Exploiting dark autoionizing states for enhancing extreme ultraviolet lasers

An international research team led by Professors Tsuneyuki Ozaki and François Légaré at the Institut national de la recherche scientifique (INRS), has developed a unique method to enhance the power of a laser source emitting extreme ultraviolet light pulses. The underlying mechanism of the newly observed phenomenon involves the unique role of dark-autoionizing states through coupling with other pertinent electronic states.

Thanks to this work, the team will be able to study the ultrafast dynamics of a single dark autoionizing state at the femtosecond timescale, which was previously impossible due to its inability to undergo single-photon emission or absorption, combined with the ultrashort lifetime of these states.

Recently published in the journal Physical Review Letters, their results allow the generation of ultrafast extreme ultraviolet light relevant for advanced ultrafast science applications such as angle-resolved photoemission spectroscopy and photoemission electron microscopy.

This work was done in collaboration with Professor Vasily Strelkov at the Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia, and Research Assistant Professor Muhammad Ashiq Fareed at the University of Nebraska-Lincoln, USA.

Unraveling the mysteries of the dark-autoionizing states

In their laboratories at the Énergie Matériaux Télécommunications Research Centre, Professors Tsuneyuki Ozaki and François Légaré, along with Ph.D. student Mangaljit Singh, have been exploiting special types of electronic states, known as dark-autoionizing states. Their work was accomplished using high-order harmonic generation, an optical phenomenon unconventional to laser physics.

“The newly published results are a step forward not only in understanding the behavior of dark autoionizing states under intense ultrafast laser-matter interactions, but also in bringing intense extreme-ultraviolet laser sources from large-scale synchrotron and free-electron laser facilities to the moderate-sized laser laboratories,” says Ph.D. student Mangaljit Singh, first author of the study.

Many limitations imposed by the fundamentals of laser physics restrict most lasers used in medicine, communications, or industry. Likewise, they tend to operate only in the ultraviolet, visible (from violet to red), or the invisible near and mid-infrared wavelength range. However, many advanced scientific applications require lasers to operate at shorter wavelengths in the extreme ultraviolet range.

The state-of-the-art systems employ commercially available primary laser sources for high-order harmonic generation from noble gases to develop secondary sources of coherent extreme ultraviolet light.

In this study, instead of noble gases, Singh and colleagues used a laser-ablated plume (obtained from the laser ablation of a solid material) for the high-order harmonic generation in sync with the unique response of dark-autoionizing states.

They found that under certain resonance conditions governed by the primary laser parameters and the electronic structure of the atomic and ionic species in the laser-ablated plume, the conversion efficiency, and hence the power of the extreme ultraviolet laser source is enhanced by more than ten times. This implies that the same extreme ultraviolet power can be obtained using a primary laser with power that is one-tenth of the power required for a typical noble gas.

In addition to providing an intense extreme ultraviolet light source, this study also shows for the first time the prospect of studying the dynamics of dark autoionizing states at the femtosecond timescale using the technique of high harmonic spectroscopy. Such dark states could be the basis of several quantum technologies, especially in improving the performance of quantum computation.