  Short-wavelength imaging, spectroscopy, and lithography scale down the
characteristic length-scale to nano meters. This poses tight constraints on the
optics finishing tolerances, which is often difficult to characterize. Indeed,
even a tiny surface defect degrades the reflectivity and spatial projection of
such optics. In this study, we demonstrate experimentally that a Hartmann
wavefront sensor for extreme ultraviolet (XUV) wavelengths is an effective
non-contact analytical method for inspecting the surface of multilayer optics.
The experiment was carried out in a tabletop laboratory using a high-order
harmonic generation as an XUV source. The wavefront sensor was used to measure
the wavefront errors after the reflection of the XUV beam on a spherical Ru/B4C
multilayer mirror, scanning a large surface of approximately 40 mm in diameter.
The results showed that the technique detects the aberrations in the nanometer
range.
