Design, Implementation and Commissioning of an Ultrafast XUV Ptychography Beamline

This thesis work focuses on the design, implementation and commissioning of a tabletop beamline for Ultrafast Ptychography experiments using eXtreme UltraViolet (XUV) coherent light. Through High-Harmonic Generation (HHG), femtosecond pulses from a titanium-sapphire regenerative amplifier are coherently upconverted to produce harmonic combs centered at 42.6eV and 96.5eV. The generated radiation retains high spatial and temporal coherence, enabling the implementation of Ptychography, a technique for Coherent Diffractive Imaging (CDI) in which multiple diffraction patterns from overlapping fields of view are processed by iterative algorithms to separately reconstruct amplitude and phase images of both the sample and of the image-forming beam. The experimental set-up was designed for pump-probe imaging with ptychography, using femtosecond laser pulses from the same ultrafast laser system to drive the sample out-of-equilibrium, and leveraging the XUV pulses to reconstruct quantitatively its dynamics at the nanoscale with high spatiotemporal resolution. The system design and implementation was performed through the creation of its digital twin, a virtual model of our physical microscope connected to its real-world counterpart through experimental optics metrology results and ray tracing data. We considered three commonly used XUV delivery schemes based on spherical, ellipsoidal and toroidal optics, and constrained by realistic source parameters, optics metrology and detector response. We implemented their probes through a full ptychographic reconstruction workflow, comparing directly how misalignments and realistic surface figure errors shaped the probe, and quantifying how probe properties controlled Michelson contrast, spatial resolution, oversampling, convergence behavior and computational cost. With this approach, we demonstrated that a z-fold geometry offered a reliable compromise between image fidelity and reconstruction speed, with high tolerance to misalignments. Based on this choice, the set-up was built and the digital twin was updated from experimentally measured quantities, including source size and divergence, distances, surface errors, detector response, creating a high fidelity replica of the real system, including imperfections, tolerances, drifts, and realistic noise, so its outputs are directly comparable to actual measurements. The ultrafast XUV microscope was deployed in a custom ultra-high-vacuum chamber with motorized stages for optics, sample, detector alignment. Fine control on the optics surface profile was ensured through metrology measurements conducted in collaboration with the optics group at the ELETTRA-Sincrotrone Trieste. The detector noise sources were quantified and the photon transfer curve was implemented in our digital twin to create a realistic camera noise model for accurate comparison with experimental data. The pump branch was designed and implemented to provide delay ranges up to several hundred picoseconds, tunable fluence, adjustable polarization, and few-tens-of-femtoseconds temporal resolution. Commissioning results, obtained after the completion of my PhD appointment, are demonstrating in-plane sub-20nm transverse resolution, 0.5Å axial precision, and ≈30fs temporal resolution, with high predictive accuracy of probe wavefront and samples transmission function between experimental reality and the digital twin model.