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Journal of Physics B Atomic Molecular and Optical Physics

Experimental determination of atomic state-dependent optical forces

G. S. Summy, Benjamin T. H. Varcoe, W. R. MacGillivray, M. C. Standage

Published Aug 28, 1997• 5 citations
Publisher

Abstract

An experimental demonstration of the state-dependent nature of the forces exerted on an atom by resonant radiation is reported. Sodium atoms, which are contained in a highly collimated beam, are initially prepared in a particular ground hyperfine substate by optical pumping. They are then deflected by resonant laser radiation of known polarization. Measurements of the magnitude of the deflection show a dependence on the way the atoms were initially prepared, confirming the predictions of a recently developed theoretical model. The experimental data show good agreement with calculations of the theory using a full quantum electrodynamic description of the sodium transition involved.

DOI
10.1088/0953-4075/30/16/001

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Physical Review Letters•May 25, 1992•25 citations
An experimental demonstration of the state-dependent nature of the forces exerted on an atom by resonant radiation is reported. Sodium atoms, which are contained in a highly collimated beam, are initially prepared in a particular ground hyperfine substate by optical pumping. They are then deflected by resonant laser radiation of known polarization. Measurements of the magnitude of the deflection show a dependence on the way the atoms were initially prepared, confirming the predictions of a recently developed theoretical model. The experimental data show good agreement with calculations of the theory using a full quantum electrodynamic description of the sodium transition involved.
This thesis presents a theoretical and experimental investigation of the deflection of an atomic beam by an interaction with a near resonant laser. This investigation focuses on the sensitivity of this force to the initial state of an atom. The term deflection spectroscopy is applied to a specific application of the deflection force as a probe of a collisionally created density matrix. For the purpose of demonstrating the state sensitivity, a method of state preparation is presented that can produce both an and off diagonal density matrix elements in the ground state of sodium, simulating those occurring in a collisional excitation. The atomic collision parameter L1 was determined from a measurement of the deflected momentum of a state prepared atomic beam. A full quantum electrodynamic description of the interaction is used to describe the laser - atom interaction. Incorporated in the description is both the preparation of a ground state and a full description of the momentum exchange. The QED equations are applied to the 32S112 to 32P312 transition of atomic sodium for all possible modes of excitation from the "pure" polarisations, circular (a) and linear (it) to an elliptical polarisation involving combinations and relative phase shifts of these two polarisations. In addition the deflection of a J=2 metastable state is considered, and a set of deflection pseudo -Stokes parameters are developed for both the metastable state and the optically prepared sodium ground state. Comparisons between theory and experiment are made. Both fluorescence measurements and transverse momentum measurements are performed and were compared with theory. All aspects of the interaction were investigated with the theory performing well under all experimental conditions. It was found the deflection was sensitive to the preparation of the atomic ground state of sodium and hence it was demonstrated that atom deflection can be used to determine atomic collision parameters.
A detailed investigation of the momentum absorbed by an atom from resonant light is presented in this thesis. The major topic to be addressed concerns the dependence of absorbed momentum upon the initial quantum state of the atom. This dependence is exploited for the purpose of showing that the absorbed momentum can be used to investigate processes where determining this initial state is important. A specific example where this technique can be applied, electron -atom collision studies, is discussed. It is found that in this case, where the determination of coherences between substates of the electron excited level is critical, the method based upon momentum absorption produces the same general formalism as experiments using the superelastic and electron -photon coincidence techniques. A method for creating coherences between substates in the ground level with rf magnetic fields is described. This is proposed as a way of testing the sensitivity of the optical forces to ground state coherences. A quantum electrodynamic theory is used to describe the photon -atom interaction. This allows all of the atomic states that are important in the interaction to be included. The QED theory provides a solution to the density matrix of the atoms as a function of time, which in turn is used to calculate the average momentum transferred to the atom. Several different atomic systems are considered some hypothetical and some real. The systems to be examined include: a two level system, a J=1 J=2 transition, and the 32S111 (F=2) -> 32P312 transition of atomic sodium. Although the latter example is a complex system requiring the solution to a set of at least 59 first order partial differential equations, its theoretical consideration is vital for comparison to the experimental data.
We have performed experimental and theoretical studies of the deflection of a sodium atomic beam by a laser tuned to the 3 2{\mathrm{}}^{2}2S1/2{S}_{1/2}S1/2​,F=2--3 2{\mathrm{}}^{2}2P3/2{P}_{3/2}P3/2​,F=3 transition. The intensity profile of the deflected beam was measured at different laser power levels and for different laser polarizations. Calculations using a constant force derived from a two-level atom yield deflections which are too large. The results of more realistic simulation calculations agree very well with the measured deflection. Those calculations incorporate the atomic-beam spatial and velocity distributions as well as the laser frequency distribution and polarization state. Optical pumping, loss of atoms to the nonresonant F=1 ground state, and Doppler shifting of the atoms out of resonance are all considered, as also are the statistics and angular distributions of the spontaneous photons.
A simple technique for producing a single-state sodium atomic beam is described. A single laser both optically pumps the F=2 ground-state atoms into mF{m}_{F}mF​=+2 and deflects the pumped atoms away from residual F=1 atoms. Data demonstrating the technique are presented, and a practical design for an apparatus based on these principles is described.
The quantum electrodynamic description of circularly and linearly polarized, single mode radiation exciting atomic transitions in a hyperfine manifold is extended to the more general case of elliptic polarization. Calculations for the sodium D2 line are tested by observing the line polarization of the resonance flourescence scattered perpendicular to the plane containing laser and atomic beams as a function of the laser polarization. Very good agreement between the calculations and the measurements is achieved. It is demonstrated that predetermined populations and substate coherences can be engineered by optically pumping the transition with the appropriately polarized radiation.
A sodium atomic beam is deflected by two optical standing-wave fields which excite near resonance Raman transitions. Observed deflections are consistent with theoretical predictions of a long-range dc component of the force on a three-level atom in the \ensuremath{\Lambda} configuration. The low laser intensities (and small detunings) used, combined with observed evidence of damping in the three-level \ensuremath{\Lambda} system, may ultimately lead to the design of high-density, all optical traps wherein the atoms would be mostly in the dark state.