Intense XUV sub-fs pulses using extreme soliton dynamics in hollow-core fibers

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Generation of bright attosecond-long XUV pulses is of immanent importance for various applications. Currently, the only way is to employ high harmonic generation (HHG), which suffers from rather low conversion efficiencies. Here, an alternative approach is proposed, allowing, according to preliminary simulations, for up to 5 % conversion efficiency to pulses with durations shorter than 500 as at wavelengths down to 50 nm. The method is based on the optical Kerr effect and exploits dynamics of few-cycle intense pulses in modern hollow-core "kagome" fibers filled with helium. Generation of new frequencies from soliton propagation in fibers is well known, but here a fundamentally new regime is exploited by using a) extremely short input pulses and b) highest possible intensities. After a few cm of propagation, such pulses will emit resonant radiation in the XUV range. The XUV pulses chirp can be compensated by linear propagation in a pre-ionized gas. Critical for success is the preparation of few-cycle short pulses of approx. 5 fs duration with around 20 µJ of energy, which will be accomplished by a home-built OPCPA setup.