Twin Femtosecond Pulses Interferometry in Plasmas

Abstract

A key issue in the field of laser-matter interaction using very intense femtosecond optical pulses is the understanding of the initial processes leading to the generation of dense hot plasmas. For instance, in the case of the interaction with metallic targets, it is important to obtain a time- and space-resolved picture of the critical density layer of very small gradient scale length 1/λ<0.1, since it defines the region where most of the energy transfer takes place. Likewise, in the case of an initially transparent target, such as a dielectric solid or a neutral gas, study of the early stages of the evolution towards a plasma requires a space- and time-resolved picture of the creation of free carriers. Using standard all-optical pump/probe detection methods it is relatively easy to obtain a spatially-integrated evolution of the system with subpicosecond time resolution. It is much more difficult to obtain the same information with a good spatial discrimination. For instance, time-resolved Schlieren measurements can determine the location of the critical density layer of an expanding plasma created at the surface of a metal with femtosecond accuracy.[1] However diffraction effects limit the spatial accuracy to a value of the order of the incident wavelength.