Subcycle terahertz nanoscopy of ultrafast interlayer dynamics in van der Waals heterostructures

Tunneling is one of the most fundamental manifestations of quantum mechanics determining elementary physical processes, chemical reaction pathways and shaping life as we know it. Moreover, it is crucial for modern data storage and electronics, and is essential for highly efficient solar technology. In this work, we introduce a novel, non-invasive concept to resolve electron tunneling on the relevant length- and timescales that even works on insulating samples. The central idea is to monitor the evolution of the local polarizability of electron-hole pairs during the tunneling process with evanescent terahertz nearfields, which are detected with subcycle temporal resolution. In a proof of concept, we resolve the ultrafast interlayer dynamics in van der Waals heterobilayers utilizing our new technique of subcycle contact-free nanoscopy to access the full life cycle of photo-induced spatially separated interlayer electron-hole pairs. Our approach builds on the drastic change of the polarizability of the electron-hole pairs during interlayer tunneling as explained by ab initio density functional theory calculations. We confirm the temporal dynamics using a complementary terahertz emission experiment that is directly linked to the ultrafast charge separation. Moreover, we reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer lengthscales. Such contact-free nanoscopy of tunneling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems