LAUSR

Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work, we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the 2D semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electron-hole interaction and screening. Our work establishes a new benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function. Functionality in electronic and optoelectronic devices is based on the control of the flow of charge carriers under out-of-equilibrium conditions. At the microscopic level, charge transport and device operation rely upon generating non-equilibrium electron distributions controlled by external fields to achieve the desired electronic response. The propagation of electrons in a crystal and the evolution of their energy distributions are governed by the details of the electronic structure as well as the efficiency of elastic and inelastic scattering processes. Time-resolved ARPES (trARPES) addresses this problem by observing the spectral function of a material after excitation via a femtosecond optical pulse [1]. The momentumresolved distribution of excited states combined with the dynamical information on state lifetimes provides a powerful view into excited solids [2], extending the scope of ARPES and allowing to observe out-of-equilibrium electronic properties which can be used to extract the electronic coupling with phonons and other degrees of freedom [3, 4]. Ultimately, understanding matter out-of-equilibrium is mandatory for achieving optical control in complex materials [5]. TrARPES can resolve states unoccupied at equilibrium, and has been used to reveal the unoccupied band structure of topological materials [6], to measure optically∗ [email protected] † [email protected]