Collective excitations and instability of an optical lattice due to   unbalanced pumping
  We solve self-consistently the coupled equations of motion for trapped particles and the field of a one-dimensional optical lattice. Optomechanical coupling creates long-range interaction between the particles, whose nature depends crucially on the relative power of the pump beams. For asymmetric pumping, traveling density wave-like collective oscillations arise in the lattice, even in the overdamped limit. Increasing the lattice size or pump asymmetry these waves can destabilize the lattice. 
J. K. Asboth, H. Ritsch, P. Domokos