  Controlling optical fields on the subwavelength scale is at the core of any
nanophotonic device. Of particular interest are nanophotonic particle
accelerators that promise a compact alternative to conventional
radiofrequency-based accelerators. Efficient electron acceleration in such
compact devices critically depends on achieving nanometer control of electron
trajectories by precisely designed optical nearfields inside the device.
However, these nearfields have so far been inaccessible due to the complexity
of the devices and their geometrical constraints, hampering efforts to design
and optimize future nanophotonic particle accelerators. Here we present the
first measurement of the field distribution inside a nanophotonic accelerator.
We develop a novel microscopy approach based on photon-induced nearfield
electron microscopy (PINEM) to achieve frequency-tunable deep-subwavelength
imaging of the nearfield inside nanophotonic accelerators. We compare the two
leading designs of nanophotonic accelerators, also known as dielectric laser
accelerators (DLAs): a dual-pillar structure with distributed Bragg reflector
and an inverse-designed resonant structure. Our experiments are complemented by
full 3D simulations, unveiling surprising deviations from the expected designs,
showing complex field distributions related to intricate 3D features in the
device and its fabrication tolerances. We further envision a tomography method
to image the 3D field distribution, key for the future development of
high-precision and hence high-efficiency DLA devices as well as other
nanophotonic devices.
