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Physical Review A

Dressed-atom theory of stimulated pair transitions

Juan Francisco Lam, Stephen Colby Rand

Published Mar 1, 1987• 18 citations
Publisher

Abstract

We present an exact, analytic solution for the probability of radiative, cooperative transitions of stationary, weakly coupled atoms in strong light fields. Very simple expressions are obtained for pair-emission and -absorption probabilities by use of the ``dressed-atom'' formalism applied to atoms in solids or nearly at rest in optical traps. Analogies between radiative collisions in gases and the pair transitions encountered in solids, which may be viewed as ``frozen'' radiative collisions, are explored. Conditions are predicted for the occurrence of electric dipole decoupling in the presence of a suitable rotating electric field, similar to the well-known phenomenon of ``magic-angle'' decoupling of magnetic spins due to a rotating magnetic field. Observations of this new effect should permit precision determinations of interaction Hamiltonians in optical pair systems.

DOI
10.1103/physreva.35.2164

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A strong radio-frequency field couples two excited atomic states which also interact with the virtual radiation field. The radio-frequency field dresses the atomic states and the virtual radiation field causes them to decay spontaneously. The spectrum of the emitted radiation is calculated by Fourier transforming the differential equations for the amplitudes. The solutions are given in terms of recurrence relationships which can be evaluated to any desired accuracy. The optical radiation field acts as a probe on the dressed atomic states. The spectrum shows all the radio-frequency induced resonances. The frequencies of the states and the Bloch-Siegert shifts of the multi-photon resonances can be deduced from this spectrum.

Journal of Physics B Atomic and Molecular Physics•Sep 4, 1982•1 citation
We present an exact, analytic solution for the probability of radiative, cooperative transitions of stationary, weakly coupled atoms in strong light fields. Very simple expressions are obtained for pair-emission and -absorption probabilities by use of the dressed-atom'' formalism applied to atoms in solids or nearly at rest in optical traps. Analogies between radiative collisions in gases and the pair transitions encountered in solids, which may be viewed as frozen'' radiative collisions, are explored. Conditions are predicted for the occurrence of electric dipole decoupling in the presence of a suitable rotating electric field, similar to the well-known phenomenon of ``magic-angle'' decoupling of magnetic spins due to a rotating magnetic field. Observations of this new effect should permit precision determinations of interaction Hamiltonians in optical pair systems.
The dressed atom approach provides a tool to investigate the dynamics of an atom-laser system by fully retaining the quantum nature of the coherent mode. In its standard derivation, the internal atom-laser evolution is described within the rotating-wave approximation, which determines a doublet structure of the spectrum and the peculiar fluorescence triplet in the steady state. However, the rotating wave approximation may fail to apply to atomic systems subject to femtosecond light pulses, light-matter systems in the strong-coupling regime or sustaining permanent dipole moments. This work aims to demonstrate how the general features of the steady-state radiative cascade are affected by the interaction of the dressed atom with propagating radiation modes. Rather than focusing on a specific model, we analyze how these features depend on the parameters characterizing the dressed eigenstates in arbitrary atom-laser dynamics, given that a set of general hypotheses is satisfied. Our findings clarify the general conditions in which a description of the radiative cascade in terms of transition between dressed states is self-consistent. We provide a guideline to determine the properties of photon emission for any atom-laser interaction model, which can be particularly relevant when the model should be tailored to enhance a specific line. We apply the general results to the case in which a permanent dipole moment is a source of low-energy emission, whose frequency is of the order of the Rabi coupling.
The absorption of single photons by coupled pairs of ions in rare-earth solids was first observed by Varsanyi and Dieke.1 They noticed that weak absorption of light occurred in PrCl3 at frequencies close to the sum of transition frequencies of individual Pr ions, giving rise to rare-earth fluorescence. This effect was explained by the perturbation theory of Dexter.2 He showed that weak multipole or exchange interactions between rare-earth ions permitted the simultaneous promotion of two ions to excited states by the action of one absorbed photon in an electric dipole allowed transition. Similarly, two ions initially in excited states could emit one sum frequency photon in a cooperative manner.
The collision-induced absorption and emission coefficients for electric-dipole forbidden atomic transitions are calculated for weak radiation fields. The approximations used are valid for $\ensuremath{\hbar}\ensuremath{\omega}$ near $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$, the atomic energy differences. The example case of $S\ensuremath{-}D$ transitions induced by a spherically symmetric perturber (e.g., a noble-gas atom) is treated in detail and compared to measurements. The case of "radiative collisions," in which both colliding atoms change their state, is included in the theory and is also compared to experiment.
Radiation can change the energy state of matter via three optical processes: spontaneous emission, absorption and stimulated emission. In spontaneous emission, an excited atomic or molecular state decays to a lower energy state with the emission of a photon of energy equal to the difference in the quantum state energies. Photon absorption is the reverse process, with a photon causing excitation from a lower to an excited quantum state. Stimulated emission was first proposed by Einstein in a paper published in 1917. He attempted to balance spontaneous emission and photon absorption, but could not get the correct balance without the postulated process of stimulated emission (see Section 4.2). With stimulated emission, an excited quantum state is stimulated to decay to a lower quantum state by an impinging photon. The existing photon survives with another identical photon being created (so that energy is conserved). The rates at which radiative transitions occur can be calculated using quantum mechanics. The time-dependent Schrodinger equation is used with the system wavefunction comprising time-varying linear combinations of the two wavefunctions for the lower and upper quantum states. In semi-classical treatments, an atom is treated quantum mechanically with the radiation field treated classically. There is a perturbation in energy associated with, say, the electric field of the radiation interacting with the electric dipole of the atom, though other interactions can be significant if the electric dipole interaction energy is small. In absorption, for example, a differential equation representing the change in ‘weighting’ of the wavefunction from the lower to upper state is obtained. A ‘time constant’ for the speed of change represents the rate of the radiative process. We examine the quantum mechanical nature of radiation absorption in Section 10.1 before discussing (Section 10.3) another important parameter arising with radiative transitions between discrete bound quantum states: the lineshape function. Quantum Theory of the Atom–Radiation Interaction An expression for the Einstein B-coefficient for photo-absorption is derived using quantum mechanics in this section. Einstein determined the detailed balance relationship between the rates of spontaneous emission, photo-absorption and stimulated emission when light interacts with an atom by considering an atomic system of two energy levels which are in equilibrium with a black-body radiation field. We use a semi-classical treatment where the atom is treated quantum mechanically and the radiation field is treated as a classical oscillating electric field.
Abstract The new quantum mechanics could at first be used to answer questions concerning radiation only through analogies with the classical theory. In Heisenberg’s original matrix theory, for instance, it is assumed that the matrix elements of the polarisation of an atom determine the emission and absorption of radiation analogously to the Fourier components in the classical theory. In more recent theories a certain expression for the electric density obtained from the quantum mechanics is used to determine the emitted radiation by the same formulæ as in the classical theory. These methods give satisfactory results in many cases, but cannot even be applied to problems where the classical analogies are obscure or non-existent, such as resonance radiation and the breadths of spectral lines. A theory of radiation has been given by the author which rests on a more definite basis. It appears that one can treat a field of radiation as a dynamical system, whose interaction with an ordinary atomic system may be described by a Hamiltonian function. The dynamical variables specifying the field are the energies and phases of its various harmonic conrponents, each of which is effectively a simple harmonic oscillator. One must, of course, in the quantum theory take these variables to be q-numbers satisfying the proper quantum conditions. One finds then that the Hamiltonian for the interaction of the field with an atom is of the same form as that for the interaction of an assembly of light-quanta with the atom. There is thus a complete formal reconciliation between the wave and light-quantum points of view.
A strong radio-frequency field couples two excited atomic states which also interact with the virtual radiation field. The radio-frequency field dresses the atomic states and the virtual radiation field causes them to decay spontaneously. The spectrum of the emitted radiation is calculated by Fourier transforming the differential equations for the amplitudes. The solutions are given in terms of recurrence relationships which can be evaluated to any desired accuracy. The optical radiation field acts as a probe on the dressed atomic states. The spectrum shows all the radio-frequency induced resonances. The frequencies of the states and the Bloch-Siegert shifts of the multi-photon resonances can be deduced from this spectrum.