Three-dimensional vectorial imaging of surface phonon polaritons

Surface phonon polaritons (SPhPs) are coupled photon-phonon excitations that emerge at the surfacesof nanostructured materials. Although they strongly influence the optical and thermal behavior ofnanomaterials, no technique has been able to reveal the complete three-dimensional (3D) vectorialpicture of their electromagnetic density of states. Using a highly monochromated electron beam in ascanning transmission electron microscope, we could visualize varying SPhP signatures fromnanoscale MgO cubes as a function of the beam position, energy loss, and tilt angle. The SPhPs’response was described in terms of eigenmodes and used to tomographically reconstruct thephononic surface electromagnetic fields of the object. Such 3D information promises insights innanoscale physical phenomena and is invaluable to the design and optimization of nanostructuresfor fascinating new uses.