KEYWORDS: Terahertz spectroscopy, Near infrared, Excitons, Crystals, Waveguide modes, Near field, Dielectrics, Near field optics, Waveguides, Second harmonic generation, Microscopy
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.
Select quantum materials can support polaritons, hybrid light matter waves, with sub-diffraction-limited confinement. In this talk I will overview recent progress on polaritons in hyperbolic materials, which propagate as conical rays throughout the bulk of these crystals. I will discuss polaritons in a class of hyperbolic hetero-bicrystals. Our data reveals negative refraction, spectral gaps and wave localization can occur in these systems.
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.
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