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

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.