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 , probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness ) irradiated by a duration pulse of wavelength and peak irradiance . 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 () and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in .
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