Excited-state band structure mapping

TL;DR

This paper introduces a novel method using ultrafast extreme ultraviolet light to map the unoccupied electronic states and band structure of materials, providing insights into excited states and electron interactions.

Contribution

It extends traditional band structure mapping to include unoccupied states and excited states using a high-repetition-rate EUV source, demonstrated on WSe2.

Findings

Mapped valence and conduction bands in a single experiment

Observed out-of-equilibrium band gap and its renormalization

Established a new benchmark for excited-state band structure measurement

Abstract

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 fem- tosecond 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.