Micro-Focusing of Broadband High-Order Harmonic Radiation by a Double Toroidal Mirror

We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband extreme ultraviolet (XUV) radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 x 4.0 mu m(2) full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 x 3.2 mu m(2). This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime.