Ultrafast band renormalizations in 2D materials

Two-dimensional (2D) materials provide a versatile platform to explore and control emergent electronic phenomena arising from reduced dimensionality and strong many-body interactions. In particular, their electronic structure can be efficiently engineered via external fields, charge transfer, and coupling to lattice degrees of freedom, opening pathways toward novel electronic and optoelectronic functionalities. In this talk, I will present time- and angle-resolved photoemission spectroscopy (tr-ARPES) studies that reveal how ultrafast non-equilibrium processes can be used to dynamically renormalize electronic band structures in prototypical 2D systems on femtosecond timescales. First, I will discuss bilayer graphene, where photoinduced interlayer charge transfer in a van der Waals heterostructure enables a transient modification of the interlayer potential asymmetry, effectively acting as an ultrafast optoelectronic gate [1]. This mechanism allows for femtosecond control of the band gap, complemented by additional band renormalization effects arising from non-equilibrium screening through via hot carriers. In a second example, I will focus on the role of electron-phonon interactions in the Weyl semimetal Td-WTe₂. By tracking coherent phonon excitations in tr-ARPES, we resolve mode-selective couplings between lattice vibrations and electronic bands. In particular, we observe a transient modulation of a Dresselhaus-type spin splitting driven by an interlayer shear mode, providing real-time insight into electron-phonon coupling mechanisms relevant for light-induced topological phase transitions [2]. Together, these results demonstrate how ultrafast optical excitation enables selective control over charge, lattice, and spin degrees of freedom in 2D materials, offering new perspectives for the manipulation of quantum states and the design of novel ultrafast devices.
[1] E. Moos et al., arXiv:2602.18065 (2026)
[2] P. Hein et al., Nat. Commun. 11, 2613 (2020)