Phase dynamics of reflected probe pulses from sub-100-fs laser-produced plasmas

Using the technique of frequency-domain interferometry, we measure the phase shift between a pair of reflected probe pulses from a plasma produced by 80-fs, 10¹⁵ W/cm² laser interaction on a solid target. A numerical model shows that one cannot interpret phase dynamics merely by adding the phase-shift contributions of the propagation in the underdense plasma and of the moving reflection turning point. The phase-shift difference between S- and P-polarized light at a given angle of incidence is found to vary with the electron density gradient scale length and to be weakly dependent on the plasma collision rate. This phase difference is used to make a snapshot measurement of the radial variations of the density scale length of an aluminum plasma.