Imaging the field inside nanophotonic devices

TL;DR

This paper introduces a novel microscopy technique to directly image the electromagnetic fields inside nanophotonic accelerators, providing insights crucial for optimizing device design and performance.

Contribution

It presents the first measurement of internal fields in nanophotonic accelerators using photon-induced nearfield electron microscopy (PINEM), revealing complex field distributions and deviations from expected designs.

Findings

Successful deep-subwavelength imaging of accelerator fields

Discovery of complex field patterns due to 3D device features

Identification of fabrication-related deviations in device performance

Abstract

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.