## 1 Introduction
Furthermore, modelocking using semiconductor saturable absorber mirrors (SESAMs) is currently the best-suited approach for high-power ultrafast laser oscillators. The invention of the SESAM nearly 20 years ago [13, 14, 15] represented an important breakthrough in the development of more practical and robust ultrafast laser sources. Today, SESAMs have become key devices for modelocking numerous laser types, including diode-pumped solid-state lasers, fiber lasers, and semiconductor lasers. Semiconductors are ideally suited for saturable absorbers because they can cover a broad wavelength range and yield short recovery times, supporting the generation of ultrashort pulse durations. The macroscopic parameters for modelocking can be optimized over a wide range by the design and growth conditions of the mirror structure and the choice of the semiconductor absorber. The combination of the SESAM and the TDL concepts results in a power scalable ultrafast technology. Simply increasing the pump power with both an increased pump spot area on the disk and laser spot area on the SESAM allows one to increase the output power of SESAM modelocked TDLs. This resulted in a steady increase of the average power and pulse energy available from such modelocked sources since their first demonstration in the year 2000 [16] (Figure 1). Currently, SESAM modelocked TDLs achieve higher average powers (\(>\)275 W [17]) and pulse energies (\(>\)40 \(\upmu\)J [18]) than any other oscillator technology. In this paper, we will review recent progress in the performance of modelocked TDLs. The average power of ultrafast oscillators recently reached a new limit with the demonstration of a SESAM  
Figure 1: Evolution of pulse energy (**a**) and average power (**b**) of ultrafast semiconductor saturable absorber mirrors (SESAM)-modelocked thin-disk laser (TDLs). Most power and energy scaling was achieved using the well-established gain material Yb:YAG, but promising new materials for this application are currently the topic of important research efforts. modelocked TDL with 275 W--the highest average power reported from an ultrafast oscillator to date. The laser was based on the gain material Yb:YAG and operated with a pulse duration of 583 fs and a pulse energy of 16.9 \(\upmu\)J. This performance was obtained by operating the modelocked laser in a vacuum environment to eliminate the parasitic nonlinearity of the air inside the oscillator. We will summarize the results obtained using this new approach, which represents an important step forward towards the kilowatt average power level milestone. Another key element to achieve this latest step in power scaling was the availability of SESAMs with high-damage threshold and parameters suitable for high-power operation. A recent investigation, which targeted to specifically explore damage behavior of SESAMs for such high-power oscillators [19], enabled the fabrication of such robust samples. We will review the guidelines developed in this recent study and discuss future improvements. Another major milestone for modelocked TDLs is to extend the high-power capabilities of TDLs to the sub-100 fs regime. This topic is intimately linked to the development of novel broadband materials suitable for this geometry [20]. Recently, the limits in terms of pulse duration of modelocked TDLs based on different gain materials were explored. As a result, a first important step in this direction was achieved, with the demonstration of sub-100 fs pulses from a TDL based on the sesquioxide gain material Yb:LuSoO\({}_{3}\) (Yb:LuSoO) [21]. In this first experiment, sub-100 fs operation was achieved at moderate average output powers (5 W). However, preliminary power scaling experiments indicate that much higher output powers are within reach. Furthermore, the intracavity peak power levels achieved are already high enough for preliminary intralaser nonlinear optics experiments, which is a promising application [22].