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Journal of the Optical Society of America B

Multiphoton transitions in a colored vacuum: coupling of the ac Stark shift with spontaneous decay and the Lamb shift

James C. Camparo, P. Lambropoulos

Published May 1, 2002• 2 citations
Publisher

Abstract

In free space, where the vacuum mode density varies relatively slowly in the spectral region of an atomic transition, an atom’s real-field and vacuum interactions are effectively decoupled. Consequently, spontaneous decay and the Lamb shift are, for all practical purposes, independent of any real-field atomic perturbation. However, in a colored vacuum (i.e., a low-Q cavity) the vacuum mode density can change significantly in the vicinity of an atomic transition, so that a Stark shift will alter an atom’s vacuum environment and thereby couple real-field and vacuum effects. Since the ac Stark shift is an inherent aspect of multiphoton processes, this coupling is unavoidable for highly nonlinear field–atom interactions that occur in cavities. Here we consider this effect for 3+2 resonance-enhanced multiphoton ionization of xenon.

DOI
10.1364/josab.19.001169

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In free space, where the vacuum mode density varies relatively slowly in the spectral region of an atomic transition, an atom’s real-field and vacuum interactions are effectively decoupled. Consequently, spontaneous decay and the Lamb shift are, for all practical purposes, independent of any real-field atomic perturbation. However, in a colored vacuum (i.e., a low-Q cavity) the vacuum mode density can change significantly in the vicinity of an atomic transition, so that a Stark shift will alter an atom’s vacuum environment and thereby couple real-field and vacuum effects. Since the ac Stark shift is an inherent aspect of multiphoton processes, this coupling is unavoidable for highly nonlinear field–atom interactions that occur in cavities. Here we consider this effect for 3+2 resonance-enhanced multiphoton ionization of xenon.
The boundary conditions imposed on the vacuum field by the superconfocal cavity (SCC) create a region of space, the active volume, at the cavity center in which the spectral structure of the vacuum field is perturbed. These perturbations transform the normally white spectrum of the free-space vacuum field into a colored vacuum whose spectrum is a comb of peaks coincident with the cavity resonance frequencies. Operating as designed, atoms resonant with the cavity and in the active region will experience a vacuum field mode density 1.5 times that in free-space. Perturbations in the vacuum field mode density of this magnitude and operative over so large an active volume (1.4 × 107µm3) have not, to our knowledge, been previously achieved in an open optical cavity. We expect to observe a plethora of new and novel phenomena associated with laser–atom interactions in the presence of a colored vacuum. The first experiment will focus on the dynamic suppression of spontaneous emission1 which predicts a dramatic narrowing of the central Mollow peak as the Rabi frequency becomes larger than the FWHM of the SCC-induced peaks in the vacuum field mode density.
We have investigated the ac Stark shift of a two-photon transition induced by a model stochastic field by using a Monte Carlo technique. The model field has a stochastic single mode, which can possess both colored amplitude and colored frequency fluctuations. The stochastic field’s modification of the ac Stark shift is quantified by a parameter M, which is the ratio of the Stark shift in the model stochastic field to the Stark shift that would be obtained in a phase-diffusion field of equivalent linewidth (i.e., a field with frequency fluctuations but no amplitude fluctuations). In the regime of weak fields, below saturation of the bound-bound transition, we find that M is greater than unity: the Stark shift is enhanced by the stochastic field. Moreover, in this regime M is an increasing function of the field’s degree of photon bunching. In strong fields, where the bound–bound transition is nearly saturated, the enhancement of the Stark shift is diminished, so much so that in extremely strong fields M is actually less than unity. Our calculations indicate that the ac Stark shift’s modification occurs through two distinct processes, depending on the strength of the field. In weak fields enhancement of a multiphoton transition’s Stark shift is influenced primarily by the intrinsic correlation between ac-Stark-shift fluctuations and Rabi-frequency fluctuations. In strong fields the fluctuating Stark shifts give rise to an asymmetric resonance line shape in a fashion analogous to inhomogeneous broadening. The line shape’s peak position then has a sublinear dependence on the stochastic field’s intensity, and this yields a diminished value of M.
We have developed a computational framework for the study of the effects of field fluctuations on strongly driven nonlinear transitions. Nonlinearity may stem from cither the strong coupling (saturation) of a resonant transition or its multiphoton character, or it may stem from both in the most general case. Our approach is based on a Monte Carlo simulation of stochastic field fluctuations, which allows maximum flexibility in the stochastic nature of the field and in the type of process that is studied. Here, we report on a more general problem than has previously been considered in the interaction of atoms with stochastic fields, namely, the three-photon resonant five- photon ionization of xenon, 3 + 2 photoionization of xenon has been the subject of a fair number of experimental investigations. It involves nonlinearity in with a bound-bound and a bound-free transition as well as an ac-Stark shift. Consequently, there are correlations among three fluctuating parameters: the three-photon Rabi frequency, the two-photon ionization rate, and the intensity-dependent ac-Stark shift, and it is these correlations that make the stochastic-field problem almost intractable to traditional methods of analysis.
Abstract A quantum system consisting of a single-mode microcavity and a two-level quantum dot or atom placed inside this microcavity, the interaction of monoenergetic photons in the microcavity with the electron in the two-level quantum dot or atom being the allowed electrical dipole transition between two energy levels, is a basic element of prospective devices for quantum information processing. Its time evolution is governed by cavity quantum electrodynamics (CQED). The photon–electron coupling induces the shifts of energy levels of the electron in the two-level quantum dot or atom. If there is no photon in the microcavity, then the shift of electron energy levels is induced by the radiative corrections and it is called the Lamb shift. The presence of photons in the microcavity induces another type of electron energy level shift—the ac Stark shift. In this paper, we present the exact derivation of the formulae for the Lamb and ac Stark shifts of electron energy levels in the two-level quantum dot or atom placed inside a single-mode microcavity.
The resonant multiphoton resonances shifts and widths and ionization cross-sections for multielectron atoms in a intense laser radiation field are studied. It is carried out a new consistent approach to atom in a strong realistic laser field, based on the relativistic energy formalism (S-matrix adiabatic formalism), relativistic Dirac equation Green function method and relativistic many-body perturbation theory with the Dirac-Kohn-Sham zeroth approximation. In relativistic theory, the Gell-Mann and Low adiabatic formula for energy shift is connected with electrodynamical scattering matrice, which includes interaction with as a laser field as a photon vacuum field (radiative decay). The optimized basis of the relativistic orbitals is generated with using a minimization procedure for the gauge-non-invariant contribution (the fourth-order of the QED perturbation theory) to the radiation width of atomic state. An approach to treating the multiphoton atomic processes is outlined on example of H, Cs, Kr,Mg etc. Analysis shows that the shift and width of the multi-photon resonance line for the interaction “atom- multimode laser pulse” is greater than the corresponding shift and width for a case of the “atom- single-mode pulse” (the Lorenz pulse model) interaction. From the physical point of view it is naturally provided by action of the photon-correlation effects and influence of the laser pulse multi-modity.