When low-velocity three-level atoms are incident at a glancing angle to a nearly-resonant standing wave light field, the centre of mass motion is affected by light-pressure forces which depend on the internal state of the atoms.
This thesis reports an investigation of the interaction of a thermal sodium beam with a variety of near-resonant evanescent field configurations which was performed between March 1990 and September 1993.The evanescent fields were tuned above the sodium D2 transition, in order to repel atoms at glancing incident angles through the gradient force.Three evanescent field configurations, corresponding to various optic elements, were employed.The first configuration was a travelling evanescent field, from which reflection of the beam was observed at glancing angles up to 4.5mrad.This configuration corresponds to a mirror for atoms.The second configuration was a standing evanescent wave generated by retroreflecting the travelling evanescent wave.The resulting evanescent wave consisted of closely spaced reflective regions separated by non-reflecting nodes of the standing wave.This configuration corresponds to a mirror grating with periodicity 7J2.The atomic beam was reflected from the evanescent field.No diffraction was observed, however.The final configuration was a moving standing evanescent grating, generated by interference between two counterpropagating laser beams.The relative velocity between the atoms and the grating could be continuously varied by adjusting the frequency of one laser.Coherent exchange of photons between the two fields in the moving evanescent grating was observed.This exchange exhibited velocity dependent resonant behaviour corresponding to doppleron resonances.The results are explained in terms of a dressed-state model for diffraction of atoms from such a grating.A parameter search was performed in an attempt to observe the diffraction of the thermal sodium beam, with null results.However, deflections which were dependent on grating velocity are reported.
We consider the velocity-dependent light pressure force on a two-level atom interacting with a two-dimensional nearly resonant standing-wave light field. We show that there can be anisotropic cooling-heating forces that will cool the atoms along one field axis while heating them along the perpendicular axis. The force is identified as a spontaneous vortex force associated with the local traveling-wave character of the two-dimensional field. We provide a simple interpretation of this anomalous force in terms of the interplay between the vortical momentum flow of the light field and a motion-induced population transfer between the ground and excited atomic states.
The interaction of a beam of two level atoms with a standing wave laser field has been the subject of extensive investigation in recent times. A standing wave perpendicular to an atomic beam allows the manipulation of the transverse velocity distribution. The interaction of an atomic beam with a strong standing wave laser field has been systematically investigated by both theoretical simulation and experiment in this thesis. We have developed a numerical model based on a continued fraction solution for the light pressure forces to simulate the near perpendicular interaction of an atomic beam with a strong standing wave laser field. The reshaping of the transverse spatial profile of an atomic beam by a perpendicular, near resonant, standing wave laser field is investigated for red detunings using a longitudinal velocity selective detection system. The experimental dependence of the reshaped profile on the longitudinal and transverse atomic velocities agrees well with a continued fraction solution for the light force. In particular, a regime is found where channelling of the atoms near the antinodes of the standing wave produces a central peak in the spatial distribution of the atomic beam in the far field which have also been observed elsewhere.