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Journal of Physics B Atomic Molecular and Optical Physics

Control of the atom (nucleus) lifetime in the excited state by means of a low-frequency external field

И. Д. Феранчук, L I Komarov, Alexander P. Ulyanenkov

Published Sep 17, 2002• 5 citations
Publisher

Abstract

The radiative decay dynamics of an atomic (nuclear) excited state split into two neighbouring sublevels are considered for the case of interaction with a low-frequency electromagnetic field. The conditions of cancellation of the spontaneous emission in this system are analysed beyond the rotating wave approximation.
DOI
10.1088/0953-4075/35/19/301

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Spontaneous emission can be modified if the atom is driven by an external coherent field. Such modification include the three-peak spectrum of resonance fluorescence from a two level atom. The spectral line elimination and cancellation of spontaneous emission is possible under conditions that the spontaneous emission from an excited atom can be trapped in the upper states without decaying. The amount of fluorescence in this case as well as the population of the upper levels can be controlled. If spontaneous emission along with atomic transition is suppressed, it is possible to create a population in version on this transition by using a very weak incoherent pumping.
A quantum model of N two-level atoms coupled by a single-mode radiation field under a restricted rotating-wave approximation is studied, assuming a dipole-interaction approximation and that the total number of excitations of the system is conserved. A solution is found for the case of spontaneous emission when the atoms are initially prepared in the state of complete inversion. This solution is valid for all times. A general solution of the model is also found for any initial configuration of the system, in the cases of both spontaneous and stimulated emission. This solution is in a closed form and valid for times tau < tau max .
We study the dynamics of a system consisting of a two-level atom and a quantum oscillator coupled by the vacuum radiation field. The oscillator and the atom are assumed to have the same resonance frequency. In contrast to the two-atom or two-oscillator case, the atom-oscillator system leads to an unclosed hierarchy of equations for the operators that determine the radiation pattern emitted by the atom-oscillator system. Instead of using the operator equations, we formulate the problem in terms of density-matrix equations which can be solved once the initial conditions are specified. As an example, we looked at the radiation pattern, the decay dynamics, and the spectrum for an initial condition in which the oscillator is in its n=1n=1n=1 state and the atom is in its excited state. Dressed states (dressed by the static interaction between the atom and oscillator) prove to be especially useful for interpreting the results and comparing them with the two-atom and two-oscillator systems. All calculations are carried out using the resonance or rotating-wave approximation. The relationship of our work to damped Jaynes-Cummings models is noted.
The quantum-electrodynamical problem of N identical two-level atoms interacting with M field modes is considered, and an exact solution for the case of spontaneous emission from a single atom is obtained. The behavior of this system is shown to be quite different from the exponential decay of a single excited atom radiating into free space. A ``ringing'' behavior at the enigmatic frequency \ensuremath{\sim} \ensuremath{\surd}N which occurs when only one mode is accessible to the field persists when many modes are present. When a continuum of modes is accessible, the long-time limit shows that the energy of the originally excited atom is equally likely to be shared by the N-1 originally unexcited atoms and the electromagnetic field when M/N\ensuremath{\rightarrow}1. ``Radiation suppression'' occurs whenever N\ensuremath{\gg}1 and M\ensuremath{\ll}N, when the originally excited atom does not emit its energy. When M=N\ensuremath{\gg}1, the single atom decays monotonically to zero (and approximately exponentially) but with a decay rate that is \ensuremath{\surd}N times the Rabi frequency. Such unexpected effects are presently within experimental range.
In this paper we improve a theory that we have previously developed in order to deal with the spontaneous decay of excited systems. These improvements allows us to eliminate some unpleasant features of the previous approach as well as to study the decay of excited systems in the presence of external irradiation fields and the response of matter to electromagnetic excitations. The present approach allows for a quantitative evaluation of the irradiation field influence on the state lifetimes, which (when the interaction under study is endowed with a ‘strong memory’) is shown to be noticeable even in the cases where the irradiation intensity is small with respect to the unperturbed linewidth. We expect that, taking into account also the third-order perturbation contributions, here disregarded, the slow-motion lineshape problem will be effectively treated by the present approach even in the saturation regime.
For pt.I see ibid., vol.12, no.11, p.2205 (1979). A numerical analysis of the solution of the problem of the interaction of N atoms via the electromagnetic field is presented, using the analytical results of paper I. Several initial conditions are considered for the cases of both spontaneous and stimulated emission. Results show that there is an excellent agreement between the present work and previous numerical results (Walls and Baraka (1970)) in the super-radiant case, while there are differences when the system is initially prepared in the state of complete inversion. These differences are attributed to the restricted rotating-wave approximation, which is the starting assumption in the present work.