Peculiarities of supercontinuum generation in high-pressure He, N2, Ar, and CO2 gases

Abstract

The generation of coherent optical radiation with an anomalously broadened spectral content (supercontinuum) during the nonlinear propagation of high-power femtosecond laser pulses in transparent media, including high-pressure gases, holds considerable practical interest for the obtaining of extremally ultrashort (attosecond) optical pulses. This capability is crucial for advancing modern attosecond spectroscopy and the study of matter under extreme conditions. In this work, we report the results of systematic experiments on the generation of broadband supercontinuum in an optical cell filled with various atomic and molecular gases (He, Ar, N₂, CO₂) at pressures ranging from 1 to 50 bar achieved through the focused filamentation of 40 fs pulses from a titanium-sapphire laser. Our measurements demonstrate that, with the exception of helium, all supercontinuum spectra exhibit similar pressure-dependent behavior: an initial strong spectral broadening is followed by saturation of the spectral bandwidth and, in the case of CO₂, even a noticeable reduction relative to its maximum achievable width. To elucidate these findings, we carry out theoretical simulations based on the unidirectional propagation equation for an ultrashort wave packet (UPPE). This analysis reveals that the most likely explanation for the observed effects is the enhanced role of kinetic processes, such as absorption and refraction of the laser pulse within the self-generated plasma as the gas pressure increases. These findings provide valuable insights into the mechanisms governing supercontinuum generation in high-pressure gaseous media.