Frequency comb laser spectroscopy in the vacuum-ultraviolet region, Phys

Frequency comb sources based on modelocked lasers have revolutionized many fields of physics, including high-resolu-tion spectroscopic tests, e.g. of QED effects in atoms, or to investigate if fundamental constants are in fact constant or not. Frequency combs are also a vital component in the most precise atomic clocks, based single ions in a Paul trap or multiple atoms in optical lattices. The impact of frequency combs could be even bigger if also transitions in the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) spectral regions could be excited or calibrated with comb lasers. The traditional way of using a frequency comb is excitation with a narrowband CW source and calibration with a comb laser. However, this mode of operation is hampered by the lack of suitable CW sources in the VUV and XUV An alternative technique for precision spectroscopy, that is rapidly gaining interest, is direct excitation of atomic transitions with a train of pulses from a frequency comb laser. As the high peak intensity of such ultrashort pulses allows efficient upconversion of an infrared frequency comb output to higher frequencies, it seems very promising for extending high-resolution spectroscopy to the VUV and XUV spectral region. Upconversion of frequency combs has been demonstrated using external enhancement cavities in combination with intracavity harmonic generation using a gas jet. We employ an alternative scheme where Ramsey-type quantum in-terference spectroscopy is performed with a train of amplified and upconverted frequency comb pulses. To this end a train of 2 to 6 pulses from a frequency comb oscillator are first amplified to tens of microjoules energy per pulse in a multi-pas