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Ultra-fast dynamics of the refractive index in laser-excited water

We report the experimental study of the ultra-fast modification of the dielectric function of pure water by an intense femtosecond laser pulse. Using a time-resolved optical interferometric technique, we measured the variation of the phase shift, which is proportional to the modification of the real part of the refractive index, as well as the variation of the fringes contrast, proportional to the modification of the absorption coefficient. We first observe a positive phase shift due to Kerr effect and immediately followed a negative one. After 200 fs, the phase shift becomes positive and remains so for at least 3 ps. Using the simple Drude -Lorentz model, we interpret this evolution as the result of electron self-trapping.