## 4 Conclusions and Outlook  
In the last few years, the performance of modelocked TDLs was pushed to new frontiers. As a result, cutting-edge ultrafast sources were developed that will enable exciting applications, for example, in strong-field physics.  
An important number of the latest achievements that were summarized in this paper were possible thanks to a thorough investigation on damage limits and crucial parameters of SESAMs for high-power oscillators. As a result of this investigation, SESAMs with ideal parameters and high-damage thresholds were achieved that enabled this technology to reach these new frontiers.  
The limits in terms of average power of ultrafast oscillators were recently pushed to a new limit with the demonstration of an Yb:YAG ultrafast TDL with 275 W of average power. Prior to these experiments, the nonlinearity of air was recognized as one of the main contributions to the total phase shift experienced by the pulses within one round trip in the laser cavity. At high peak power, this phase shift can become too large and destabilize the modelocking mechanism. In this experiment, we reduced the influence of the nonlinearity of the ambient environment by operating the oscillator in a medium vacuum environment (0.5 mbar). In this way, 275 W of average power and a pulse energy of 16.9 \(\upmu\)J at a pulse duration of 583 fs were achieved. In the near future, we expect this approach to enable a further increase in pulse energy and average power of such sources. Currently, the only limitations to higher average powers appear to be thermal effects that occur in the dispersive mirrors. Future thermally improved mirror designs will therefore enable the next steps in average power and pulse energy scaling.  
Another important achievement was the recent demonstration of TDLs with nearly bandwidth-limited pulses that reach pulse durations as short as 96 fs. Although in these preliminary experiments we achieve moderate output powers (typically \(<\)10 W), we expect further power scaling to the 100-W level to be within reach, in particular with tailored high-damage threshold SESAM designs, and given the latest power scaling results obtained by operating the oscillator in a vacuum environment. Furthermore, promising materials with significantly larger emission bandwidths than the ones used in this experiment should soon yield even shorter pulses at high average powers. This paves the way to highly nonlinear optics experiments driven directly by table-top oscillators, at high repetition rates and with high efficiency. Another promising possibility that is already enabled by the current proof-of-principle sub-100 fs medium power oscillators is to exploit the high intracavity peak powers inside TDLs to drive highly nonlinear experiments.  
The record short pulses demonstrated in the context of this work enabled us to perform the first CEO frequency measurement of a modelocked TDL. The detection of this characteristic frequency is the first crucial step to fully stabilize this type of laser source, which is the key in applications such as spectroscopy or metrology.  
In addition to future experiments enabled by the state-of-the-art sources presented here, the performance milestones of these modelocked thin-disk oscillators will continue to evolve to higher average power, pulse energy, peak power and shorter pulse durations.  
We acknowledge financial support by the Swiss National Science Foundation (SNF) and support from the FIRST cleanroom facilities of ETH Zurich for the SESAM fabrication. [PERSON] and [PERSON] acknowledge financial support by the Joachim Herz Stiftung in the framework of the Excellence Cluster \"Frontiers in Quantum Photon Science\" of the federal state of Hamburg. [PERSON] acknowledges support from the European Research Council for the project \"Efficient megahertz XUV light source\" (ERC Starting Grant 2011 #279545).