Researchers Achieve Continuous Frequency Tuning of X-ray Light Using Modified Laser Technique

Scientists at TU Wien and the University of California San Diego have developed a method for continuously tuning the frequency of X-ray light - closing a longstanding gap in high-harmonic generation technology where researchers could previously only access discrete, fixed frequency steps with no way to move smoothly between them.

The technique uses a six-metre gas-filled hollow waveguide to modify the output of an ytterbium infrared laser. As the beam propagates through the waveguide, nonlinear interactions with the gas alter the light's spectral composition. The modified beam is then converted to visible wavelengths ranging from blue to yellow-green, and that visible light strikes helium atoms to produce the final X-ray output. Adjusting gas type and pressure controls both the direction and degree of frequency shift.

The approach contrasts with conventional high-harmonic generation, where the emitted X-ray frequencies are determined by fixed harmonics of the driving laser - integer multiples of the fundamental frequency with nothing in between. The new method allows continuous movement across the entire gap between harmonics.

Prof. Tenio Popmintchev of TU Wien described the advance using an analogy: "What we have now built is more like a slide guitar in the X-ray range: we can move continuously between the harmonics, in either direction, and select exactly the energy we need."

The choice of driving light in the visible range rather than infrared carries additional advantages. Visible wavelengths accelerate electrons more efficiently and improve phase-matching conditions, which governs how efficiently the X-ray output builds up during propagation. The result is maintained brightness and narrow spectral linewidth throughout the tuning range.

The practical implications span multiple fields. Precision spectroscopy and nuclear clock development require X-ray sources that can be tuned to match specific atomic transition energies with high accuracy. Semiconductor metrology, used in chip fabrication quality control, similarly benefits from tailored radiation. In each case the new method could replace or supplement large-scale synchrotron facilities with a compact laboratory-scale instrument.

The study was published in Communications Physics. The DOI is 10.1038/s42005-026-02791-5.

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