High-order harmonic generation (HHG) in noble gases serves as the foundation of short-wavelength
ultrafast optics and extensive research over the past 30 years has led to a comprehensive
understanding of the involved generation processes. For the majority of theses studies, titaniumdoped
sapphire lasers were deployed, emitting in the near infrared region at around 800 nm.
Photon energies up to around 150 eV with unprecedented pulse durations down to hundreds of
attoseconds were reached. However, the low conversion efficiency of the extremely nonlinear
process is impeding a high flux of the short-wavelength radiation while intrinsic limitations in
the generation process prevent higher photon energies at these driving field wavelengths.
More recently, HHG was extended to well-ordered crystalline solids, both semiconductors
and dielectrics, with continuing debate about the involved generation mechanism. A conclusive
understanding of such high density targets is of particular interest, since it can potentially
overcome the limit of the low conversion efficiency.
This work describes the extension of high-order harmonic generation light sources to novel
regimes with the aim to lift these restriction. In the scope of this work, three projects were taken
forward.
To detect and fully characterize the generated short-wavelength radiation, a high-order harmonic
generation beamline was set up. A custom built and optimized charge detector system
was developed and installed, enabling to determine the absolute photon flux of the harmonic
emission.
Deploying a short-wavelength infrared (1800 nm) few-cylce pulse with an excellent spatial
and temporal beam quality and a high pulse energy of 750 μJ extended the accessible photon
energy of the emitted soft X-ray radiation to the water window region (284 to 540 eV) and
above. The emitted radiation was furthermore fully characterized, confirming its excellent spatial
and temporal quality.
Further, high-order harmonic generation in the liquid phase was studied, representing a
largely unexplored generation medium. The liquid phase constitutes the missing link between
a high density well-ordered solid target and an atomic or molecular gas, thus allowing to gain
novel insights into the process of HHG. By scanning a number of laser and target parameters,
the underlying mechanisms, including the extent of generation in liquid rather than surrounding
gas, were investigated.