## 2 Average Power Scaling of Modelocked Thin-Disk Lasers: Challenges and Milestones
### SESAMs for High-Power Femtosecond Modelocking
#### 2.3.6 Guidelines and Outlook
* Materials: Different dielectric material pairs with a higher refractive index contrast are beneficial to reduce the necessary number of layers to achieve a given field enhancement in the absorbers. Some examples are SiO\({}_{2}\)/Ta\({}_{2}\)O\({}_{5}\) (which is already commonly used as a topsection for SESAMs [32]), SiO\({}_{2}\)/HfO\({}_{2}\) or SiO\({}_{2}\)/TiO\({}_{2}\). Using the maximum possible number of QWs in one antinode of the electric field is preferred to limit the amount of material used in the structure. However, the QWs need to be separated by a large enough distance such that they do not interact between each other. This sets a limit to the number of QWs in one antinode for achieving efficient absorption. Furthermore, the fabrication of SESAMs with large amount of QWs is challenging due to stress resulting from the difference in lattice constants of the absorbers and the spacer layers. Although the exact consequences of this stress on SESAM properties have not been studied in detail, strain-compensation methods might be necessary to maintain the losses in the sample low. * Upside-down growth of large-scale SESAMs for improved thermal management Until now, thermal effects in SESAMs have not been the main limitation for power scaling of modelocked TDLs. Deposited heat in such structures is very low and is mainly due to the residual low nonsaturable losses of the samples. However, in future kW level oscillators, this small fraction of absorbed power can become significant, and lead to thermal aberrations and beam degradation [53]. The modelocked TDL power-scaling concept relies on increasing the spot sizes both on the disk and on the SESAM. Although for SESAMs we have the additional design freedom of increasing the saturation fluence to keep the saturation level constant, spot sizes will most likely also become larger as we move towards several kW of intracavity powers. One important consequence is that possible thermal lensing of the SESAM will then become a critical issue. Therefore, the design of large (several centimeters squared) SESAMs with improved thermal capabilities is a key point in this direction. One possible improvement for better thermal management of SESAMs is substrate removal. In standard SESAMs, the DBR and the absorber section are grown in this order on a GaAs substrate. In this case, the GaAs substrate plays no optical role, and only limits heat transport due to its low thermal conductivity (\(\kappa_{\mathrm{GaAs}}\approx\) 44 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)). A common approach to improve the heat removal of the sample is to grow the structure upside down, contact the sample to a heatsink with a higher thermal conductivity (for example diamond \(\kappa_{\text{diamond}}\approx\) 2200 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)) and remove the substrate. This technique is usually referred to as \"flip-chip\" bonding, and is most commonly used for power scaling of VECSELs [54; 55; 56; 57], where the very high pump absorption makes this a more critical issue. In this way, heatsinks with better heat removal properties such as copper or diamond can be chosen. The resulting structure is very thin (typically a few \(\upmu\)m thick) and can be very efficiently cooled. This technique is illustrated in Figure 13 for a metal heatsink (typically copper). Using large-scale upside-down SESAMs will then be the key to limit thermal distortions of the SESAM and achieve stable modelocking in future kW-level modelocked TDLs.