Extreme ultraviolet frequency comb spectroscopy
Frequency comb lasers have brought a revolution of precision and control in many fields of physics. Originally, frequency combs existed only in the near-infrared, based on Ti:sapphire modelocked lasers or fiber laser systems. However, by employing nonlinear optics, the wavelength coverage of frequency combs has expanded dramatically over the past few years. Comb generation is now possible from infrared to extreme ultraviolet (XUV) wavelengths, which provides e.g. interesting possibilities for precision spectroscopy in helium and helium+ ions to test quantum-electrodynamical effects. At LaserLaB Amsterdam we have performed direct frequency comb spectroscopy in the extreme ultraviolet at wavelengths ranging from 85-51 nm on argon, neon and helium, based on selective amplification and high-harmonic generation of near-infrared frequency comb laser pulses [1,2]. With this method the ground state of helium was determined with an 8-fold improved accuracy of 6 MHz, providing a stringent test of QED in the low-energy limit. In the presentation the latest developments will be discussed, including a newly developed laser system to reach kHz-level accuracy in the XUV, and a new method of spatial coherent control for improved two-photon spectroscopy with frequency comb lasers.
 D. Z. Kandula et al., PRL 105, 063001 (2010)
 T.J. Pinkert et al., OL 36, 2026 (2011)