External screening and lifetime of exciton population in single-layer ReSe$_2$ probed by time- and angle-resolved photoemission spectroscopy

TL;DR

This study investigates the ultrafast dynamics and spatial extent of excitons in single-layer ReSe$_2$ using time- and angle-resolved photoemission spectroscopy, revealing polarization-tunable exciton decay and delocalization effects.

Contribution

It provides the first femtosecond-scale measurements of exciton lifetime, polarization dependence, and real-space size in single-layer ReSe$_2$, advancing understanding of its optoelectronic properties.

Findings

Exciton decay times are approximately 110 fs and 650 fs.

The exciton distribution can be tuned by pump pulse polarization.

Excitons are delocalized over more than 17.1 Å due to substrate screening.

Abstract

The semiconductor ReSe$_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ReSe$_2$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53 \pm 0.02)$ eV, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by $\tau_1 = (110 \pm 10)$ fs and $\tau_2 = (650 \pm 70)$ fs, facilitated by recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than 17.1 \AA, implying significant exciton delocalization due to screening from the bilayer graphene substrate.