  Diffraction gratings synthetically moving at trans-luminal velocities contain
points where wave and grating velocities are equal. We show these points can be
understood as a series of optical event horizons where wave energy can be
trapped and amplified, leading to radiation from the quantum vacuum state. We
calculate the spectrum of this emitted radiation, finding a quasi-thermal
spectrum with features that depend on the grating profile, and an effective
temperature that scales exponentially with the length of the grating, emitting
a measurable flux even for very small grating contrast.
