Abstract

In the decade preceding this thesis, femtosecond enhancement cavities had emerged as a highly promising technology in the context of extreme ultraviolet light (XUV) sources for frequency comb metrology and attosecond physics. These applications require light of laser-like coherence, which can be provided by high-order harmonic generation (HHG), a highly nonlinear frequency conversion process driven by intense ultrashort laser pulses. The laser systems commonly used to drive HHG are limited to pulse repetition rates in the kilohertz range. In contrast, the enhancement of femtosecond pulses in passive optical cavities to average powers of many kilowatts delivers the necessary intensities even at repetition rates of tens to hundreds of megahertz. Achieving sufficient XUV flux with megahertz repetition rates would enable the extension of frequency comb metrology to the XUV, and dramatically reduce data acquisition times for experiments in attosecond physics. However, cavity-enhanced HHG comes with unique challenges, imposing cavity-related limitations to the power, peak intensity, and minimum duration of the driving pulses. In this thesis, several novel approaches to extending the capabilities of femtosecond enhancement cavities are presented. In a first experiment, we demonstrated the compensation of thermal lensing effects in enhancement cavities. Using intracavity Brewster plates, which also offer a robust solution for XUV output coupling in cavity-enhanced HHG setups, we gained control over the thermally-induced mode change at average powers of up to 160 kW. Subsequently, we investigated the effects of nonlinear phase modulations caused by ionization in an intracavity gas target, which is a prerequisite for HHG. We experimentally validated a numerical model of the plasma-cavity interaction, leading to a scaling law allowing for the layout of optimized cavity HHG systems, and a proposal for tailoring the spectral finesse of cavities to exploit the nonlinear phase modulation for intracavity pulse compression. In parallel, we worked on the design and characterization of highly reflective multilayer mirrors to optimize the cavity dispersion. Combining different mirrors with compatible spectral phase characteristics, we demonstrated enhancement cavities supporting waveform-stable pulses, and cavities supporting pulse durations approaching the few-cycle regime. These results represent vital technological developments towards the goal of isolated attosecond pulse generation with enhancement cavities. Finally, we applied the developed methods of dispersion control to design an enhancement cavity for intracavity pulse compression using self-phase modulation in a Brewster plate. Implementing a flexible locking scheme, we demonstrated for the first time the generation of temporal cavity solitons in free-space enhancement cavities. The temporal compression from 350 fs to 37 fs together with the spectrally tailored finesse resulted in a peak power enhancement factor of over 3000, significantly surpassing the enhancement in linear cavities supporting similar pulse durations. This intriguing result opens the door to a novel regime of nonlinear cavity operation, with potentially significant benefits to cavity-enhanced HHG. In addition, we proposed a concept for optomechanical cavity dumping, with the potential to aid efforts employing enhancement cavities for a new generation of high-pulse-energy lasers.

Abstract