  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 $\pi/2$ 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-P\'{e}rot 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 $\pi/2$ 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
