Fig. 3: Visualization of light-speed propagation of an air plasma ionization front by COFT measurement of the probe pulse.
a Schematic diagram of the pump-probe experiment visualizing the light-speed propagation of the air plasma ionization front in air. The pump pulse (1 mJ, 40 fs) propagating along the x-direction is focused and ionizes the air. The probe pulse detects the pump-induced refractive index shift transversely and enters a COFT system based on wavefront retrieval and non-collinear FROG, with the interacting region as CP. The inset cartoon shows the process of the probe picking up the pump-induced phase shift. At time t1, the pump and the plasma channel overlap with the leading edge of the probe; at time t2, with the probe central part; at time t3, with the trailing edge. b Representative frames of the probe pulse transverse phase shift profiles in the x-y plane at \(t = 0,285,579\) fs. The yellow dashed line shows the position of the ionization front at different time. c Linear fitting of the ionization front positions at different time, and the slope is consistent with light speed. d Free electron density profile along the black dashed line in b, reconstructed via a standard Abel inversion scheme
