  A novel atomic beam splitter, using reflection of atoms off an evanescent
light wave, is investigated theoretically. The intensity or frequency of the
light is modulated in order to create sidebands on the reflected de Broglie
wave. The weights and phases of the various sidebands are calculated using
three different approaches: the Born approximation, a semiclassical path
integral approach, and a numerical solution of the time-dependent Schr\"odinger
equation. We show how this modulated mirror could be used to build practical
atomic interferometers.
