Nano-optical imaging of van der Waals crystals: from ultrafast exciton dynamics to anisotropic waveguide modes

Near-field microscopy has emerged as a powerful tool for investigating the optoelectronic properties of van der Waals crystals on deeply subwavelength length scales. Complementary information may be obtained by interrogating the layered materials with electromagnetic radiation oscillating at vastly different frequencies: In the terahertz spectral range, for example, the polarizability of excitons in transition metal dichalcogenide (TMDC) heterostructures can be recorded on subcycle timescales - granting access to ultrafast formation dynamics or the exciton Mott transition with nanometer precision. In contrast, visible or near-infrared light propagates through thin van der Waals slabs in the form of waveguide modes (WMs). By resolving interference patterns in maps of the scattered electric field, the anisotropic dielectric tensor of layered materials is retrieved and signatures of strong light-matter coupling in the dispersion of the WMs are revealed. This approach also allows for boosting the potential of 3R-stacked TMDCs for applications in nonlinear optics by quantifying their birefringence, thus providing essential parameters for future phase-matched waveguide second harmonic generation and compact on-chip optical devices in general.