Fig. 1. Experimental setup for pump-probe-based diffractometry.

Fig. 2. Images of the probe pulses recorded by CCD at different delay times. The negative sign in (a) and (b) denotes that the probe pulse is ahead of the pump pulse in time.

Fig. 3. Schematic diagram of calculating the propagation of the probe beam along the filament.

Fig. 4. (a) Radial intensity distribution recorded by CCD at delay time −396 ps for low-density air channel. (b) Extracted transverse distribution of the refractive index variation in (a).

Fig. 5. (a) Simulated radial intensity distribution of probe beam at exit of the plasma channel when the delay time is 0.98 ps. (b) The diffraction fringe imaged on the CCD camera along radial direction at 0.98 ps. Blue solid-circle line is experimental data; orange solid line is the best fitting curve extracted from our simulation.

Fig. 6. Temporal evolutions of (a) peak electron density and (b) filament diameter. The diamonds and squares are extracted from experimental results. The black solid curve is obtained by solving the rate equations of plasma decay. The inset figures are the zoom-in results near zero delay. (c) Transverse distribution of electron density at different delay times.