The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adpr.202100124.

Transition-metal dichalcogenides with their exciton-dominated optical behavior emerge as promising materials for realizing strong light–matter interactions in the visible range and at ambient conditions. When these materials are combined with metals, the energy confining ability of plasmon polaritons in metals below the diffraction limit allows for further enhancing and tailoring the light–matter interaction due to the formation of plexcitons in hybrid metal-TMDC structures at the interface. Herein, we demonstrate that the coupling between quasi-propagating plasmons in plasmonic crystals and excitons in WSe2 provides a multioscillator playground for tailoring the band structure of plasmonic crystal structures and results in emerging flat bands. The cathodoluminescence spectroscopy and angle-resolved measurements combined with the numerically calculated photonic band structure confirm a strong exciton–plasmon coupling, leading to significant changes in the band diagram of the hybrid lattice and the ability to tailor the band diagram via strong coupling. The hybrid plexcitonic crystal structures investigated here sustain optical waves with remarkably low group velocities. These results could be used for designing tunable slow-light structures based on the strong coupling effect and pave the way for plexcitonic topological photonic structures.

Strong electron-light interactions supported by the surface plasmon polaritons excited in metallic thin films can lead to faster optoelectronic devices. Merging surface polaritons with photonic crystals leads to the formation of Bloch plasmons, allowing for the molding of the flow of polaritons and the controlling of the optical density of states for even stronger electron-light interactions. Here, we use a two-dimensional square lattice of holes incorporated inside a plasmonic gold layer to investigate the interaction of surface plasmon polaritons with the square lattice and the formation of plasmonic Bloch modes. Cathodoluminescence spectroscopy and hyperspectral imaging are used for imaging the spatio-spectral near-field distribution of the optical Bloch modes in the visible to near infrared spectral ranges. In addition, the higher-order Brillouin zones of the plasmonic lattice are demonstrated by using angle-resolved cathodoluminescence mapping. We further complement our experimental results with numerical simulations of the optical modes supported by the plasmonic lattice that helps to better resolve the superposition of the various modes excited by the electron beam. Next to previous works in this context, our results thus place cathodoluminescence scanning spectroscopy and angle-resolved mapping as complementary techniques to uncover the spatio-spectral distribution of optical Bloch modes in real and reciprocal spaces.