  Transmission electron microscopy (TEM) of vitrified biological macromolecules
(cryo-EM) is limited by the weak phase contrast signal that is available from
such samples. Using a phase plate would thus substantially improve the
signal-to-noise ratio. We have previously demonstrated the use of a high-power
Fabry-Perot cavity as a phase plate for TEM. We now report improvements to our
laser cavity that allow us to achieve record continuous-wave intensities of
over 450 GW/cm$^{2}$, sufficient to produce the optimal 90{\deg} phase shift
for 300 keV electrons. In addition, we have performed the first cryo-EM
reconstruction using a laser phase plate, demonstrating that the stability of
this laser phase plate is sufficient for use during standard cryo-EM data
collection.
