  We describe propagation of light in a gas with periodic density modulation,
demonstrating photonic-crystal-like refraction with negative refraction angles.
We address the role of poorly defined boundaries and damping, and derive an
optical analog of the quantum adiabatic theorem. For Cs atoms in an optical
lattice, we show that relying on semi-adiabatic propagation one can excite and
spatially split positively and negatively refracting modes at experimentally
available gas densities.
