Extreme-ultraviolet and nonlinear spectroscopy of thin-film functional materials

This thesis explores different aspects of extreme-ultraviolet (XUV) and nonlinear spectroscopy and applies them to several relevant thin film materials. Both of these rely on ultrafast lasers to be made visible. The first research chapter investigates the fluorescent response of XUV-excited photoresist materials and explores a setup capable of characterizing such a response. The second research chapter follows up on this concept by investigating a thin film of lead halide perovskite methylammonium lead bromide. Here we reveal an interesting self-healing property that manifests itself through continuous XUV exposure. We also explore a tentative method to improve this effect through ozone exposure. The third research chapter uses ultrashort infrared pulses to generate solid harmonics in the same perovskite material and then uses a pump pulse to interfere with the generating process. This results in the rapid suppression and slow recovery of the signal intensity of the harmonics. We demonstrate through qualitative agreement and theoretical methods that this suppression and recovery are linked to the photophysical primary process. We also demonstrate another, as of yet unreported, effect in methylammonium lead bromide: electron impact ionization. The final research chapter summarizes an explorative study and the first tentative results of a pump-probe study on a thin film of carefully structured samarium nickelate, which demonstrates a solid-to-solid transition that results in a large change in conductivity. This insulator-to-metal transition can be excited optically through an infrared pump laser pulse and is known to occur on a timescale of less than half a picosecond. We tentatively put a new bound on this timescale through the use of XUV transient absorption spectroscopy at the nickel M-absorption edge, where a rapid change in signal is observable already after 100 fs. Further research and theoretical investigations are required to confirm this result. The final, concluding chapter explores a number of research options that could be pursued based on the results presented in the previous chapters.