Nonlinear propagation of intense ultrashort laser pulses in gases is mostly understood to be influenced only by the pulse itself, i.e., by single-shot dynamics, because relaxation processes are typically faster than the time between pulses. Using single-ring hollow-core photonic crystal fibre (PCF) , strong-field intensities can be reached by µJ-level pulses, available from MHz-level repetition rate fibre lasers. At these high repetition rates it is not clear that pulse-to-pulse interactions can still be neglected. Indeed, experiments involving photoionization show a strong repetition rate-dependence, suggesting that the pulse dynamics are influenced by inter-pulse effects . Recently it has been shown, using interferometric side-probing , that fs photoionization and subsequent recombination-induced heating leads to a drop in refractive index tens of μs long, caused by a gas density depression in the core after an acoustic shock wave has propagated away. Here we report that this modifies the characteristics of the fibre, influencing pulse propagation even at 100 kHz repetition rate.
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