Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

Abstract

The process of high harmonic generation (HHG) enables the development of table-top sources of coherent extreme ultraviolet (XUV) light. Although these are now matured sources, they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime. Moreover, many of the emerging applications of such light sources (e.g., photoelectron spectroscopy and microscopy, coherent diffractive imaging, or frequency metrology in the XUV spectral region) require an increase in the repetition rate. Ideally, these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously. So far, this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers. Here, a novel route with significantly reduced complexity (omitting the requirement of an external actively stabilized resonator) is demonstrated that achieves the previously mentioned demanding parameters. A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 µJ, 250 fs pulses leading to ∼7 µJ, 31 fs pulses at 10.7 MHz repetition rate. The compressed pulses are used for HHG in a gas jet. Particular attention is devoted to achieving phase-matched (transiently) generation yielding >1013 photons s−1 (>50 µW) at 27.7 eV. This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments, thus demonstrating the considerable potential of this source. A photonic-crystal fibre filled with krypton gas has been used to realize an extreme-ultraviolet source with a megahertz repetition rate. Applications such as photoelectron spectroscopy and coherent diffractive imaging ideally require extreme-ultraviolet sources with a high photon flux and a high repetition rate, but achieving both properties simultaneously is challenging. Now, scientists in Jena, Germany, have solved this problem by using a krypton-filled kagome fibre to nonlinearly compress laser pulses at a rate of 10.7 megahertz. They then used the rapid stream of compressed pulses to drive high-harmonic generation in a gas jet. This resulted in the generation of over 50 microwatts at a photon energy of 27.7 electron volts. The approach could lead to a new breed of more compact and efficient high-harmonic-generation sources.