Low image contrast is a major limitation in transmission electron microscopy, since sampleswith 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-Perot 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 590GW/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 bythe 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.