Direct frequency metrology Up To PHz frequencies

The capability of frequency-comb (FC) lasers to precisely measure optical frequencies has been extended to the to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), corresponding to frequencies of multiple PHz. We demonstrate “broad frequency comb generation” for a wavelength range of 51-85 nm. Our method is based on amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. Excitation of argon, neon, and helium with these upconverted laser pulses in the XUV lead to Ramsey-like signals with up to 61% contrast. From these signals an accuracy of 6 MHz has been achieved in the determination of the ground state ionization energy of helium at 51 nm. Further improvement to a kHz-level accuracy is expected based on a new pump laser for the employed parametric amplification system.