  High-intensity laser pulses covering the ultraviolet to terahertz spectral
regions are nowadays routinely generated in a large number of laboratories. In
contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated
using a small number of sources including free-electron laser facilities [1-3]
and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a
concept for a compact intense XUV source based on HHG that is focused to an
intensity of $2 \times 10^{14}$ W/cm$^2$, with a potential increase up to
$10^{17}$ W/cm$^2$ in the future. Our approach uses tight focusing of the
near-infrared (NIR) driving laser and minimizes the XUV virtual source size by
generating harmonics several Rayleigh lengths away from the NIR focus.
Accordingly, the XUV pulses can be refocused to a small beam waist radius of
600 nm, enabling the absorption of up to four XUV photons by a single Ar atom
in a setup that fits on a modest (2 m) laser table. Our concept represents a
straightforward approach for the generation of intense XUV pulses in many
laboratories, providing novel opportunities for XUV strong-field and nonlinear
optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent
diffractive imaging of nanoscale structures.
