Speaker
Description
Excitons are realizations of a correlated many-body wavefunction, consisting of a Coulomb-bound electron-hole pair. They are the dominant excitations in semiconducting organic and low-dimensional quantum materials and, thus, govern their optoelectronic response. To unlock the full optoelectronic potential and to control exciton-mediated energy conversion pathways, a microscopic understanding of excitons is crucial. Ultimately, this relies on access to the correlated exciton wavefunction, which has hardly been realized in experiments.
In this presentation, I will show how time-resolved photoemission orbital tomography can directly probe correlated exciton wavefunctions. I will demonstrate the power of this technique using the prototypical organic semiconductor C$_{60}$ as an example, unraveling the exciton’s multiorbital electron-hole contributions [1]. Building upon this, I will present our results on ultrafast exciton dynamics at the interface of the organic molecule PTCDA and monolayer WSe$_2$ [2]. Based on their unique momentum fingerprints, we can unambiguously identify the different excitonic states formed after optical excitation of WSe$_2$. Notably, our findings reveal a hybrid exciton state characterized by concomitant intra- and interlayer electron-hole transitions within the molecular layer and across the 2D-organic interface, respectively, which gives rise to an exciton wavefunction with a mixed Frenkel-Wannier character.
Finally, I will discuss our recent realization of a table-top three-dimensional photoemission orbital tomography scheme [3]. In this approach, we extend the photoemission momentum microscope with a spectrally tunable femtosecond high-harmonic generation (HHG) source and a tailored 3D reconstruction algorithm. This enabled us to image the frontier orbitals of PTCDA with full 3D resolution at strongly reduced experimental cost and paves the way for future time-resolved 3D wavefunction imaging.
[1] Bennecke, et al., Nat. Commun. 15, 1804 (2024)
[2] Bennecke, et al., Nat. Phys. 21, 1973–1980 (2025)
[3] Bennecke, et al., arXiv:2502.18269 (2025)