High-power, ultrafast source for XUV frequency comb spectroscopy

This work is devoted to creation of a unique laser source for high-precision spectroscopy in the extreme ultraviolet (XUV) spectral region, where no continuous wave lasers exist. The optical region contains a vast of intriguing atomic and electron transitions that still need to be investigated, such as an isomeric nuclear transition in 229Th and the 1S-2S transition in trapped He+ ions. Namely, the work focuses on developing a high average and peak power laser system at a multi-megahertz repetition rate, which should drive high harmonics in a noble gas to transfer the optical frequency comb into the short wavelength region. Since the conversion efficiencies in high harmonic generation processes typically do not exceed ~10^-6, a driver should possess high average power and energy in combination with low noise. While high pulse peak power could improve the conversion efficiency, the high average power would ensure reasonable power in the generated XUV comb lines for spectroscopy applications. Thin-disk Kerr-lens mode-locked oscillators were chosen as robust and powerful femtosecond sources possessing high peak and average powers with the proven potential for scalability. In combination with a new approach of nonlinear spectral broadening and pulse compression in Herriott-type multipass cells, the thin-disk oscillators can provide the necessary peak power (>1 GW) in combination with short pulses to efficiently drive high harmonics in gas. The work is divided into two packages. Firstly, a powerful Kerr-lens mode-locked thin-disk oscillator was developed. Thanks to the scalability of this type of oscillators, it was possible to reach the unprecedented output parameters in a few iterative steps. The final version of the oscillator set a world record on output peak power, namely 110 MW, corresponding to 115 fs-long pulses and 202 W average power at a 14 MHz repetition rate. Additionally, it was demonstrated that the scaling of this type of oscillator can be well predicted and realized beyond 100 MW peak powers. The second part of the thesis focused on boosting the pulse peak power of the developed oscillator for subsequent high harmonic generation in gas jets. To achieve this, the oscillator output pulses underwent nonlinear spectral broadening and pulse compression in Herriott-type multipass cells. As a proof of principle, pulse compression was initially demonstrated with a commercial Pharos laser at reduced average power but equivalent to the oscillator peak power. Finally, the approach was transferred to the high peak- and average power oscillator. The 120 fs pulses were compressed in two cascaded multipass cells by a factor of 15 down to 8.0 fs, corresponding to 148 W average power, 0.9 GW pulse peak power with 82% overall throughput. Additionally, a sub-two-cycle operation with the compressed 6.2 fs long pulses was demonstrated. The multipass cells relying on the all-dielectrically coated mirrors and gas as a nonlinear medium proved to be highly suitable as well as scalable for spectral broadening and compression of high average- and peak power Yb-based lasers. Moreover, several proof-of-principle applications of the developed systems were demonstrated. Among them were mid-infrared generation spanning the range of 2 – 20 µm, multiphoton imaging of the biological tissues, and the first experiments on high harmonic generation.