Germanium Sulfide Nano-Optics Probed by STEM-Cathodoluminescence Spectroscopy.

Nano-optical studies of confined modes in planar waveguides have attracted significant interest as a means to probe exciton-polaritons and other hybrid light-matter quasiparticles in layered semiconductors, such as transition metal dichalcogenides or boron nitride. There is a need to broaden such studies to other materials and to identify alternatives to scanning near-field optical microscopy for exciting and measuring confined waveguide modes. Here, we establish an approach for probing the dispersion of traveling waveguide modes by cathodoluminescence spectroscopy excited by the focused electron beam in scanning transmission electron microscopy (STEM-CL) and apply it to solid-state resonators consisting of mesoscale monocrystalline prisms and plates composed of GeS, an anisotropic layered semiconductor with direct bandgap in the near-infrared spectral range. Structure, crystallography, and chemical composition of the mesostructures are analyzed by analytical electron microscopy. STEM-CL maps and spectra show pronounced interference effects and sharp emission peaks at photon energies below the fundamental bandgap of GeS. Our analysis demonstrates that locally excited light emission in STEM-CL launches in-plane waveguide modes in the mesoscale GeS structures, which are internally reflected by highly specular GeS edges to cause interference of the waveguide modes. Reabsorption and secondary luminescence give rise to the intensity modulations detected in the far field. Our results highlight avenues for probing light-matter interactions below the diffraction limit in a wide range of quantum materials and open up the possibility of tuning light emission geometrically using interference rather than by the conventional bandgap engineering.