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Journal of Physics B Atomic and Molecular Physics

Population trapping in the neon 2p3to 1s4transition

Russell McLean, R. J. Ballagh, D. M. Warrington

Published Jun 28, 1985• 24 citations
Publisher

Abstract

The fluorescence observed when a single-mode laser drives the 2p 3 (J=0) to 1s 4 (J=1) neon transition displays a narrow minimum as an applied magnetic field is varied. A detailed theoretical analysis of the system confirms that this resonance is due to the formation of a Zeeman coherence in the lower level which traps the population there. The dominant contribution to the population trapping comes from the atomic velocity classes for which the atom-laser detuning is close to zero, and for the case of exact resonance analytic expressions are obtained for dip width and depth. A broad minimum which also appears in the profile is shown to originate from incoherent processes in the atomic ensemble.

DOI
10.1088/0022-3700/18/12/016

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The fluorescence observed when a single-mode laser drives the 2p 3 (J=0) to 1s 4 (J=1) neon transition displays a narrow minimum as an applied magnetic field is varied. A detailed theoretical analysis of the system confirms that this resonance is due to the formation of a Zeeman coherence in the lower level which traps the population there. The dominant contribution to the population trapping comes from the atomic velocity classes for which the atom-laser detuning is close to zero, and for the case of exact resonance analytic expressions are obtained for dip width and depth. A broad minimum which also appears in the profile is shown to originate from incoherent processes in the atomic ensemble.
Laser-induced Zeeman coherences and non-linear processes in the neon 2p 5 (J = 1) to 1s 5 (J = 2) transition have been studied experimentally and theoretically. By varying the observation geometry and choice of polarisation, the fluorescence observed in the 2p 5 to 1s 3 (J = 0) transition becomes sensitive to different processes within the atom. A detailed theoretical treatment enables profiles to be matched to the experiment using an appropriate choice of parameters. Moreover, analysis of the equations of motion for the system allows the origins of the various features of the fluorescence profiles to be precisely identified in terms of the generation of higher-order Zeeman coherences in the 1s 5 level and coherent trapping of part of the atomic population there. The population trapping can be interpreted and analysed, for a particular choice of geometry, in terms of simpler subsystems of the full transition.
The authors have investigated the zero-field level crossing lineshape in resonance absorption. A single-mode HeNe laser beam at 633 nm is applied to an external neon-gas absorption cell in a longitudinal magnetic field. They compare the measured lineshapes with theoretical predictions. Two superposed nonlinear Lorentzian-shaped resonances, known as the population effect of the 2p 4 -3s 2 transition and the Zeeman coherence effect of the lower 2p 4 level, appear on the background caused by the inhomogeneous broadening. Taking into account the background and the power broadening the authors obtain the linewidths at zero pressure gamma (2p 4 ,3s 2 )=(7.7+or-2.1) MHz and gamma (2p 4 )=(8.4+or-1.2) MHz. For the neon pressure broadening the authors get for pressures below 200 Pa (1.5 Torr) d gamma (2p 4 ,3s 2 )/dp=0.20+or-0.02 MHz Pa -1 and d(2p 4 )/dp=0.040+or-0.007 MHz Pa -1 . Their values are in acceptable agreement with the values obtained with other spectroscopic methods.
Coherent population trapping in a gas discharge was studied for transitions between excited levels of neon atoms. Resonances corresponding to the arising of coherent population trapping in the Λ and V schemes for Zeeman sublevels of the lower and upper working states were observed in the presence of a longitudinal magnetic field. The effect of nonlinear polarization plane rotation under coherent population trapping conditions was studied. The possibility of using the results obtained in this work for diagnostics of local magnetic fields and other plasma parameters in gas discharges was considered.
The simultaneous action of a RF perturbation between the Zeeman sublevels of an atomic transition, which is also sustaining laser oscillations, is given further consideration. General equations are derived that describe the phenomena in a rotating effective magnetic field basis. These are valid for any level of the RF perturbation, being then solved by iteration to third order in the laser electric field and integrated exactly over the atomic velocity distribution. Results are given for single-π-mode laser operation on aJ = 1 \rightarrow 0transition and for single-mode operation on each of the well-resolved σ components of the Zeeman splitting. The extension to more complex transitions, such as aJ = 1 \rightarrow 2, is then given. A dominant resonance peak in the laser intensity occurs when the RF equals the Zeeman splitting of either the upper or lower states of the transition. Population differences amongst the upper sublevels are more effective than those between the lower sublevels and give the smallest resonance widths. For operation on the σ modes, the main effect is a reduction in the saturation coefficient β, together with smaller changes in the coupling coefficients θ between the oscillations. Such changes will affect the general behavior near the singular points of the nonlinear equations, and will change the shape of the curves of mode intensities as functions of cavity detuning and magnetic field.
We observe the population dynamics within a metastable neon magneto-optical trap (MOT) through the measurement of the average squared Clebsch-Gordan coefficient C2{C}^{2}C2 over a range of laser detunings. The magnitude of C2{C}^{2}C2 is dependent on the internal quantum state of an atom interacting with the light field and is found to show a strong dependence on the applied laser detuning. Previously it has been reported [Townsend et al., Phys. Rev. A 52, 1423 (1995)] that trapped atoms in a MOT are pumped towards the states that interact most strongly with the local field and therefore the measured value of C2{C}^{2}C2 is larger than the average over all possible transitions. For the 3P2{}^{3}{P}_{2}3P2​-to-3D3{}^{3}{D}_{3}3D3​ cooling transition in metastable neon the average C2{C}^{2}C2 value is equal to 0.46; however, we have measured $0.29\ifmmode\pm\else\textpm\fi{}0.03<{C}^{2}<0.73\ifmmode\pm\else\textpm\fi{}0.09$. We explain this range of values for C2{C}^{2}C2 by considering the possible transition rates between the different magnetic sublevels in the system. This result has significant consequences when trap populations are measured via fluorescence in a MOT.
We observe coherent population trapping (CPT) in a two-electron atom— 174 Yb—using the transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a Λ -type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips—the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.