From the Article

>For the first time, scientists were successful at driving at a thousand shots per second a so-called plasma mirror in the relativistic regime, i.e. with a laser field so strong that hurls the plasma electrons back and forth at nearly the speed of light. The feat was accomplished at the LOA (Laboratoire d’Optique Appliquée) in France.
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>When an intense laser pulse ionizes the surface of a solid target, it creates plasma so dense that it is impenetrable to the laser, even if the target was initially transparent. The laser now gets reflected off this “plasma mirror.” In the relativistic regime, the mirror surface no longer just sits stills but is driven to oscillate so fast that, through a process called relativistic surface high-harmonic generation (SHHG), it temporally compresses the laser’s electromagnetic field cycles. This concentrates the laser energy further in time and makes plasma mirrors a promising path for the generation of ever more intense and shorter laser pulses.