processes in action requires bright and short X-ray pulses – a task in which free-electron lasers such as SwissFEL excel. However, within these X-ray pulses the light is internally disordered: its temporal structure is randomly distributed and varies from shot to shot. This limits the accuracy of certain experiments.

To tame this inherent randomness, a team of PSI researchers has succeeded in implementing a technique known as mode-locking to generate trains of pulses that are coherent in time. “We can now obtain fully ordered pulses in time and frequency in a very controlled manner,” says accelerator physicist Eduard Prat, who led the study, published in Physical Review Letters. Selected by the journal as Editor’s Suggestion, the study, funded by the EU/ERC project “HERO”, represents a significant step towards the generation of tailored attosecond X-ray pulses and a range of new experiments that are only possible with precisely timed, synchronized light pulses.

A train of coherent X-ray pulses (top and middle plots) is produced with from a modulated electron beam (bottom plot). The spacing of the X-ray pulses is defined by an external optical laser, which a periodic pattern on the electrons.
Image Credit: © Paul Scherrer Institute PSI/Wenxiang Hu

Exploring the attosecond-scale world

X-ray pulses in the attosecond regime give a window into the motion of electrons – the fastest processes in atoms and molecules, which underpin all chemical and physical changes. The 2023 Nobel Prize for Physics recognised the development of the attosecond field with techniques that used optical light, not X-rays. Probing the attosecond timescale with X-ray light demands a similar level of control and precision as needed for optical lasers – yet offers far higher photon energies that can probe core electrons and specific atomic elements.

This is what fully coherent X-ray pulses would represent: controlled and reproducible pulses of light that behave like those from an optical laser.

The consequences of combining of this degree of control with the capabilities of free-electron laser X-ray light – such as exceptional brightness – would be significant. With this development, not only would attosecond X-ray science be enabled, but also other techniques that until now have been limited to the realm of optical lasers:

“With custom-designed pulses in the X-ray regime, new experiments inspired by laser-based quantum optics become possible,” says Gabriel Aeppli, head of the Centre for Photon Science at PSI. “Also, we now have the prospect of a very exact clock according to which attosecond pulses arrive at a sample, which means that we should be able to time phenomena seen by X-rays in gases, liquids and solids with unprecedented precision.” 

A perfectly orchestrated experimental setup

The generation of X-ray pulses at SwissFEL is based on a technique called Self-Amplified Spontaneous Emission (SASE). The electron beam accelerated at the SwissFEL travels through a series of undulators, which are magnets that cause the electrons to wiggle from side to side and emit X-rays. The emitted photons interact with the electrons, causing them to gain or lose energy. In this situation, fast electrons can catch up with slower ones, causing them to bunch together. This microbunching amplifies the emitted X-ray light and increases its coherence. However, this coherence occurs only in space across the beam width, while the longitudinal coherence – that is, the coherence along the pulse – is limited to a small fraction of the electron beam duration, thus temporal and spectral profiles still consist of multiple randomly distributed spikes. 

Temporal coherence can be improved simply by shortening the electron bunch – but this comes at the cost of a broader, less defined spectrum. At the soft X-ray ATHOS beamline of SwissFEL, PSI researchers demonstrated an alternative method: a technique known as mode-locking. In this approach, the electrons pass through a series of tuned magnetic chicanes interleaved between the undulators that delay the trip of the charged particles and increase their temporal coherence. At the same time, an external laser limits the lasing parts of the electron beam with a period matching the chicane delays. 

As a result of both the chicanes and the matching laser, the radiation consists of a train of equally separated, phase-locked, ultrashort pulses. The corresponding photon spectra display a characteristic comb structure of evenly spaced spectral lines. 

As well as exhibiting improved coherence, the pulses generated were in the attosecond time range: “The experiment used a 790-nanometre wavelength laser to modulate the electron beam, producing a train of X-ray pulses separated by 2.6 fs, each pulse lasting well below one femtosecond,” says Prat.

This marks the first experimental demonstration of mode-locking at an X-ray free electron laser, achieved through the precise combination of undulators, lasers, and chicanes in one experimental setup. Prat adds: “This scheme is beautiful. Everything is perfectly in its place, and it is easily controllable.” 

Diagnostics provide the proof

In a study published last year, the PSI team already demonstrated a mode-coupled scheme, which improved coherence in the spectral distribution - that is, it produced a more regular and stable distribution of X-ray frequencies. The main difference between mode-coupled and mode-locked schemes is that the latter, thanks to the external laser, cleans the time domain and further enhances the coherence.

Successfully implementing the mode-locking scheme required proving that the pulses were coherent in time. To this end, advanced diagnostic instrumentation was needed to characterise the pulses. A photon spectrometer downstream of the chicanes determines the energy distribution of the X-ray pulse, while a radiofrequency (RF) deflector device uses information contained in the electron bunch to infer the temporal structure of the X-ray pulse. 

One of the main challenges was to measure the femtosecond periodic structure of the pulses, which required a special setup to achieve attosecond resolution with the RF device. “This study demonstrates the careful orchestration of many aspects that need to work in harmony – from the chicane and the external laser to the fidelity of the diagnostics,” notes Prat.

The next step will be to test the new capabilities to generate time-coherent X-ray with real experimental samples, rather than purely diagnostic setups. Future work will further enhance the coherence of X-ray pulses by using more chicanes and reducing electron pulse duration. In the long term, the ability to make highly coherent and tailored X-ray pulses will benefit users in wide-ranging applications, from attosecond science to quantum optics. 

Published in journal: Physical Review Letters

Title: Demonstration of Mode-Locked Frequency Comb for an X-Ray Free-Electron Laser

Authors: Simon Gerber, Martin Huppert, Stefan Neppl, Sven Reiche, Thomas Schietinger, Neil Thompson, Alexandre Trisorio, Carlo Vicario, Alexander Zholents, and Eduard Prat

Source/Credit: Paul Scherrer Institute | Hector Garcia Morales and Miriam Arrell

Reference Number: phy010526_01

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