Ultrafast Nano-Imaging of Dark Excitons

Dynamical processes in the condensed matter occur on the femtosecond time- and the nanometer length-scale. This very general statement holds true for fundamental processes such as the dissipation of energy after an optical excitation or the creation of light-matter coupled phases. At the same time, it showcases that any nanoscale inhomogeneity that affects the quasiparticle dynamics can, in the end, limit the performance of a real-world device. In consequence, there are a multitude of research efforts that work towards the development of new experimental techniques that can provide a holistic picture of dynamical processes on ultrashort time- and length-scales.<br/><br/>In this contribution, we discuss our recent efforts to extend the full capabilities of time- and angle-resolved photoemission spectroscopy (trARPES) to the nanoscale. In short, we will show how we can monitor the femtosecond evolution of spectral weight at a specific in-plane momentum k and kinetic energy E with 50 fs time- and 500 nm spatial-resolution [1]. Experimentally, this is achieved by the development of ultrafast dark-field photoelectron microscopy: We employ our table-top high-repetition rate high-harmonic generation beamline and time-of-flight momentum microscope [2] and combine it with the full capabilities of dark-field imaging techniques [1].<br/><br/>In the presentation, we then present the capabilities of ultrafast dark-field photoelectron microscopy in three steps: First, in a spatially integrated mode of the experiment, we will discuss momentum-resolved photoemission signatures of intralayer, interlayer and hybrid excitons and identify distinct hallmarks of the moiré superlattice [3]. Second, we will show that interlayer excitons are effectively formed via exciton-phonon scattering, and subsequent interlayer tunneling at the interlayer hybridized Σ_W valleys on the sub-100 fs timescale [3,4]. And third, we will report on the spatio-temporal and spatio-spectral dynamics of bright and dark excitons in a laterally inhomogeneous TMD heterostructure. Most interestingly, we find that the rate of charge transfer across the type-II band aligned WSe2/MoS2 interface can vary by more than a factor 2. We explain this discrepancy with a locally varying energy landscape of excitons, as directly accessible with ultrafast dark-field photoelectron microscopy [1].<br/><br/>[1] Schmitt <i>et al</i>., arXiv:2305.18908 (2023).<br/>[2] Keunecke <i>et al</i>., Rev. Sci. Ins. <b>91</b>, 063905 (2020).<br/>[3] Schmitt <i>et al</i>., Nature 608, 499 (2022).<br/>[4] Bange <i>et al</i>., 2D Materials 10, 035039 (2023).