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

One of the main difficulties of efficiently generating high-order harmonics in long neutral-gas targets is to reach the phase-matching conditions. The issue is that the medium cannot be sufficiently ionized by the driving laser due to plasma defocusing. We propose a method to improve the phase-matching by pre-ionizing the gas using a weak capillary discharge. We have demonstrated this mechanism, for the first time, in absorption-limited XUV generation by an 800 nm femtosecond laser in argon and krypton. The ability to control phase-mismatch is confirmed by an analytical model and numerical simulations of the entire generation process. Our method allows to increase the efficiency of the harmonic generation significantly, paving the way towards photon-hungry applications of these compact short-wavelength sources.

Automated summary

One of the main challenges in efficiently generating high-order harmonics in long neutral-gas targets is achieving phase-matching conditions. The issue lies in the fact that the driving laser is unable to sufficiently ionize the medium due to plasma defocusing. In order to improve phase-matching, we propose a method of pre-ionizing the gas using a weak capillary discharge. Through experimental demonstrations in argon and krypton with an 800 nm femtosecond laser, we have successfully shown this mechanism for the first time. The ability to control phase-mismatch has been validated through an analytical model and numerical simulations of the entire generation process. Our method significantly increases the efficiency of harmonic generation and opens up possibilities for photon-hungry applications of these compact short-wavelength sources.