Real space imaging of nano-tip plasmons using electron energy-loss spectroscopy

Focusing light beyond the diffraction limit is of great interest in photonics. Metallic nano-tips offer the ability to concentrate light in nanometric volumes on femtosecond timescales, facilitating applications in tip-enhanced Raman scattering, optical near field microscopy or ultrafast photoemission. Here, we report the spatial mapping of surface plasmon polaritons on conical gold nano-tips by electron energy-loss spectroscopy (EELS) in combination with scanning transmission electron microscopy. We observe the formation of standing wave patterns formed by reflection of surface plasmon modes in the apex. The experimental EELS maps are in agreement with analytical computations based on a cylindrical mode expansion and full numerical calculations using the discontinuous Galerkin time-domain method. Our results highlight the importance of considering surface plasmon reflection and violations of adiabaticity in nanofocusing.