Micro-focusing of broadband high-order harmonic radiation by a double toroidal mirror

TL;DR

This paper demonstrates a novel optical system using double toroidal mirrors in a Wolter configuration to focus broadband high-order harmonic radiation, achieving a small focal spot suitable for nonlinear interactions.

Contribution

The work introduces a new focusing setup with optimized alignment and wavefront correction for broadband XUV radiation, enabling high-intensity applications.

Findings

Achieved a focal spot of 3.6×4.0 μm² FWHM.

Wavefront optimization aligns with ray-tracing predictions.

Enhances the potential for nonlinear high-order harmonic interactions.

Abstract

We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband 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$\times$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$\times$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.