Let there be extreme ultraviolet light

When an intense laser is focused into a noble gas, it initiates a nonlinear process called high-order harmonic generation (HHG). Under certain conditions, odd harmonics of the laser’s original frequency are emitted, extending into the extreme ultraviolet range. This process can be performed in small laboratories, enabling accessible, advanced nanoscale imaging or the study of ultrafast atomic and molecular systems.

However, HHG is a highly inefficient process. It strongly depends on intense laser properties, such as its pulse duration, the influence of which has not been systematically explored.

Westerberg et al. developed a setup using an industrial-grade laser delivering ultrashort pulses paired with a compact multi-pass cell, an optical resonator formed by two spherical mirrors.

By bouncing between mirrors, the pulses interact with a nonlinear medium and spectrally broaden.

“Since the broadening process depends on laser intensity, we can control how much we change the spectrum and therefore the laser duration,” said author Saga Westerberg. “Our system lets us tune pulse durations without changing any other property of the laser light.”

The researchers investigated HHG in argon gas with pulses ranging from 45 to 180 femtoseconds and successfully reproduced their experiments numerically. They found that required laser intensities depended on pulse duration, and that there are optimal pulse durations that maximize efficiency at these intensities.

“Our results show that efficient buildup of the extreme UV light can be achieved for both long and short pulses, but at significantly different laser intensities, leading to drastically different pulse energy requirements for the laser source,” said Westerberg. “This can serve as a guideline when utilizing a specific laser system to efficiently generate table-top extreme UV light.”

Source: “Influence of the laser pulse duration in high-order harmonic generation,” by S. Westerberg, M. Redon, A.-K. Raab, G. Beaufort, M. Arias Velasco, C. Guo, I. Sytcevich, R. Weissenbilder, D. O’Dwyer, P. Smorenburg, C. L. Arnold, A. L’Huillier, and A.-L. Viotti, APL Photonics (2025). The article can be accessed at https://doi.org/10.1063/5.0272968 .

This paper is part of the Advances enabled by Ytterbium: from advanced laser technology to breakthrough applications collection, learn more here .