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