EUV off-axis focusing using a high harmonic source

High Harmonic Generation is a well established technique for generating Extreme Ultraviolet radiation. It is a promising technique for both structure and spectroscopic imaging due to both the high flux and coherence of the source, and the existence of multiple absorption edges at the generated wavelengths. To increase the flux, a focussing device can be used. Here we present focussing results for a Mo/Si spherical mirror that has been used in an off-axis arrangement, and give extensive analysis of the resulting astigmatic focus and its consequence on diffractive imaging. The astigmatic beam exists as a vertical and horizontal focus, separated by a circle of least confusion. With the help of a theoretical model we show that the most intense part of the beam is always the second line foci and that the phase at the focus is strongly saddle-shaped. However, this phase distortion cannot explain the significant interference peak splitting that is experimentally observed in our diffraction patterns. Instead we propose that the beam quality is degraded upon reflection from the multilayer mirror and it is this asymmetric phase distortion that causes the diffraction peak splitting.