Sources of short wavelength radiation are becoming increasingly important for studies of the so-called nano-cosmos that has implications in driving fields such as physics, chemistry, biology, materials science or medicine. These high demand experiments are often performed at large scale facilities such as synchrotrons or free electron lasers. Unfortunately, beam time availability is highly competitive, leading to enormous interest in table-top systems that can fulfill at least some of the requirements for sophisticated experiments. In that regard, the process of high harmonic generation (HHG) has attracted a lot of attention, since it allows obtaining femto- to attosecond pulses in the extreme ultraviolet to soft x-ray regions. For decades this process has been driven by kilohertz repetition rate ultrashort pulse laser systems.

In recent years there has been a growing demand for high repetition rate HHG sources to address applications in photoelectron spectroscopy (PES), diffractive imaging, coincidence detection and frequency metrology in the extreme ultraviolet spectral region. Achieving a multi-MHz repetition rate (as required, e.g., for PES) in combination with a high average power in the XUV presents a formidable challenge, which has been addressed by three different concepts. First, there have been attempts to utilize field enhancement in nanostructures so as to directly use laser oscillators. A second approach relies on using high average power ultrashort pulse lasers and phase-matching strategies in tight focusing geometries. The third approach, as used by Ozawa et al., exploits an external passive resonator to enhance the in-coupled power by orders of magnitude.

In their work Ozawa et al. demonstrate a passive enhancement cavity and intra-cavity HHG at a repetition rate of 10 MHz and demonstrate up to 0.5 MW of out-coupled power at 149 nm. The use of a four-mirror ring cavity with an optical path length of 30 m, where the long arm of the bow-tie cavity was 15 m, creates specific demands on stability of the cavity setup itself, which are met by using a specially designed and damped large-scale optical table with temperature stabilization. This allows reproducible day-to-day operation and long-term stability of the out-coupled radiation over more than 15 minutes limited only by the degradation of the out-coupling elements. However, the use of a 10 MHz resonator has several advantages over the commonly used 50 MHz to 200 MHz, since it mitigates one of the most severe limitations of this approach, i.e. the accumulation of plasma in the cavity focus. A careful characterization of the generated radiation under different experimental conditions and generation gases shows that wavelengths as short as 30 nm can be generated. Therefore, such a source can be readily used for different ultraviolet photoelectron spectroscopy methods and with further improvements in sight will be a viable tool for the exploration of the nano-cosmos.

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