Generation of Coherent sub-20 nm XUV Radiation at 78 MHz via Cavity-Based HHG

¹ Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
² Ludwig-Maximilians-Universität München, Fakultät für Physik, Am Coulombwall 1, 85748 Garching, Germany
³ Friedrich-Schiller-Universität Jena, Institut für Angewandte Physik, Albert- Einstein-Str. 15, 07745 Jena, Germany
⁴ Fraunhofer-Institut für Lasertechnik ILT, Steinbachstr. 15, 52074 Aachen, Germany
⁵ RWTH Aachen University, Lehrstuhl für Lasertechnik LLT, Steinbachstr. 15, 52074 Aachen, Germany

^a e-mail: ioachim.pupeza@mpq.mpg.de

Abstract

We present two major advances of enhancement-cavity-based high-order harmonic generation (HHG). First, the generated extreme ultraviolet (XUV) radiation is coupled out collinearly through an on-axis opening in the mirror following the HHG focus. This minimizes the interaction of both the fundamental and the intracavity generated ra diation with the output coupler while simultaneously enabling a large enhancement and an output coupling efficiency that increases with the harmonic order. Second, we use the nonlinearly compressed pulses of an Yb-based laser to drive intracavity HHG allowing for a unique power regime combining short pulses with high average powers. Together, these advances overcome fundamental limitations of current enhancement cavity setups and extend intracavity HHG towards higher photon energies. In a proof-of-principle experiment we use a 3-kW and 78-MHz train of 54-fs to generate and couple out coherent sub-20 nm radiation.