A Laser Phase Plate for Transmission Electron Microscopy

Abstract

Low image contrast is a major limitation in transmission electron microscopy, since samples with low atomic number only weakly phase-modulate the illuminating electron beam, and beam-induced sample damage limits the usable electron dose. The contrast can be increased by converting the electron beam's phase modulation into amplitude modulation using a phase plate, a device that applies a $π/2$ radian phase shift to part of the electron beam after it has passed through the sample. Previous phase plate designs rely on material placed in or near the electron beam to provide this phase shift. This results in image aberrations, an inconsistent time-varying phase shift, and resolution loss when the electron beam charges, damages, or is scattered from the material. In this thesis, I present the theory, design, and implementation of the laser phase plate, which instead uses a focused continuous-wave laser beam to phase shift the electron beam. A near-concentric Fabry-Pérot optical cavity focuses and resonantly enhances the power of the laser beam in order to achieve the high intensity required to provide the phase shift. We demonstrate that the cavity can surpass this requirement and generate a record-high continuous-wave laser intensity of $590 \, \mathrm{GW}/\mathrm{cm}^{-2}$. By integrating the cavity into a transmission electron microscope, we show that the ponderomotive potential of the laser beam applies a spatially selective phase shift to the electron beam. This enables us to make the first experimental observation of the relativistic reversal of the ponderomotive potential. We then theoretically analyze the properties of the contrast transfer function generated by the laser phase plate. We experimentally determine that resolution loss caused by thermal magnetic field noise emanating from electrically conductive materials in the cavity can be eliminated by designing the cavity with a sufficiently large electron beam aperture. Finally, we show that the laser phase plate provides a stable $π/2$ phase shift and concomitant contrast enhancement when imaging frozen hydrated biological macromolecules. We use these images to successfully determine the structure of the molecules. This demonstrates the laser phase plate as the first stable and lossless phase plate for transmission electron microscopy