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Physical Review A

Effects of magnetic sublevel degeneracy on Rabi oscillations

Bruce W. Shore

Published May 1, 1978• 35 citations
Publisher

Abstract

This paper describes the dynamics of multilevel excitation by coherent radiatin, in the $N$-level rotating-wave approximation, when the levels have nonzero angular momentum $J$ and consequent spatial-orientation degeneracy $2J+1$. Observable populations average over $2J+1$ independent sublevel excitation sequences. The phenomenon of inaccessible sublevels is noted, as is the effect of polarization. The paper illustrates how averaging over orientation can, in some cases, render illegible the Rabi oscillations and the ac Stark effect, whereas in other cases ($J\ensuremath{\rightarrow}J$, circular polarization on $J\ensuremath{\rightarrow}J\ifmmode\pm\else\textpm\fi{}1$ linear polarization) appreciable structure remains.

DOI
10.1103/physreva.17.1739

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Coherent population transfer in multilevel systems with magnetic sublevels. I. Numerical studies

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The technique of stimulated Raman adiabatic passage has become an established procedure for producing complete population inversion in atoms or molecules via application of Stokes and pump pulses in a counterintuitive sequence. We discuss here some of the new and important phenomena that arise when some of the states involved have nonzero angular momentum, the field polarization directions have no simplifying symmetries, and Zeeman splitting lifts the magnetic sublevel degeneracy. We consider the effects of the linkage structure of the Hamiltonian, the effects of various polarization directions, and the effects of various choices for carrier frequencies, as expressed by detunings from single-photon resonance. In particular, we consider an example with angular momentum J=0\ensuremath{\leftrightarrows}J=1\ensuremath{\leftrightarrows}J=2 in which nine magnetic sublevels occur. We show that under suitable conditions it is possible, simply through the appropriate tuning of pump and Stokes carrier frequencies, to place the entire population into any specified single magnetic sublevel.

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Physical Review A•Mar 7, 2001•8 citations
This paper describes the dynamics of multilevel excitation by coherent radiatin, in the NNN-level rotating-wave approximation, when the levels have nonzero angular momentum JJJ and consequent spatial-orientation degeneracy 2J+12J+12J+1. Observable populations average over 2J+12J+12J+1 independent sublevel excitation sequences. The phenomenon of inaccessible sublevels is noted, as is the effect of polarization. The paper illustrates how averaging over orientation can, in some cases, render illegible the Rabi oscillations and the ac Stark effect, whereas in other cases (J\ensuremath→JJ\ensuremath{\rightarrow}JJ\ensuremath→J, circular polarization on J\ensuremath→J\ifmmode±\else\textpm\fi1J\ensuremath{\rightarrow}J\ifmmode\pm\else\textpm\fi{}1J\ensuremath→J\ifmmode±\else\textpm\fi1 linear polarization) appreciable structure remains.
The technique of stimulated Raman adiabatic passage has become an established procedure for producing complete population inversion in atoms or molecules via application of Stokes and pump pulses in a counterintuitive sequence. We discuss here some of the new and important phenomena that arise when some of the states involved have nonzero angular momentum, the field polarization directions have no simplifying symmetries, and Zeeman splitting lifts the magnetic sublevel degeneracy. We consider the effects of the linkage structure of the Hamiltonian, the effects of various polarization directions, and the effects of various choices for carrier frequencies, as expressed by detunings from single-photon resonance. In particular, we consider an example with angular momentum J=0\ensuremath{\leftrightarrows}J=1\ensuremath{\leftrightarrows}J=2 in which nine magnetic sublevels occur. We show that under suitable conditions it is possible, simply through the appropriate tuning of pump and Stokes carrier frequencies, to place the entire population into any specified single magnetic sublevel.
We extend previous theoretical work on coherent population transfer by stimulated Raman adiabatic passage for states involving nonzero angular momentum. The pump and Stokes fields are either copropagating or counterpropagating with the corresponding linearly polarized electric-field vectors lying in a common plane with the magnetic-field direction. Zeeman splitting lifts the magnetic sublevel degeneracy. We present an algebraic analysis of dressed-state properties to explain the behavior noted in numerical studies. In particular, we discuss conditions which are likely to lead to a failure of complete population transfer. The applied strategy, based on simple methods of linear algebra, will also be successful for other types of discrete multilevel systems, provided the rotating-wave and adiabatic approximation are valid.
In recent years interest has significantly quickened in investigation of nonlinear optical and spectroscopic phenomena in such unique objects as Rydberg atoms (the atoms in the energy states near the ionization limit) and van der Waals complexes (dimers). These objects are characterized, in particular, by that their energy states are strongly degenerate due to a large value of angular (rotational) momentum. Here, the problem arises of adequate theoretical description of experimental results, especially when polarization peculiarities of nonlinear radiation processes are considered. The problem is caused by the existence of a huge number of quantum magnetic sublevels of energy states, which are, in the general case, coherently coupled to each other and take part in the radiation processes. Figure 1 gives a vivid presentation of the problem arising even in such a seemingly simple case when one need describe the radiation transitions caused by a strong monochromatic field between two energy levels with large values of the angular momentum J. If the field polarization is arbitrary, the whole set of the magnetic sublevels turn out to be coupled by radiative transitions. Formally, the problem becomes essentially multilevel. In this case not only the hope fades of solving it analytically using the traditional quantum-mechanical approach, but the problems of numerical solution grow abruptly with increasing quantum number J as well. On the other hand, as is often the case, every cloud has a silver lining. With large J we approach the situation when the rotational motion of particles can be described classically. It turns out that the transition from the quantum description of orientation states of the angular momentum to the classical one simplifies the problem radically: it reduces to the problem of radiation processes in the simplest model of nondegenerate states. In this case the rotational motion manifests itself in a dependence of the density matrix elements on orientation angles of the angular momentum as on a parameter.
The technique of stimulated Raman adiabatic passage (STIRAP) has become an established procedure for producing complete population inversion in atoms or molecules via application of Stokes and pump pulses in a counterintuitive sequence. Previously we reported on numerical and analytical investigations showing some of the additional and important phenomena that arise when some of the levels involved have nonzero angular momentum, the field polarization directions have no simplifying symmetries, and Zeeman splitting lifts the magnetic sublevel degeneracy. Here we verify our theoretical findings using the metastable neon system: 3^{3}3P0{\mathit{P}}_{0}P0​?3{\mathrm{?}}^{3}?3P1{\mathit{P}}_{1}P1​?3{\mathrm{?}}^{3}?3P2{\mathit{P}}_{2}P2​ with the corresponding angular momentum sequence J=0?J=1?J=2. In particular, we demonstrate that it is possible to transfer all population from the single initial state to any single selectable final magnetic sublevel M. Selectivity can be achieved either by choosing special laser polarization angles or, for fixed polarizations, by tuning the two lasers into two-photon resonance between the initial and desired final state. The complete control of magnetic sublevel population enabled by this procedure extends the customary experimental methods for producing oriented or aligned beams of atoms. In addition, we demonstrate very clearly the previous prediction that population transfer may fail for certain sets of parameters (detunings, Rabi frequencies, and polarization angles). \textcopyright{} 1996 The American Physical Society.
Previously published papers have noted the existence of two-level periodicities in idealized theoretical models of monochromatic excitation of multilevel systems. The present paper provides a theoretical derivation of such two-level dynamics from the more general NNN-level rotating-wave-approximation Schr\"odinger equation and applies the resulting formulas to different extremes of excitation in a nondegenerate system of sequential (ladderlike) excitation: first, when intermediate levels have much larger cumulative detunings than Rabi frequencies, and second, when intermediate Rabi frequencies are much larger than detunings. In each case one has simple analytic expressions for the effective two-level Rabi frequencies and detunings.
The Wigner representation of angular-momentum orientation in the classical limit on $J\ensuremath{\gg}1$ has been used to derive an equation of angular-momentum motion in the case of a closed two-level system interacting with an arbitrary polarized radiation. This equation enables us to carry out a theoretical analysis of angular-momentum motion and consider final states of the quantum system for all types of dipole transitions. We have found that in the case of a zero magnetic field in the final state of the system with $\stackrel{\ensuremath{\rightarrow}}{J}J$ or $\stackrel{\ensuremath{\rightarrow}}{J}J+1$ optical transition, the angular momentum is directed along or opposite to the wave vector of the radiation depending on the sign of the polarization ellipsity, while for a linear polarization the angular momentum is isotropically distributed on the plane orthogonal to the polarization vector. For the $\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1$ transition two directions of angular momentum are possible in the final state. These directions are determined by the ellipsity of polarization. The spatial size of the final distribution of the angular momentum has been defined by the quantum uncertainty, 1/J,1/\sqrt{J},1/J​, of the angular-momentum orientation. In the case of the $\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1$ or $\stackrel{\ensuremath{\rightarrow}}{J}J,$ transition particles come into ``dark'' states while for the $\stackrel{\ensuremath{\rightarrow}}{J}J+1$ transition they occupy the ``brightest'' state. In the presence of a nonzero magnetic field, particles with $\stackrel{\ensuremath{\rightarrow}}{J}J$ transition have in the final state only one angular-momentum orientation, i.e., along or opposite to the direction of the magnetic field depending on the polarization ellipsity sign. For the $\stackrel{\ensuremath{\rightarrow}}{J}J+1$ transition both directions are possible, i.e., along and opposite to the magnetic field. In the case of the $\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1$ transition the directions of the angular momentum form a cone around the magnetic field.