Electron beam shaping with light

The precise transverse control of electrons is at the heart of any high-resolution electron microscope where electron lenses and multipole aberration correctors have enabled sub-Angström spatial resolution. This thesis describes a novel way of achieving transverse control over the electrons by interaction with shaped light fields. An experimental setup has been built from scratch and is based on a scanning electron microscope, which has been modified to allow ultra-short laser-triggered electron pulses and operation in transmission mode. In the modified specimen chamber of the scanning electron microscope, the electrons interact in free space with a counter-propagating transversely shaped laser. During the interaction with the laser, the electrons acquire a local phase shift proportional to the laser's local pulse energy density. Electron displacements are measured with a position-sensitive detector. The transverse profile of the laser beam is shaped by diffractive optical phase masks, which are displayed on a spatial light modulator. A Gerchberg-Saxton algorithm has been modified to calculate the required phase mask, which allows for minimizing heating effects along the beam path and produces speckle-free laser distributions. For the electron-light interaction to take place, temporal and spatial overlap between pulses is established in a two-step process without an energy filter. First, a fast-timing scintillator is placed in the interaction plane. The arrival time of electron-triggered photons and laser is recorded with a silicon photomultiplier, and the delay stage is adjusted accordingly. Second, pump-probe measurements are carried out with copper grid in the interaction plane, synchronizing the pulses with an uncertainty of ∼ 1ps. Programmable lossless transverse electron beam-shaping and both concave and convex electron lensing via ponderomotive scattering in free space is demonstrated. Furthermore, an electron Ronchigram in pulsed mode is recorded, which opens up the possibility of doing coherent experiments with the modified microscope. Selected applications that can arise from the electron phase modulation with light are programmable phase plates for contrast enhancement, generation of exotic beams, aberration correction of traditional electron lenses, and more versatile electron microscopes with lenses made out of light. (author)