We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).

Aluminium K α emission (1.5 keV) produced by an 8 J, 500 ps, Nd:Glass laser incident at 45° onto a layered target of 0.8 µm thick aluminium (front side) and 1µm thick iron (back side) has been used to probe the opacity of iron plasma. Source broadened spectroscopy and continuum emission analysis shows that whole beam self focussing within the aluminium plasma results in a two temperature spatial distribution. Thermal conduction from the laser-irradiated aluminium into the iron layer, enhanced by the whole beam self focusing, results in iron at a temperature of ~ 10 -150 eV. The iron opacity at photon energy of 1.5 keV is shown to be strongly modified from cold values in agreement with IMP code opacities. Results presented here represent a feasibility study to seed future work using table top laser systems for plasma opacity experiments.

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