1 XUV FREQUENCY COMB SPECTROSCOPY

Frequency comb lasers in the infrared region of the spectrum have revolution-ized many fields of physics. We demonstrate for the first time direct frequency comb spectroscopy at XUV wavelengths. Generation of an XUV comb is real-ized by amplification of two pulses from a frequency comb laser in a parametric amplifier, and subsequent high-harmonic generation to 51 nm (15th harmonic). These XUV pulses, with a time separation between 5.4 and 10 ns, are then used to directly excite helium on the 1s2 1S0 – 1s5p 1P1 transition. The resulting Ramsey-like signal has up to 60 % modulation contrast, indicating a high phase coherence of the generated XUV comb light