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Journal of Physics B Atomic and Molecular Physics

Geometry and polarisation in two-photon absorption followed by fluorescent decay

Noureddine Melikechi, L. Allen

Published Dec 14, 1987• 2 citations
Publisher

Abstract

Racah algebra and standard irreducible tensor techniques show that the probability of two-photon absorption followed by fluorescent decay can be written as a product of two terms with in a series of summations. One term embraces the electric-dipole interactions between the radiation fields and the atomic system and the other, a geometric term, depends upon the choice of experimental orientation and the polarisation of the fields. Examination of the geometric term allows the angular distribution of the radiation emitted following two-photon absorption to be investigated for different atomic coupling schemes for counterpropagating light beams of arbitrary polarisation.

DOI
10.1088/0022-3700/20/23/013

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Racah algebra and standard irreducible tensor techniques show that the probability of two-photon absorption followed by fluorescent decay can be written as a product of two terms with in a series of summations. One term embraces the electric-dipole interactions between the radiation fields and the atomic system and the other, a geometric term, depends upon the choice of experimental orientation and the polarisation of the fields. Examination of the geometric term allows the angular distribution of the radiation emitted following two-photon absorption to be investigated for different atomic coupling schemes for counterpropagating light beams of arbitrary polarisation.
Correlations between photons emitted by an atom in a laser field and near a metal surface are studied. With polarisation-dependent detection it is feasible to select photons which are emitted in a specific transition between degenerate substates. Both the Einstein coefficient for spontaneous decay of a particular excited substate and its branching towards the various ground states depend on the distance between the atom and the surface. A combination of these notions to design a geometry for the correlated detection of polarised photons is employed, in order to predict a strong dependence of the correlation functions on the atom-surface distance. In general, an enhancement of the correlations between emitted photons due to the presence of the metal surface is found if the atom-surface distance is (roughly) less than 20% of the wavelength of the fluorescence radiation. In particular the correlations between circularly polarised photons with the same helicity are modified dramatically, and the correlation time tends to infinity if the atom approaches the surface. It is pointed out how the different photon correlations can be understood from a simple interpretation of transition diagrams.
Equations are derived for the polarisation of light emitted by impulsively excited atoms. A spontaneous electric dipole transition between Russell-Saunders terms is considered. The density matrix of the excited term is described by a double tensor expansion which allows for anisotropy in the directional distribution of the spin as well as the orbital angular momentum. Conditions are discussed under which one may test for the presence of spin anisotropy by polarisation measurements or zero-field quantum beats.
A theory appropriate for investigating the angular spectra of atoms scattered out of an atomic beam by resonance fluorescence due to interactions with an orthogonal uniform light beam is presented. Particular emphasis is made on the scattering in the plane containing the two beams. The atomic distribution is shown to be intimately related to the angular distribution of the spontaneously emitted photons. Both distributions show marked variations with the detuning and field intensity parameters, leading to the possibility of maximum scattering of atoms to occur on either side of the atomic beam, while the accompanying photons are always emitted on one particular side of the light beam. Since the angular distributions reflect the momentum changes suffered by both the atoms and the electromagnetic field, this lends insight into the basic mechanisms involved in resonance fluorescence.
A method alternative to the density matrix formalism is applied to derive a general expression for the radiative decay probability following the photoionization of polarized atoms in two-step approximation. The graphical technique of the angular momentum theory is used for the integration over angular and spin variables. The general expression joining both steps of photoionization and fluorescence contains the possibility to investigate the polarization of all particles in the initial and final states and the angular distributions as well as angular correlations of photoelectrons and radiation. A simple way to get the expressions for special cases of the angular distribution and polarization of fluorescence radiation in the photoionization of randomly orientated and polarized atoms is given as an example of the practical application of the general formula.