## 2 Average Power Scaling of Modelocked Thin-Disk Lasers: Challenges and Milestones
### SESAMs for High-Power Femtosecond Modelocking
#### 2.3.5 Influence of Growth Temperature  
In some specific cases, additional requirements on the SESAM parameters can be beneficial to push the oscillator performance to its limits. For example, in the case of TDLs where short pulses are targeted and a large fraction of the bandwidth needs to be exploited, a somewhat higher modulation depth and faster recovery time are beneficial [20; 28; 29]. Achieving shorter recovery times is straightforward with low-temperature (LT) growth [52]. The results presented in the previous paragraph indicate that growing the QWs at lower temperature should only have a small influence on the damage threshold of the samples, since the damage threshold is nearly independent of the absorber section. However, an experimental verification of this point is crucial, in particular given the higher nonsaturable losses expected from such LT-grown QW-SESAMs. In this paragraph, we will present measurements that confirm that the growth temperature of the QWs has only a small influence on the damage threshold, indicating that the guidelines presented in the previous paragraph can also be applied to fast samples with non-negligible nonsaturable losses. * Experimental Setup In order to carry out this study, we measured the nonlinear reflectivity, damage threshold and recovery parameters of a set of representative samples. For the nonlinear reflectivity and the damage threshold measurements, we used the same setup and experimental procedure described in 2.3.2. To measure the recovery time of the samples, we used a standard pump-probe setup (Figure 11) seeded by a bulk Yb:YAG laser delivering 1-ps long pulses at a repetition rate of 38 MHz and 150 mW of average power. The laser operates at a central wavelength of 1030 nm. For the study of the influence of the absorber growth temperature, we used samples with a larger number of QWs than in the damage and lifetime investigation presented in the previous paragraph in order to reach an initially larger modulation depth. The structure consists of a standard DBR and four InGaAs QW absorbers embedded in GaAs. The absorbers were placed two-by-two in two consecutive antinodes of the electric field pattern to optimize their absorption. This resulted in samples with more than 3% modulation depth, which can be further coated to decrease their modulation depth. The QW absorbers of the four samples used for this study were grown at different temperatures (245 \({}^{\circ}\)C, 270 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 385 \({}^{\circ}\)C). The samples were designed for an operation wavelength of 1030 nm. * Results In Figure 12a, pump-probe measurements of these samples are shown, confirming as expected that samples grown at lower temperatures have faster recovery times [52]. In Figure 12b, we plot the characteristic decay time to 1/e (\(\tau_{\rm 1/e}\)) as a function of QW growth temperature. The observed behavior confirms previous studies carried out with LT-grown GaAs [52]. Although the parameter \(\tau_{\rm 1/e}\) is not sufficient to fully characterize the dynamics of the absorber, it is an appropriate simplified parameter to compare the recovery of the absorber in a passively modelocked solid-state laser. In Figure 12c, we plot nonsaturable losses and damage threshold of these samples as a function of the growth temperature. The total nonsaturable losses of the samples decrease exponentially with the growth temperature, which also confirms the observations made in reference [52]. It is interesting to point out that even for our fastest sample with arecovery time of \(\approx\)2 ps the nonsaturable losses stay low in comparison to the modulation depth of the sample (1.2% nonsaturable losses for 3.9% modulation depth). For use in most common TDLs, lower modulation depth is required. Therefore a dielectric topcoating that will reduce the field in the absorber will be applied, further reducing the nonsaturable losses.