An Exploration of Hybrid Photonic-Plasmonic and Topologically-Protected Resonances in Van der Waals Nanostructures on Gold

Recently, there has been significant movement towards photonic computing chips, offering lower losses, and additional functionality over conventional electronic systems. Layered van der Waals materials are emerging as ideal candidates for nanophotonics, with interlayer adhesive forces that facilitate simple fabrication of multi-layer heterostructures and devices, without the need for lattice matching. Furthermore, transition metal dichalcogenides in particular offer higher refractive indices than silicon and III-V materials, as well as effcient direct band gap emission in the monolayer limit.
Here, I simulate, fabricate, and optically characterise WS2-based nanostructures to investigate their exotic resonance behaviours. WS2 nanoantennas on gold are shown to host hybrid Mie-plasmonic modes, with higher quality factors than purely dielectric systems. Via tuning of geometrical parameters, strong mode coupling is observed, resulting in a highly confined supercavity mode with an experimental quality factor of over 260. In simulation, a WS2 nanoantenna atop a few-layer hBN spacer on gold is characterised, yielding strong Purcell enhancement of coupled single photon emitters, directivity tailoring, and an overall emission enhancement of over 10^7 times stronger than the same emitter in vacuum. WS2 gratings on gold are further investigated experimentally, where placing structures with opposite topologies together produces a robust Jackiw-Rebbi edge state. This topologically-protected state is highly localised in energy and wave vector space, with a respective 6 meV linewidth and 7° angular emission bandwidth. An active grating heterostructure incorporating an hBN-encapsulated monolayer WSe2 is further fabricated, exhibiting directional coupling of the photoluminescence to the edge state, with an enhancement of 16 times compared to uncoupled monolayer. Scattering-type scanning near-field optical microscopy measurements of both the Mie-plasmonic modes and Jackiw Rebbi edge states are finally presented, providing insight into the near-field properties of such resonances.
These results highlight applications of van der Waals nanophotonics in controllable quantum light emission, robust unidirectional waveguiding, lasing, sensing, and more, with easy integration into a range of other material systems.