Enhancement cavities for attosecond physics

The work presented in this thesis was aimed at developing a high-repetition rate source of coherent radiation in the extreme ultra-violet (XUV) spectral region, envisaging applications in attosecond physics or precision metrology in the XUV. Due to the lack of laser oscillators operating in the XUV, the method of choice was the frequency upconversion of a near-infrared laser via the nonlinear process of high-order harmonic generation. Obtaining sufficient XUV photon flux per pulse at repetition rates of several tens of MHz, despite the inherently low conversion efficiency, requires a powerful driving source. To date, passive enhancement of ultrashort pulses in an external resonator has been the most successful strategy in meeting this demand. In this thesis four main achievements towards extending this technique and understanding its limitations are presented. A first experiment was dedicated to obtaining shorter intracavity pulses without compromising the high average power available from Yb-based laser technology. To this end, we spectrally broadened and temporally compressed the pulses prior to the enhancement in a broadband resonator. Aside from being a prerequisite for time-domain applications, shorter intracavity pulses led to improved conditions for the harmonic generation process. Furthermore, we addressed the task of extracting the intracavity generated XUV light. We established two methods for geometrical XUV output coupling, one employing the fundamental mode of the cavity, and the other a tailored transverse mode, which offers additional degrees of freedom to shape the harmonic emission. Both techniques are particularly suited for the intracavity generation of attosecond pulses, because they afford an unparalleled flexibility for the resonator design, and exhibit a broadband output coupling efficiency approaching unity for short-wavelength radiation. This enabled a significant improvement of the crucial parameters, photon flux and photon energy. In a combined experimental and theoretical study, we investigated the ionization-related intensity limitations observed in state-of-the-art enhancement cavities. The quantitative modeling of the nonlinear interaction allows for an estimation of the achievable intracavity parameters and for a global optimization of the XUV photon flux. Based on this model, we proposed a strategy to mitigate this limitation by using the nonlinearity in combination with customized cavity optics for a further spectral broadening and temporal compression of the pulse in the resonator. More importantly, this work establishes enhancement cavities as a tool to investigate nonlinear light-matter interactions with the increased sensitivity provided by the resonator. The last study was dedicated to the technological challenge of building a resonator in which the electric field of the circulating pulse is reproduced at each round-trip. This is an essential prerequisite to generate identical XUV emission with each driving pulse. By tailoring the spectral phase of the cavity mirrors we succeeded in enhancing pulses of less than 30 fs (less than nine cycles of the driving field) to a few kilowatts of average power with zero pulse-to-pulse carrier-to-envelope phase slip. At similar pulse durations, the generation of isolated attosecond pulses has already been demonstrated in single-pass geometries. In conclusion, the results presented in this thesis are milestones on the way to a powerful, compact and coherent source of ultrashort XUV radiation. The unique property of the source, that is, its high repetition rate lays the foundation for advancing attosecond physics and precision spectroscopy in the XUV region