  The force exerted on a material by an incident beam of light is dependent
upon the material's velocity in the laboratory frame of reference. This
velocity dependence is known to be diffcult to measure, as it is proportional
to the incident optical power multiplied by the ratio of the material velocity
to the speed of light. Here we show that this typically tiny effect is greatly
amplified in multilayer systems composed of resonantly absorbing atoms (e.g.
optically trapped 87Rb), which may exhibit ultra-narrow photonic band gaps. The
amplification of the effect is shown to be three orders of magnitude greater
than previous estimates for conventional photonic-band-gap materials, and
significant for material velocities of a few ms/s.
