Ultrafast plasma studies by phase and amplitude measurements with femtosecond spectral interferometry

Using the technique of frequency-domain interferometry, we study the dynamics of ultrafast laser-produced plasmas at moderate laser irradiances. From the complex reflection coefficient measured with the two in-quadrature probe pulse polarizations, we determine the peak electron temperature during the pump laser pulse, the gradient scale length at critical density, the variation of the real and imaginary part of the dielectric constant of the plasma, and, using the Drude model, the collisionality of the plasma as a function of time and temperature. We find electron relaxation collision frequencies which depart from those predicted by the model of Lee and More (Phys. Fluids 1984;27:1273.) by no more than a factor of two.