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Project A.1.2

Fig. A.1.1 Different geometries for coupling out the XUV radiation

XUV frequency combs

Our project will explore new ways to develop high-repetition-rate (100 MHz) ultra-short pulse laser sources in the XUV spectral domain. Focusing intense femtosecond laser pulses on a gas target leads to the generation of high-order harmonics (HHG) of the fundamental radiation reaching into the XUV or even soft-X-ray range. The conversion efficiency can be significantly increased if the HHG process takes place in an external enhancement cavity, as has already been demonstrated.

A power enhancement of the visible seeding comb of several hundred and intensities of 1013 W/cm2 or more, which are necessary for HHG, can be reached in the focus of the resonant passive cavity. This scheme holds much promise for generating XUV combs with unprecedentedly high powers and will be investigated and further optimised in this project. We will develop improved highreflectivity mirrors for dispersion management in close collaboration with Project A.2.1. This will allow for the full spectral bandwidth of the seeding comb to be coupled into the resonator and to sustain the required high intra-cavity average circulating power.

Novel approaches to coupling out the XUV radiation from the enhancement cavity will be explored. These include non-collinear high harmonic generation, new cavity geometries, new output coupler designs (e.g. small-period diffraction gratings) and are schematically shown in Fig. A.1.1. The XUV radiation generated will be carefully characterised to confirm that the frequency-comb coherence has been conserved through the HHG process.

Collaborations with A. Ozawa and Y. Kobayashi (Tsukuba, Japan) and A. L’Huillier (Lund, Sweden) will majorly support our investigations.