Widely tunable extreme ultraviolet frequency comb generation

Frequency comb lasers [1] have led to great advances in fields such as precision spectroscopy, optical atomic clocks, and attosecond science. We transfer the remarkable precision of frequency combs to extreme ultraviolet (XUV) wavelengths by parametric amplification and high-harmonic generation (HHG) of two subsequent Ti:Sapphire comb laser pulses (see Fig. 1b). As a result a pair of phase-locked extreme ultraviolet pulses is generated, which can be used directly for precision spectroscopy without the need for an additional spectroscopy laser. Viewed in the frequency domain, the spectrum of the upconverted pulse sequence in the XUV still resembles a frequency comb, but now in the form of a cosine-modulated spectrum (see Fig. 1a). From a timedomain perspective, excitation with phase-locked pulses is a form of Ramsey excitation (see e.g. [2,3]).