## 1 Introduction  
Ultrafast lasers sources are one of the main scientific achievements of the past decades. In addition to an important number of industrial applications, such as high-speed and high-precision micromachining they have tremendous impact on many disciplines of scientific research for example in biology, chemistry, physics, materials science and medicine, where they have become essential tools [1]. These sources generate intense and very short laser light bursts (with durations of femtoseconds = \(10^{-15}\) s to picoseconds = \(10^{-12}\) s) and can be tightly focused in space, reaching electric field strengths comparable to those binding electrons to atoms and molecules. This enables the study of the interaction of matter with these strong fields and allows us to temporally resolve complex dynamic processes that occur on this timescale. This research topic is most widely referred to as strong-field physics. Nowadays, most laboratory-based ultrafast laser systems for such applications rely on complex amplifier systems based on Ti:sapphire technology that can deliver ultrashort pulses (\(<\)30 fs) with GWs of peak power, sufficient to reach the necessary electric field strength to carry out the targeted experiments. However, in addition to being complex and expensive, these sources are limited to repetition rates in the kilohertz range, with only a few watts of average power. The development of novel ultrafast sources that operate at higher average power is currently a topic of important research efforts. The combination of high peak power and high average power is very attractive for the above-mentioned strong-field applications. Driving such experiments at multimegahertz repetition rate results in a reduced measurement time and a higher signal-to-noise ratio. Furthermore, photoionization studies in noble gases show that space charge issues are also reduced at higher repetition rates [2]. An important example where such sources would have major impact is high harmonic generation (HHG) [3,4] where driving the experiments with high-average power sources opens new avenues for increasing the vacuum ultraviolet and extreme ultraviolet (VUV/XUV) photon flux [5], possibly in combination with enhanced phase-matching techniques (such as hollow-core photonic crystal fibers (HC-PCF) [6], or resonant field enhancement in nanostructured targets [7]). Such compact megahertz sources of radiation in the VUV/XUV spectral region would enable coherent sources to be available at a wavelength range where laser transitions are not known to date. The main technological challenges that arise from the combination of high average and peak powers are mainly an excessive heat deposition in the gain medium and a too large nonlinearity accumulated by the pulses during propagation. In the past few years, several clever amplifier geometries have been suggested to overcome these limitations such as slab amplifiers [8], fiber based chirped pulse amplifiers (CPA) [9], and thin-disk laser (TDL) amplifiers [10]. However, these amplifier technologies require a low-power seed oscillator and several amplifying stages to reach the targeted high average power, resulting in overall complex systems. In contrast, semiconductor saturable absorber mirror (SESAM) modelocked TDLs enable high average powers and femtosecond operation directly from a single oscillator, without the need for additional amplification stages. Unlike bulk oscillators, where thermal aberrations that occur in the gain medium limit the achievable output power, the TDL concept [11,12] is based on a very thin disk-shaped gain medium that can be efficiently cooled through the backside. The resulting outstanding heat removal capabilities allow for high average powers and excellent beam quality suitable for SESAM modelocking. In addition, the very thin gain medium is ideally suited for small amounts of accumulated nonlinearity even at very high peak powers.