Mapping optical Bloch modes of a plasmonic square lattice in real and reciprocal spaces using cathodoluminescence spectroscopy.

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.