Plasmonic-Nanofocusing-Based Electron Holography

Point-projection microscopy (PPM) with low energy electrons is developing into a powerful tool for holographic reconstruction of individual proteins and solid state nanostructures. In combination with laser-based photoemission schemes, it offers the exciting prospect of ultrafast coherent electron holography of single nanostructures. Such experiments would greatly benefit from a freestanding electron source with femtosecond time resolution, few-nm emitter size, and good coherence properties. Here, we use plasmonic nanofocusing on a conical gold taper and multiphoton photoemission from the taper apex to create such a source. It is implemented in a PPM setup and used to record in-line holograms of thin bundles of single-walled carbon nanotubes, demonstrating an effective emitter radius of less than 5 nm. We show that the same concept can also be transferred to tungsten tips, offering further improvements in emitter size and brightness. Numerical simulations show that such an ultrafast, low-energy electron source presents a highly interesting tool for probing optical fields at surfaces with nanometer spatial and femtosecond time resolution.