Visualizing band structure of Bloch polaritons

Surface plasmon polaritons (SPPs) are hybrid modes of light and electrons on conducting surfaces. Analogous to Bloch electrons in crystals, SPPs traveling in a medium with spatially varied properties experience scattering and interference, which transforms SPPs into Bloch polaritons. Bloch polaritons possess the universal attributes of Bloch quasiparticles whose energy-momentum dispersion is dictated by the underlying band structure. It is therefore feasible to reconstruct the polariton bands structure by exploring the real space motion of ballistic Bloch wavepackets. We visualized the band structure of Bloch polaritons in a graphene-based polariton crystal by utilizing near-field imaging techniques to record the propagation of Bloch polaritons launched by nano-structured antennas. Bloch polaritons emanating from a point antenna reveal star-shaped patterns which encode information for the underlying polariton band structure. Fourier analysis of the near-field images reveals the isofrequency contours of the polariton bands which can subsequently be stacked to form a complete band structure. As the energy bands in graphene polariton crystals are tunable by gate voltage, we were able to switch on demand the propagation direction of Bloch polaritons and make a polariton switch.