Time- and angle- resolved photoelectron spectroscopy (tr-ARPES) offers the unmatched possibility to resolve the charge carrier dynamics upon optical excitation in solids. In parallel with the possibility to access the unoccupied band dispersion [1], tr-ARPES is capable to track the change in the materials’ band structure. This strategy has been often used for investigating strongly correlated electron systems, such as charge density waves [2] and high temperature superconductors [3, 4], for which the optical excitation triggers large change in the single-particle spectral function. The establishment of a new generation of tr-ARPES setups, like Harmonium at the EPFL, with improved experimental conditions, has made possible to extend the study of change in the materials’ band structure to weakly correlated electron systems.
In my talk I will focus on two compounds, which have recently attracted the great interest of the condensed matter community: black phosphorus [5] and the nodal line Dirac semimetal [6]. The optical excitation of high-energy electrons in the unoccupied conduction band is responsible for an ultrafast change in the electronic screening. As a consequence the band gap and band velocities are renormalized, and these change are resolved in the time, energy and momentum domains via tr-ARPES.

[1] J. C. Johannsen et al., Phys. Rev. B 91 201101 (2015)
[2] S. Hellman et al., Nat. Commun. 3 1069 (2012)
[3] R. Cortés et al., Phys. Rev. Lett. 107 097002 (2011)
[4] C. L. Smallwood et al., EPL 115 27001 (2016)
[5] S. Roth et al., 2D Materials 6 031001 (2019)
[6] G. Gatti, A. Crepaldi et al., arXiv:1912.09673 (2019)