Transition metal dichalcogenides (TMDCs) are semiconducting 2D-materials with a direct bandgap in a range of 1.0 to 2.5 eV [1] . They exhibit strong second-order nonlinearity per unit thickness, making them interesting for nonlinear light-conversion devices [2] . Due to their small thickness, an interaction enhancement is paramount for efficient operation [3] . We have previously demonstrated that such enhancement can be acchieved by embedding TMDCs into monolithic resonators [4] if a gentle coating process compliant with the sensitive nature of the monolayer TMDC flakes is utilized.
The amplification of waves in the scattering with rotating black holes is a fundamental process in gravitational physics. It was introduced by Roger Penrose in 1969 as a way to extract energy from these astrophysical objects [1] . Soon after Penrose proposal, in 1971 Zel’dovich predicted a similar superradiant amplification for electromagnetic waves scattered by a metallic rotating cylinder [2] . In Zel’dovich proposal, waves get amplified if their angular frequency ω satisfies the condition ω < m Ω, where m is the waves topological charge and Ω is the cylinder angular velocity. Despite this amplification process is ubiquitous in wave scattering physics, and not limited to electromagnetic waves and astrophysics only, direct measurements are limited to an experiment with waves in a water vortex in a draining bathtub experiment [3] . The related, but not identical, Zel’dovich amplification has been observed for acoustic waves [4] .
This tutorial will describe how ultrashort x-ray pulses are generated using free-electron lasers, including their spectral, temporal, coherence properties, and, their application to study photo-initiated electronic and nuclear dynamics in gas and liquid phase.
The generation of XUV supercontinua is of high interest for different applications in that region of the spectrum, such as spectral interferometry and spectroscopy. In high-order harmonic generation (HHG) supercontinua are usually obtained only when a single attosecond pulse is produced, but this implies gating techniques or sub-3-fs IR driving pulses. In this work we demonstrate a new approach for tailoring supercontinuum spectra by exploiting the microscopic and macroscopic properties of HHG.
