Spin selection rule for {\it S} level transitions in atomic rubidium under paraxial and nonparaxial two-photon excitation

TL;DR

This paper experimentally investigates the spin selection rule for two-photon transitions in rubidium atoms, revealing how transition rates depend on light polarization and the differences between paraxial and nonparaxial excitation.

Contribution

It provides new experimental insights into how two-photon transition rates in rubidium are affected by polarization in nonparaxial light fields, extending understanding beyond traditional paraxial regimes.

Findings

Transition rate scales quadratically with polarization degree in paraxial excitation.

Transition rate can be completely suppressed with circular polarization in paraxial setup.

In nonparaxial excitation near nanofibers, the transition cannot be fully suppressed by polarization alone.

Abstract

We report on an experimental test of the spin selection rule for two-photon transitions in atoms. In particular, we demonstrate that the $5S_{1/2}\to 6S_{1/2}$ transition rate in a rubidium gas follows a quadratic dependency on the helicity parameter linked to the polarization of the excitation light. For excitation via a single Gaussian beam or two counterpropagating beams in a hot vapor cell, the transition rate scales as the squared degree of linear polarization. The rate reaches zero when the light is circularly polarized. In contrast, when the excitation is realized via an evanescent field near an optical nanofiber, the two-photon transition cannot be completely extinguished (theoretically, not lower than 13\% of the maximum rate, under our experimental conditions) by only varying the polarization of the fiber-guided light. Our findings lead to a deeper understanding of the physics of multiphoton processes in atoms in strongly nonparaxial light.