Photonic crystal optics in cold atomic gases

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.