Selected Mode Mixing and Interference Visualized within a Single Optical Nanoantenna

Interference-based directional antennas typically consist of multiple dipoles with properly set distances and phases, which cause constructive interferences toward certain directions in radiation or reception. For nano-optical antennas, the directionality can be realized by superposition of multiple eigenmodes in a single structure. Such mode mixing creates locally strong field enhancement, which should be properly controlled for energy conversion or sensing applications. However, experimental verification of the nano-optical field, or especially the hot-spots, created by interference of selected eigenmodes is not trivial. We here visualize how optical fields are distributed when multiple modes interfere within a silver disk nanoantenna. We use angle- and polarization-resolved cathodoluminescence based on scanning transmission electron microscopy to select specific modes and visualize the field distribution at the nanoscale. The interfered field distribution significantly changes depending on the detection angles even when the detection geometry is symmetric, which can be explained by the phase differences of the excited modes. The cathodoluminescence signals are also modeled as superpositions of analytical eigenmode functions consisting of multipoles in space and complex Lorentzians in frequency to reproduce the experimentally obtained photon maps.