The Extreme Light Infrastructure ERIC

Phase-matched high-order harmonic generation in pre-ionized noble gases

A beamline based on high-order harmonic generation (HHG) is one of the secondary sources being operated at ELI Beamlines, providing a stable source of XUV radiation for user research. The team operating this instrument and investigating the generation process itself has recently published their new results in Nature’s journal Scientific Reports. Ondřej Finke, the last year Ph.D. student and first author of the article explains: “During high harmonic generation, IR laser nonlinearly interacts with a gaseous target, leading to generation of extreme ultraviolet radiation composed of odd harmonic orders of the fundamental laser field. The conversion efficiency of the generation process is, however, very low, forcing research to focus on improving this or developing entirely new schemes of generation. In the work published in Sci. Rep., we have made a significant step towards improving the conversion efficiency of the generation process”.

In the paper published by ELI Beamlines team, in collaboration with French scientists, research focuses on studying and enhancing the generation efficiency in long gaseous media. Using numerical simulations, the team identified previously unknown issues of HHG caused by the nonlinear propagation of the laser. As an intense laser pulse propagates through a long medium, it is subject to plasma-induced defocusing, which prevents the efficient generation of the XUV radiation in the later part of the medium. We have found a way to overcome this bottleneck. Using a specialized target consisting of weakly pre-ionized gas, we can artificially change the conditions of generation, pushing it into the optimal window. Within an experimental implementation of the concept, we were able to enhance the XUV signal generated by our near-infrared laser by a factor of 5 and 6 in long krypton and argon medium, respectively. Moreover, robust numerical simulations, as well as a simplified analytical model, confirmed the functionality of this method with a theoretical enhancement of up to 8 for the cases under investigation.

“Both the discovery of a new issue in HHG with a long gaseous medium and the presentation of a novel type of generating medium are important steps in advancing research of high harmonic generation and its applications,” said Jaroslav Nejdl, the principal investigator of the project.

The paper in Nature’s Scientific Reports entitled “Phase-matched high-order harmonic generation in pre-ionized noble gases”.