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An ultrastable, scalable and repeatable method for generating soft X-ray beams using a custom-built 3-micron ultrafast laser that is focused into an anti-resonant hollow-core fiber. Image credit: Steven Burrows / JILA

Producing coherent (or laser like) soft X-ray beams in a lab-scale setup represents a many decades-long challenge. Scientists in physics, chemistry, and materials science can use soft X-ray light to study the nanoscale properties of materials and biological systems, to capture behaviors that cannot be seen using visible or even ultraviolet light. But here’s the catch: soft X-rays are notoriously hard to make. To get them, most researchers must travel to large, government-funded synchrotrons—billion-dollar machines, that have limited access and stability. These trips are often rushed, competitive, and only available a few times a year.

Now, a team led by JILA Fellows and ���Ҵ�ý�ƽ������ professors Margaret Murnane and Henry Kapteyn has made a significant advance to make soft X-rays more accessible: with their research group, they have developed an ultrastable, scalable and repeatable method for generating soft X-ray beams using a custom-built 3-micron ultrafast laser that is focused into an anti-resonant hollow-core fiber. This breakthrough, detailed in a paper recently published in APL Photonics, builds on well over a decade of laser development. It presents a technological and experimental advance in high-harmonic generation (HHG), the nonlinear optical process by which high-frequency light is created from lower-frequency driving lasers. The team’s past breakthroughs had shown that the key to generating bright coherent soft X-ray beams was to use mid-infrared (2 – 4 µm) driving lasers focused into a waveguide filled with high-pressure gas. However, no good robust drive lasers existed. In this new breakthrough, the team made giant leaps in transitioning the technique from a heroic optics experiment towards a reliable, applications-oriented light source.

“We wanted to make a coherent X-ray source that doesn’t require a team of optics experts to babysit—something that could find applications in labs across various scientific disciplines and industries,” says JILA research associate Drew Morrill, one of the lead scientists on the project and the paper’s co-first author.��
Drew and the team have made a huge step forward by creating bright, ultrastable, coherent soft X-ray beams. In the future, they can enable higher-resolution microscopes that can work in a stroboscopic mode—for example, by capturing nanoscale processes in nanoelectronic, quantum, energy and biological systems, making it possible to understand and optimize them.

A Decade in the Making

Developing JILA’s compact soft X-ray source took over ten years of effort—refining a homebuilt 3-micron wavelength ultrafast laser system when no commercial options existed. From the beginning, the goal was ambitious: to build a mid-infrared laser that was not only powerful and ultrafast but stable enough to operate for entire days without interruption.

To reach that level of performance, the team had to learn how to build fiber lasers from the ground up. That meant mastering delicate tasks like fiber splicing, amplifier construction, and dispersion balancing—adapting technologies initially designed for telecommunications into a new realm of nonlinear fiber optic to seed high power lasers.

One key laser advance came during the early months of the COVID-19 pandemic when the team collaborated with ���Ҵ�ý�ƽ������ Engineering and Physics Professor Scott Diddams. “Scott’s group gave us a roadmap—parts lists, layout guidance, and design principles,” says JILA research scientist Michaël Hemmer, one of the paper’s lead authors. “Then we built it ourselves. The pulses provided by this front-end are outstandingly stable and really the cornerstone of the laser system. These pulses are then amplified using a home-built ytterbium-doped crystal amplifier, providing the high energy needed for HHG while maintaining a clean, controlled beam.”

“The cryogenic ytterbium amplifier is also a second key building block of the system, but it can only run reliably because the front-end is exceptionally reliable; otherwise, it would destroy itself all the time,” notes Hemmer.

Another key contributor was European physicist Dr. Gunnar Arisholm, who shared advanced simulation code that helped the team model complex optical interactions in nonlinear crystals.

“It saved us months of trial and error,” says Hemmer. “He helped train Drew to use the code, which was instrumental in getting the final version running.”

And finally, the key advance was to use optimized waveguides for efficiently converting the laser light into coherent soft X-ray beams.

