Plasma Profile along a Hollow-Core Photonic Crystal Fibre Measured with Counter-Propagating Probe Solitons

Short laser pulses undergoing soliton self-compression in gas-filled hollow-core photonic crystal fibre (HC-PCF) can reach intensities sufficient to significantly ionise the gas under many experimental conditions. Due to the long-lived nature of the plasma, this may affect the propagation dynamics of subsequent pulses and thus limit scaling to higher repetition rates [1, 2]. Here we report experimental measurements and a numerical study of the free electron density distribution along the length of a gas-filled HC-PCF including its temporal evolution (with a few fs resolution) over 1 ns after the ionisation event. The plasma causes the effective refractive index of the modes to decrease which, in turns, alters the phase-matching conditions for dispersive wave (DW) generation from counter-propagating probe pulse, which shifts to higher frequencies in the ultraviolet. By measuring this shift we can detect free electron densities as low as ~10
¹⁶
cm
^-3
, corresponding to a drop of refractive index of only ~10
^-6
.