Mechanical bistability of atoms cooled in an optical lattice

The dynamics of an atom localized in a periodic potential and submitted to a probe wave is studied theoretically and experimentally in a cesium optica l lattice. Because the optical potential is not harmonic, two solutions for the amplitude of motion in steady state are found in a certain range of pa rameters. The free oscillation following the forced excitation exhibits two peaks associated with these two amplitudes. The relative weight of the pea ks. depends on the probe frequency and on the lattice temperature.