Ultrafast interlayer charge transfer probed by subcycle contact-free THz nanoscopy

Tunneling is one of the most fundamental phenomena in quantum mechanics, and a hallmark of interlayer exciton formation in transition metal dichalcogenide heterostructures. Here, we introduce a new contact-free nanoscopy technique centered around probing the local polarizability of photogenerated electron–hole pairs with evanescent terahertz fields. This concept is even capable of tracing ultrafast charge carrier dynamics in insulating materials. Thus, we are able to resolve the interlayer tunneling process in a heterobilayer comprising atomically thin sheets of WSe2 and WS2 with <50nm spatial and subcycle temporal resolution. Nanoscale strain and changes in atomic registry give rise to pronounced variations of the formation and annihilation dynamics of interlayer excitons. Our results demonstrate the potential of this technique for revealing how ultrafast tunneling governs the properties of a large variety of solid-state systems.