XUV FREQUENCY COMB SPECTROSCOPY

Frequency comb lasers in the infrared region of the spectrum have revolutionized many fields of physics. We demonstrate for the first time direct frequency comb spectroscopy at XUV wavelengths. Generation of an XUV comb is realized by amplification of two pulses from a frequency comb laser in a parametric amplifier, and subsequent high-harmonic generation to 51 nm (15^th harmonic). These XUV pulses, with a time separation between 5.4 and 10 ns, are then used to directly excite helium on the 1s^{2 1}S₀ - 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.