Lifetime and Emission Characteristics of Electronic Excitations in 2D   Optical Lattices
  Collective electronic excitations "excitons" in planar optical lattices exhibit strong modifications of the radiative damping rate and directional emission pattern as compared to a single excited atom. Excitons for long wave numbers and polarizations orthogonal to the lattice plane exhibit superradiance with a very short life time and a tightly confined emission direction. For shorter wavelength and in plane polarization they can posses a long life time, which beyond a critical wave number tends to infinity. Those excitons thus become metastable and decoupled from the free radiation field. They can store a single energy quantum for a long time and transfer the excitation over long distances. In general the spatial, polarization and frequency dependence of the emission pattern can provide us with optical and electronic properties of optical lattices. 
Hashem Zoubi, and Helmut Ritsch
quant-ph