## 2 Average Power Scaling of Modelocked Thin-Disk Lasers: Challenges and Milestones
### SESAMs for High-Power Femtosecond Modelocking
#### 2.3.6 Guidelines and Outlook  
* Guidelines for high-damage threshold SESAMs In this study, important elements concerning the catastrophic damage of SESAMs that occurs at high fluences were identified. On the one hand, damage originates in the amount of energy deposited by IA on the sample. Therefore, damage threshold scales proportionally to \(\sqrt[]{F_{2}}\). In our study, we used 1-ps long pulses. The dominant mechanism involved in the IA (and therefore damage) at this pulse duration is TPA [45]. On the other hand, the damage threshold was shown to be nearly independent of the absorber section, since a DBR showed a comparable damage threshold and lifetime behavior to those of the different tested SESAMs. As a result, we can give guidelines to develop SESAMs for operation in high-power oscillators:
* Multiple QWs allow for tuning of the modulation depth of the samples without changing their saturation fluence. This is required to have an initially large \(\Delta R\) without topcoating, since the top mirror will reduce this modulation depth. When a relatively low number of QWs is used, the absorbers can be placed simultaneously in one antinode of the electric field. In this case, the absorbers have only a small influence on the damage threshold, mostly because of the additional GaAs spacer layers commonly used as barrier material. When a larger number of QW is used, the amount of material to add becomes significant. In this case, one could also consider using another material with lower \(\beta_{\rm TPA}\) (for instance AlAs) for such QW barrier layers. Without GaAs barrier layers, we expect uncoated SESAM samples to have an almost identical damage behavior to that of a DBR mirror. * Dielectric topcoatings are preferred over semiconductor topcoatings to increase the saturation fluence. The topsection of a SESAM is critical because it experiences a very strong electric field and therefore contributes strongly to the IA and to the damage. With a dielectric material, we achieve larger values for \(F_{2}\) and, consequently, higher damage fluences. * The absorber section of the samples used for the damage and lifetime study was grown at \(T\approx 400\)\({}^{\circ}\)C leading to recovery times in the order of \(\approx\)200 ps and negligible nonsaturable losses. Lower growth temperatures result in shorter recovery times but also in a larger growth defect densities and higher nonsaturable losses. Additional measurements show that the growth temperature only has a small influence on the damage threshold, which confirms the mechanism identified in this study. * Future SESAM designs
* More QWs, more topcoating layers The simple guidelines developed in this study indicate that multiplying the number of QWs and the number of dielectric layers results in high-damage threshold samples with reduced IA and high saturation fluences, which are all crucial parameters for high-power oscillators. However, the limits of this approach for example in terms of maximum amount of QWs and/or dielectric top layers were not investigated. Concerning the dielectric topcoating, we chose the pair SiO\({}_{2}\)/Si\({}_{3}\)N\({}_{4}\) simply because the coatings could be applied in-house in just a few hours using PECVD. Most of the
depositions were performed at 300 \({}^{\circ}\)C in order to achieve high optical quality layers. However, this deposition method has limits in terms of the number of layers that can be applied. * Stress: Given the high growth temperature of the PECVD topcoating, a large number of layers can result in significant stress of the coating at room temperature, which can lead to fracture. The limit in terms of number of layers can be increased up to some extent by reducing the deposition temperature, for example to 120 \({}^{\circ}\)C, at the expense of a slight degradation of the optical quality of the deposited coating. However, for a large number of layers, the use of other stress-less deposition techniques such as sputtering or evaporation should be considered.