By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 10~(18)W/cm~2 intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15 % never achieved up to date in relativistic regimes.
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