The line of the first three OPA's that amplify the 3-micron beam. The green light is the parasitic second harmonic light of the 1-micron pump, and the red light is the sum frequency of the 1-micron pump and the 1.5-micron signal. Credit: Gabriella “Gabi” Seifert / JILA

Building and Testing a New Instrument

After designing and re-designing the laser system featuring a fiber-laser-seeded optical parametric chirped-pulse amplifier (OPCPA), the team was finally able to deliver 3 µm wavelength laser pulses with exceptional power and stability. To upconvert this laser light into soft X-rays, the laser pulses are guided through an engineered anti-resonant hollow-core fiber (ARHCF) filled with high-pressure noble gas. Working as a “conductor” for the light, the fiber acts as a waveguide and a container for the interaction medium, allowing the laser and the emitted soft X-rays to travel in phase and interfere constructively over large lengths—opening the door to a new regime of compact, high-brightness sources.

“The laser light travels through the fiber, ionizes the gas, and emits harmonics—overtones of light—far above the frequency of the original beam,” explains JILA graduate student and co-first author Will Hettel.

This process, known as high-harmonic generation (HHG), converts mid-infrared laser pulses into coherent soft X-ray light—similar to how plucking a violin string produces overtones from a single note.

To support this process, the team, with the help of JILA instrument maker and co-author James Uhrich, engineered a precision target system with a modular design: a chassis that allows rapid reconfiguration for different gases and geometries, streamlining the experimental workflow.

“We designed a setup where we can swap out fiber cartridges with micron-level precision,” says Hettel. “It stays aligned even under 10 atmospheres of pressure.”

In terms of output, the system generates soft X-ray photons at energies exceeding 280 eV, reaching the carbon K-edge—a crucial spectral region for biological and materials science applications.

From their design, the researchers found that the setup can run at kilohertz-level repetition rates with continuous, stable beam output for several hours or longer with minimal fluctuation. The system is also rather robust, showing no signs of optical damage even after months of operation. This level of durability is essential for research workflows that demand high uptime and minimal maintenance.

“This isn’t a one-off result,” said Hemmer. “We can run it for days. The beam doesn’t drift. The power doesn’t degrade. That makes it incredibly useful for real experiments.”

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Simulating a Symphony

While the laser system was being constructed, another crucial component of the project unfolded in parallel: advanced simulations. To better understand and optimize the HHG process, JILA graduate student Ben Shearer helped develop a fast and flexible numerical model.

“Simulations like this normally take days or weeks to run,” Shearer explains. “We created a version that runs in hours or even minutes—without sacrificing too much of the physics.”

His code, based on a parameterized version of the strong-field approximation, allowed the team to virtually test a wide range of laser pulse durations, energies, and gas conditions before trying them in the lab.

“Ben’s work gave us a cheat sheet,” notes Hemmer. “We could avoid dead ends and prioritize ideas that had a real shot at working.”

These simulations also laid the groundwork for future upgrades, such as transitioning from argon to helium to achieve even higher photon energies.

“If you want to go to the absolute highest energy of high harmonic generation, you need to ionize helium,” says JILA graduate student Gabriella “Gabi” Seifert. “We're getting there; it’s just taking it one step at a time.”

Helium’s higher ionization potential allows stronger driving fields without over-ionizing the medium—a key requirement for pushing HHG to higher energy regimes.

A view of the argon gas cell that the laser is beamed through to produce HHG, showing the fifth harmonic (yellow) and seventh harmonic (blue). Credit: Drew Morrill and Grzegorz Golba / JILA

A World of Possibilities

By building a stable, coherent soft X-ray source that fits on a lab bench, the team has opened the door for broader scientific access to a tool that once required massive infrastructure with limited access.

“We’re really just scratching the surface of what this source can enable,” says Morrill. “With this kind of stability and control, we can start to ask questions that were previously only addressable at synchrotron or free-electron laser facilities, and even go beyond what was possible before.”

Potential applications include high-resolution soft X-ray microscopy of carbon-rich biological material—opening up the possibility of live cell imaging without the need to add light-emitting fluorescent molecules or without the need to freeze the sample.

“This spectral regime is well suited for high-resolution biological imaging,” says JILA graduate student Clay Klein��
Other uses lie in probing advanced magnetic materials, such as those explored for ultra-low-energy computing or data storage technologies based on electron spin.

“There’s a long history of new light sources unlocking unexpected science,” said Morrill. “We’re excited to see where this one leads.”
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This research was published in .
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