Frequency combs in the extreme ultraviolet
Optical frequency combs have become an indispensable tool for both optical frequency metrology and high-field phyiscs, as they uniquely combine the extremely high spectral resolution and the large peak intensities of their generating pulse trains. By nonlinear processes such as high harmonic generation (HHG), the latter can be exploited to extend frequency combs from the infrared, where they are usually generated, to shorter wavelengths down to the extreme ultraviolet (XUV) spectral region
... pectral region where direct synthesis is not possible. Among the possible applications of a high-power XUV frequency comb, high precision spectroscopy of the 1s-2s transition in He + at 61 nm is a particularly interesting example. Such an experiment promises tests of bound-state quantum electrodynamics (QED) with unprecedented precision by being more sensitive to theoretically challenging higher-order corrections than e.g. hydrogen 1s-2s spectroscopy. However, the first demonstrations of high repetition rate XUV frequency combs generated in external enhancement cavities did not generate sufficient power for spectroscopy. Therefore, the development of high power XUV frequency combs is of utmost importance for applications. In this work the generation of a high power frequency comb in wavelength regions down to 40 nm via intracavity HHG was demonstrated. The average power around 61 nm (13 th harmonic) was close to 1µW (a 10 5 fold improvement over first generation sources) which made it the most powerful XUV frequency comb demonstrated so far. The improvement was achieved by lowering the repetition rate to obtain higher pulse energies and by elaborate dispersion management. Even with 1µW of average power spectroscopy experiments are still on the verge of feasibility. Thus further efforts were made to overcome the two most important limitations that impede progress. Those are the limited available seed power and the low efficiency and high nonlinearities of the employed Brewster's plate outcoupling method. The first limitation was mitigated by the demonstration of a cryogenic high repetition rate single pass amplifier. We obtained a 4-fold improvement in average power without deforming the beam and with negligible phase noise. Such an amplifier is a versatile tool and not limited to cavity-enhanced HHG applications. The second limitation was addressed by a study of non-collinear high harmonic generation. In this outcoupling method, two infrared driving beams that cross at an angle generate an XUV frequency comb which propagates along their bisectrix. This renders the use of any output coupler unneccessary. We demonstrated efficient, stable, and well-collimated noncollinear high-harmonic emission in a setup applicable to an enhancement cavity for the first time. The estimated loss is orders of magnitude lower than with the conventional Brewter's plate method. The results obtained in the course of this work are important milestones towards reliable high-power XUV frequency combs that make high precision spectroscopy or laser cooling in this wavelength region possible. APPLICATIONS OF FREQUENCY COMBS 1. The intra-cavity dispersion has to be compensated.