Near-field nanoscopy of excitons and ultrafast interlayer dynamics in van der Waals crystals

Tip-based nanoscopy techniques have emerged as powerful tools for probing the exceptional optoelectronic properties of van der Waals crystals (vdW) on deeply sub-wavelength scales. Based on two sets of experiments, we demonstrate how bound electron-hole pairs - so-called excitons - can be interrogated with near-field microscopy. First, we build on terahertz nanoscopy with subcycle temporal resolution to access the separation of photo-carriers via interlayer tunneling and their subsequent recombination in transition metal dichalcogenide bilayers. By tracing the local polarizability of electron-hole pairs with evanescent terahertz fields, we reveal pronounced variations of the exciton dynamics on the nanoscale. This approach is uniquely suitable to reveal how ultrafast charge transfer processes shape functionalities in a variety of solid-state systems. Second, we image waveguide modes (WMs) in thin flakes of the biaxial vdW crystal ReS2 across a wide range of near-infrared frequencies. Resolving the dependence of the WM dispersion on the crystallographic direction, polarization of the electric field and sample thickness, enables us to quantify the anisotropic dielectric tensor of ReS2 including the elusive out-of-plane response. The excitonic absorption at ∼1.5 eV induces a backbending of the dispersion and increased losses of the WMs as fully supported by numerical calculations. Thus, we provide crucial insights into the optical properties of ReS2 and explore light-matter coupling in layered, anisotropic waveguides. Our findings set the stage for probing ultrafast dynamics in biaxial vdW crystals on the nanoscale.