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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. At 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 ( ∼ 26 meV) and space-charge free operation, with a time-resolution of ∼ 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 μ J, hence is easily scalable to multi-MHz 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 Bi 2 Se 3 .

A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe we are able to track the dynamics of the dielectric function, unveiling an anomalous decrease of the scattering rate of the charge carriers in a pseudogap-like region of the temperature ($T$) and hole-doping ($p$) phase diagram. In this region, delimited by a well-defined $T^*_{neq}(p)$ line, the photo-excitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasi-particles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the single-band Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between "nodal" quasiparticles and antinodal excitations., accepted for publication on Nature Communications