Experimental investigation of the reflection of thermal sodium atoms from a moving evanescent grating [article]

Bruce Stenlake, University, The Australian National, University, The Australian National
2018
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
more » ... ling 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. Chapter 2 Theoretical Considerations Chapter 2 Theoretical Considerations This chapter describes the concepts required for an understanding of the project. A historical introduction into the theory is developed, beginning with forces on atoms and concluding with the dressed state, quasipotential model. The project described in this thesis was primarily experimental. Consequently, a limited theoretical development will be presented in this chapter. The discussion is divided into four sections. The first section discusses the forces which an atom may experience in a near-resonant light field. A more rigorous discussion for this background theory is presented by Cohen-Tannoudji (1991) and Adams et al (1994) . The second section introduces the concept of the atomic mirror, first presented by Cook and Hill (1982) . The use of a standing evanescent wave to generate a diffraction grating for atoms, proposed by Hajnal and Opat (1989) , is discussed in the third section. To describe the processes occurring inside the standing evanescent wave, the quasipotential model developed by Deutschmann et al (1993) is reviewed in the final section. Near-Resonant Light Forces on Atoms This project utilises an evanescent wave to reflect neutral sodium atoms using the gradient force. There are a number of ways to derive this force, of which the simplest may be the semi-classical approach due to Dowling et al (1996) and Cook (1979) . Let the interaction potential term, V(R), between the electromagnetic field and the atom be, in the dipole approximation, Abstract -The reshaping of the transverse spatial profile of an atomic beam by a perpendicular, near-resonant, standing-wave laser field is investigated for negative frequency detunings using a longitudinal velocity-selec tive detection system. The experimental dependence of the reshaped profile on the longitudinal and transverse atomic velocities agrees qualitatively 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 atomic beam profile in the far field. The central peak is predicted when the force generated by the periodic component of the standing-wave potential is calculated using the continued fraction solution but not when approximated by the (zero velocity) gradient force model.
doi:10.25911/5d67b687b228e fatcat:6fsq75yhyfddlpwtm3a5wa5gei