Time-resolved VUV ARPES at 10.8 eV photon energy and MHz repetition rate

The quest for mapping the femtosecond dynamics of the electronic band structure of complex materials over their full first Brillouin zone is pushing the development of schemes to efficiently generate ultrashort photon pulses in the VUV energy range. M present, the critical aspect in time- and angle-resolved photoelectron spectroscopy (TR-ARPES) is to combine a high photon energy with high photoemission count rates and a narrow pulse-bandwidth, necessary to achieve high energy resolution, while preserving a good time resolution and mitigating space-charge effects. Here we describe a novel approach to produce light pulses at 10.8 eV, combining high repetition rate operation (1-4 MHz), high energy resolution (similar to 26 meV) and space-charge free operation, with a time-resolution of similar to 700 fs. These results have been achieved by generating the ninth harmonic of a Yb fiber laser, through a phase-matched process of third harmonic generation in Xenon of the laser third harmonic. The full up-conversion process is driven by a seed pulse energy as low as 10 mu J, hence is easily scalable to multiMHz operation. This source opens the way to TR-ARPES experiments for the investigation of the electron dynamics over the full first Brillouin zone of most complex materials, with unprecedented energy and momentum resolutions and high count rates. The performances of the setup are tested by TR-ARPES on the topological insulator Bi2Se3.