## 3 New Pulse Duration Limits of Modelocked Thin-Disk Lasers
### State-of-the Art and Recent Experimental Results  
Table 5 summarizes laser results achieved to date from modelocked TDLs. These results can also be found in Figure 15. A more general overview of different Yb-doped materials for the generation of ultrashort pulses can be found in [59]. In addition, a specific overview of broadband materials for TDLs is given in reference [20]. The most widely-used gain material for TDLs is Yb:YAG [76]. It is grown with excellent quality and in large sizes. Furthermore, it has a good thermal conductivity of \(\approx\)6.6 W-m-1-K-1 for an Yb  
Figure 16: **Left:** Gain cross section of Yb:LuO for different inversion levels of the gain medium; **Right:** Gain cross section of Yb: LuScO for different inversion levels of the gain medium. doping concentration of 10 at.%, which is a key ingredient for laser operation at high power levels. The first modelocked TDL was based on Yb:YAG and delivered an average power of 16.2 W with a pulse duration of 730 fs [16]. To date, the pulse duration of high-power SESAM modelocked Yb:YAG TDLs is limited to pulses longer than 500 fs, although much shorter pulses were demonstrated in low-power bulk oscillators [77, 78, 79, 80]. Recently, the first Kerr-lens modelocked (KLM) Yb:YAG TDL was demonstrated with an average power of 17 W and a pulse duration of 200 fs [61], and similar performance was obtained by operating the KLM modelocked oscillator in the positive dispersion regime [62]. One broadband material that held the record for the shortest pulses from a modelocked TDL for many years was Yb:KYW [81, 82]. In 2002, a modelocked TDL based on this material was presented with an average power of 22 W and a pulse duration of 240 fs [66]. In this result, these short pulse durations were achieved with an intracavity prism in order to tune-off the narrow maximum emission peak thus broadening and flattening the emission spectrum. The varying quality of the available material and the anisotropic nature of this crystal limited further power scaling. Some borate materials show wide gain spectra for the generation of short pulses [83, 84, 85]. Initial modelocking experiments with Yb:YCOB resulted in a pulse duration of 270 fs at a moderate average output power of 2 W. In this case, anisotropic thermal aberrations made obtaining fundamental mode operation difficult at higher power levels [70]. The first modelocking experiments using the promising material Yb:CALGO (Yb:CaGdAlO4) [86, 87] in the thin-disk configuration were recently reported [69, 88]. This material combines a very broad and smooth emission bandwidth with a thermal conductivity comparable to Yb:YAG (6.5 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\) for 2% doping). Furthermore this material benefits from anisotropic thermal properties when cut along the \(\sigma\)-axis, which are crucial for power scaling in the thin-disk configuration. In the first modelocking results, up to 28 W of average power were demonstrated with 300 fs pulses. Shorter pulses (135 fs) could only be achieved at a moderate average power of 1.3 W. The available doping concentrations and thickness of the crystal limits the average power in this first experiment. Nevertheless, this material is one of the most interesting candidates for power scaling of modelocked TDLs with short pulses, in particular given the very promising results obtained with bulk modelocked lasers based on this material [89, 90]. Very recently, promising first results were demonstrated using the oxyorthosilicate laser crystal Yb:Sc\({}_{2}\)SiO\({}_{5}\) (Yb:SSO) [91, 92], where 27.8 W and 298 fs were demonstrated [68]. In this case, further improvements on growth and polishing quality are still required for further power scaling. Finally, another very successful group of thin-disk gain materials are Yb-doped cubic sesquioxides. This family has been recognized as potentially suitable to outperform Yb:YAG in terms of efficiency and shorter pulse durations [93, 94, 95, 96, 97, 98, 99].