Dynamical Density Grating in a Hot Vapor

We create ii dark optical lattice structure in ii scanding wave tuned to the blue of the F=3 to F=3 Irtinsition ofthe XSRubidiuin DI-line. The coupling of the F=3 ground slate lo both the F=2 and F=3 excited states induces a periodic potential i n which the atoms are trapped at locations of purely circular polarisation, wherc they itre only weakly coupled to the light field. We adiabatically release the atoms rrom the optical lattice and measure their inomentuin distribution with a resolution of one third of 21 single photon recoil. During the adiabatic release the quasi-momentum and the population of the energy bands are conserved. The population of the lowest energy band is mapped on the -hk to +hk momentum interval of the free atom, and the first energy band is mapped on the two momentum intervals -2hk to -hk and hk to 2hk. The figure shows the measured populations i n comparision with those determined by a numerical simulation, which uses a rate equation approach to obtain the steady state populations of the Bloch states.