Influence of Different Gases on Ionisation-Induced Refractive Index Changes in Gas-Filled Hollow-Core PCF

Recombination in gas ionised by femtosecond laser pulses generates heating and sound waves. The resulting transient refractive index changes can, at high enough laser repetition rates, affect propagation of successive pulses. In gas-filled hollow-core photonic crystal fibre (HC-PCF), where pulses with μJ energies are sufficiently intense to ionise the gas, pulse-compression and UV-generation experiments revealed a clear dependence of the nonlinear dynamics on repetition rate [1]. In a recent study a probe laser was transmitted through a single-ring HC-PCF from the side to detect the single-shot temporal refractive index evolution [2]. The results suggest using atomically lighter gases to reduce detrimental ionisation effects, however capillary vibrations driven by plasma-induced sound waves caused additional probe-phase modulation which complicated a clear experimental proof.