Propagating Elementary Excitation in a Dilute Optical Lattice

Propagating elementary excitations characterized by a well-defined propagation velocity (such as acoustic waves) are usually considered as being typical of condensed matter or dense fluids. We show that this requirement is actually not necessary. We predict in the case of a dilute optical lattice the occurrence of a propagating excitation mode which can induce a stimulated scattering mechanism analogous to stimulated Brillouin scattering, although it does not involve any interaction between atoms. We also present the results of an experimental investigation in cesium that demonstrates the existence of this novel stimulated scattering process.