Proximity Transient Absorption in van der Waals Heterostructures

Abstract

Transient absorption spectroscopy is a widely used technique for investigating ultrafast electronic dynamics, where transient optical signals are generally interpreted as arising from excitations within the material hosting the probed optical transition. A distinct form of transient absorption spectroscopy, termed proximity transient absorption (PTA), is demonstrated, in which the probed optical transition and the monitored photoexcited carriers reside in different materials. Using graphene/hBN/WS$_2$ heterostructures, photoexcited carriers in graphene induce a strong transient optical response at the excitonic resonance of a WS$_2$ monolayer through Coulomb screening, even when the two layers remain electronically isolated by an atomically thin hBN spacer. The resulting signal follows the carrier dynamics of graphene rather than those of WS$_2$, demonstrating that the WS$_2$ exciton acts as an optical transducer of electronic processes in a neighboring material. The signal decreases systematically with increasing hBN thickness, confirming its proximity origin while remaining substantial for separations approaching 2 nm. PTA provides a noncontact approach for optically modulating one material using photoexcited carriers in another and enables probing of carrier dynamics in materials that are otherwise difficult to investigate directly. More broadly, PTA establishes a new modality of ultrafast spectroscopy and reveals that transient optical signals in van der Waals heterostructures can originate from excitations residing in spatially separated layers.