Polarization vs. magnetic field: competing eigenbases in laser-driven atoms

TL;DR

This paper investigates how the interplay between polarization and magnetic fields influences atomic fluorescence, demonstrating a crossover between regimes dominated by laser or magnetic effects through experiments and a theoretical model.

Contribution

It introduces a simple theoretical model that accurately describes the competition between polarization and magnetic field effects in laser-driven atoms, supported by experimental validation.

Findings

Crossover occurs at a laser intensity proportional to magnetic field strength

Dark states inhibit fluorescence without magnetic field

Magnetic field at an angle prevents optical pumping to dark states

Abstract

We present experimental results and a theoretical model that illustrate how competing eigenbases can determine the dynamics of a fluorescing atom. In the absence of a magnetic field, the atom can get trapped in a dark state, which inhibits fluorescence. In general, this will happen when the magnetic degeneracy of the ground state is greater than the one of the excited state. A canonical way to avoid optical pumping to dark states is to apply a magnetic field at an angle with respect to the polarization of the exciting light. This generates a competition of eigenbases which manifests as a crossover between two regimes dominated either by the laser or the magnetic field. We illustrate this crossover with fluorescence measurements on a single laser-cooled calcium ion in a Paul trap and find that it occurs at a critical laser intensity that is proportional to the external magnetic field. We contrast our results with numerical simulations of the atomic levels involved and also present a simple theoretical model that provides excellent agreement with experimental results and facilitates the understanding of the dynamics.