High Harmonic Frequency Combs for High Resolution Spectroscopy

Abstract

We generated a series of harmonics in a xenon gas jet inside a cavity seeded by pulses from a Ti:sapphire mode-locked laser with a repetition rate of 10.8 MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the $\ensuremath{\mu}\mathrm{W}$ power level. An elaborate dispersion compensation scheme and the use of a moderate repetition rate allowed for this significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results. With this power level and repetition rate, high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable. An interesting target would be the $1S\mathrm{\text{\ensuremath{-}}}2S$ transition in hydrogenlike ${\mathrm{He}}^{+}$ at 60 nm.