## 3 New Pulse Duration Limits of Modelocked Thin-Disk Lasers
### State-of-the Art and Recent Experimental Results
#### 3.2.2 Mixed Sesquioxide Materials: Sub-100 fs Pulse Duration  
The disordered mixed sesquioxide Yb:LuSoO was recently successfully introduced as an interesting candidate for short-pulse generation in the thin-disk geometry [109]. This material succeeded in combining two emission bands of Yb:LuO and Yb:ScO that lie roughly 7 nm apart, in a 22-nm-broad emission bandwidth centered around 1038 nm. In contrast to many other broadband materials that exhibit anisotropic thermal and optical properties (such as Yb:KYW or Yb:YCOB) this material has an isotropic crystal structure, which is crucial for reaching high power levels. In the first modelocking experiment, 227 fs were reached at a moderate average power of 7.2 W [71]. In the meantime, the crystal quality was significantly improved and power scaling to 23 W with a pulse duration of 235 fs was possible [72]. This was followed by the demonstration of a TDL based on this material with a pulse duration of 96 fs at an average power of 5.1 W (Figure 19). These are the shortest pulses ever obtained from a modelocked TDL, reaching for the first time the sub-100 fs milestone [21]. The use of a fast SESAM with a relatively high modulation depth was one of the keys to exploit such a large fraction of the available bandwidth. Further SESAM optimization and the use of larger spot sizes on the disk and on the SESAM will allow for higher powers. In comparison to Yb:LuO, the larger  
Figure 18: Setup for carrier-envelope offset (CEO) frequency detection of Yb:LuO modelocked TDL.  
emission bandwidth of Yb:LuSoO relaxes the requirements on the SESAM parameters, which should facilitate straightforward power scaling.  
Another mixed sesquioxide host consisting of the mixture of LuO, ScO and YO (Y\({}_{2}\)O\({}_{3}\)) was recently introduced. However, the resulting material Yb:(Sc,Y,Lu)\({}_{2}\)O\({}_{3}\) (Yb:ScYLO) showed no apparent advantage in terms of spectroscopic properties compared to Yb:LuSoO. Nevertheless, it is an interesting isotropic material that exhibits an emission bandwidth of 18.5 nm FWHM, making it a good candidate for short pulse generation. Two thin disks from the first growth run of this material were tested in modelocked operation. In the first experiment, 236 fs were demonstrated at an average power of 3.9 W [67]. The low average power was due to the very poor quality of the crystal, which showed a strongly asymmetric thermal lens and several grain boundaries, which limited the obtainable power in fundamental mode operation to only a few watts. It is worth noting that a similar problem was observed in the first results of Yb:LuSoO [71] for which the second growth run demonstrated disks of significantly higher quality.  
Given the quality of the available disks from the first growth run of Yb:ScYLO, we focused on exploring the limits in terms of pulse duration of this material, and were able to obtain 4.6 W of average power pulse duration of 101 fs. This was obtained at a repetition rate of 70 MHz, resulting in a pulse energy of 0.1 \(\upmu\)J. This result further confirms the potential of this family of materials for modelocking in the thin-disk configuration with short pulse durations.