1：Experimental Design and Method Selection:

The study employs a pump-probe technique to monitor ultrafast polarizability changes, specifically measuring plasma density in a gas-filled hollow-core photonic crystal fiber. The method involves using a counterpropagating probe soliton to emit dispersive-wave radiation, whose wavelength shift indicates plasma density changes.

2：Sample Selection and Data Sources:

Experiments are conducted on argon- and krypton-filled hollow-core photonic crystal fibers. The fibers have a core diameter of 34 μm and are kept in a pressurized gas cell.

3：List of Experimental Equipment and Materials:

Ti:sapphire laser amplifier, fused-silica beam splitter, half-wave plate, thin-film polarizer, calcium fluoride planoconvex lenses, kagomé hollow-core photonic crystal fiber, magnesium fluoride windows, spectrometer, retroreflector mounted on a linear translation stage.

4：Experimental Procedures and Operational Workflow:

Pulses from a Ti:sapphire laser are divided and delivered to two arms, controlling pulse energies with a half-wave plate and thin-film polarizer. The pulses are launched into opposite ends of the fiber, and the probe light is reflected into a spectrometer. The relative delay between pump and probe is varied to map the plasma density distribution.

5：Data Analysis Methods:

The spectral centroid of the dispersive wave is calculated over a wavelength range of 50 nm. Numerical simulations are performed to compare with experimental results, using the unidirectional full-field nonlinear wave equation and finite element modeling.