Transfer of angular momentum of guided light to an atom with an electric quadrupole transition near an optical nanofiber

TL;DR

This paper investigates how guided light's angular momentum transfers to a nearby atom with an electric quadrupole transition near an optical nanofiber, revealing dependencies on photon mode, atomic state changes, and transition strength.

Contribution

It introduces a detailed analysis of angular momentum transfer via quadrupole transitions near nanofibers, highlighting mode-dependent torque effects and internal-state selection rules.

Findings

Torque depends on photon angular momentum and atomic state change.

Higher-order mode HE_{21} generally produces larger torque than fundamental mode HE_{11}.

Torque vanishes for certain state transitions and mode combinations.

Abstract

We study the transfer of angular momentum of guided photons to a two-level atom with an electric quadrupole transition near an optical nanofiber. We show that the generation of the axial orbital torque of the driving guided field on the atom is governed by the internal-state selection rules for the quadrupole transition and by the angular momentum conservation law with the photon angular momentum given in the Minkowski formulation. We find that the torque depends on the photon angular momentum, the change in the angular momentum of the atomic internal state, and the quadrupole-transition Rabi frequency. We calculate numerically the torques for the quadrupole transitions between the sublevel $M=2$ of the hyperfine-structure level $5S_{1/2}F=2$ and the sublevels $M'=0$, 1, 2, 3, and 4 of the hyperfine-structure level $4D_{5/2}F'=4$ of a $^{87}$Rb atom. We show that the absolute value of the torque for the higher-order mode HE$_{21}$ is larger than that of the torque for the fundamental mode HE$_{11}$ except for the case $M'-M=2$, where the torque for the mode HE$_{21}$ is vanishing.