Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure

TL;DR

This study uses ultrabroadband terahertz spectroscopy to investigate how dielectric environment modifications in van der Waals heterostructures influence excitonic properties, revealing new insights into exciton dynamics and screening effects.

Contribution

It demonstrates a novel terahertz-based method to probe excitonic transitions and their evolution in heterostructures with atomic-layer precision.

Findings

Red shift and narrowing of excitonic resonances in heterostructures

Observation of dark exciton formation from bright excitons

Mapping of complex mid-infrared conductivity changes

Abstract

Heterostructures of van der Waals bonded layered materials offer unique means to tailor dielectric screening with atomic-layer precision, opening a fertile field of fundamental research. The optical analyses used so far have relied on interband spectroscopy. Here we demonstrate how a capping layer of hexagonal boron nitride (hBN) renormalizes the internal structure of excitons in a WSe$_2$ monolayer using intraband transitions. Ultrabroadband terahertz probes sensitively map out the full complex-valued mid-infrared conductivity of the heterostructure after optical injection of $1s$ A excitons. This approach allows us to trace the energies and linewidths of the atom-like $1s$-$2p$ transition of optically bright and dark excitons as well as the densities of these quasiparticles. The fundamental excitonic resonance red shifts and narrows in the WSe$_2$/hBN heterostructure compared to the bare monolayer. Furthermore, the ultrafast temporal evolution of the mid-infrared response function evidences the formation of optically dark excitons from an initial bright population. Our results provide key insight into the effect of non local screening on electron-hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures.