Relativistically intense XUV radiation from laser-illuminated near-critical plasmas

Pulses of extreme ultraviolet (XUV) light, with wavelengths between 10 and 100$$nm, can be used to image and excite ultra-fast phenomena such as the motion of atomic electrons. Here we show that the illumination of plasma with near-critical electron density may be used as a source of relativistically intense XUV radiation, providing the means for novel XUV-pump–XUV-probe experiments in the non-linear regime. We describe how the optimal regime may be reached by tailoring the laser-target interaction parameters and by the presence of preplasma. Our results indicate that currently available laser facilities are capable of producing XUV pulses with duration $\sim 10~$\text{fs}$$,brillianceinexcessof$10^{23}$photons\mathrm{/}s\mathrm{/}mm$^2$\mathrm{/}mrad$^2$(0.1$I\lambda^2 \gtrsim 10^{19}~$\text{W}$$\text{cm}$^{-2}\mu$\text{m}$^2$.