Gas Mixtures to Suppress Thermal Buildup Effects Caused by High-Repetition-Rate Photoionization of Confined Gases

Gas-based systems (e.g. based on hollow-core fibres) are extensively used over a wide spectral range to temporally compress ultrashort pulses. Recent progress in solid-state lasers and compression schemes has enabled the generation of sub-femtosecond pulses at unprecedented energies and average powers [1] – [3] . However, the latest generation of lasers deliver mJ pulses at hundreds-of-kHz repetition rates [4] , which raises new challenges for gas-based pulse compression. At high repetition rates, even weak single-shot energy deposition in the gas (e.g. by ionisation-driven heating) is sufficient to cause a transversely non-uniform gas density depression to build up pulse-by-pulse [5] , leading to thermal instabilities. Using lighter noble gases is often insufficient to mitigate this, despite the weaker nonlinear absorption. This is because sufficient nonlinearity for spectral broadening requires substantially higher pressures, resulting in much slower gas relaxation (the thermal diffusivity scales inversely with gas density).