The interaction of free electrons with light has been a continuous source of new discoveries and technologies, ranging from medical imaging  and microscopy  techniques to bright, ultrafast x-ray sources  and particle accelerators . In recent years, particular attention was devoted to such interactions occurring in platforms with a strong near-field component [5,6], revealing novel ways to manipulate the wavefunction of free electrons in an inherently quantum-mechanical way. As such, it is clear that precise control over the near-field distribution can enable new physical observations and technological capabilities, making it the main focus of our work.
By passively and actively controlling the transverse distribution of a plasmonic field, we generated various high-quality electron probability distributions and shifted between them, while altering the resulting pattern through post-selection . By employing a hybrid photonic-plasmonic waveguide design to control the longitudinal near-field distribution, 2D Cherenkov radiation and its quantized nature were observed for the first time, along with a record-high free-electron - photon coupling strength . Our investigation pushes the limit of free-electron interactions with light to new regimes, establishing free electrons as an alternative route for quantum optics and making a significant step towards the long-standing goal of arbitrary spatial modulation of electrons.