Intense quasi-monochromatic resonant harmonic generation in the multiphoton ionization regime

Resonant high-order harmonics, which result in quasi-monochromatic extreme ultraviolet light with coherent intensity enhancement involving autoionizing resonances, have been demonstrated from laser-ablated plumes in the tunnel-ionization regime. Here, we demonstrate resonant harmonics in the previously unexplored multiphoton-ionization regime. We demonstrate an intense resonant harmonic from gallium with an intensity enhancement ratio of 714 relative to the neighboring harmonics, achieved without the need for extreme ultraviolet filtering methods, thus preventing a typical photon flux loss of more than 70%. Three-dimensional time-dependent Schrodinger equation calculations reveal that this increase in the enhancement ratio is due to the low electron wave packet spreading in the multiphoton-ionization regime. These results reveal a method for increasing the intensity and monochromaticity of intense multimicrojoule femtosecond extreme ultraviolet light and will also facilitate understanding of the involvement of autoionizing resonances in generating resonant harmonics in the multiphoton-ionization regime. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

Resonant high-order harmonics have been demonstrated in the multiphoton-ionization regime, achieving intense harmonics with high intensity enhancement and monochromaticity without the need for extreme ultraviolet filtering methods, thus preventing photon flux loss. This research provides a new method for increasing the intensity of extreme ultraviolet light and facilitates a better understanding of autoionizing resonances in generating resonant harmonics in the multiphoton-ionization regime.