Abstract

Complete information about the scattering potential of a sample is in principle encoded in the distribution of scattered electrons from a localized beam propagating through it. A new generation of high-speed, momentum-resolved electron microscope detectors brings us closer to realizing this general goal and in doing so enables new imaging modes spanning sub-Angstrom to multi-micron length scales. Most of these new detectors were adapted from x-ray sensors where fluxes are low. Accordingly, they perform well at the very low count rates appropriate for biological, but are limited to count rates of less than 0.1 pA/pixel by the transit time in the silicon sensor. Our electron-microscope pixel array detector (EMPAD) developed at Cornell is designed to overcome this fundamental weakness of pulse counting with a hybrid architecture that preserves the single electron sensitivity but also remains linear and unsaturated when exposed to the full electron beam. This enables not only measurements of probability current flow that can be used to map electric and magnetic fields in thin samples, but also the orbital angular momentum of an electron beam and resultant torque transfer. The detector has proved useful for a wide range of quantitative applications including the imaging of strain fields in 2D materials, polarization vortices in ferroelectrics, and robust demonstrations of super-resolution imaging by ptychography.