Enhancement of the quadrupole interaction of an atom with guided light of an ultrathin optical fiber

TL;DR

This paper analyzes how guided light in ultrathin optical fibers enhances the electric quadrupole interaction with alkali-metal atoms, revealing dependencies on mode type, atom position, and fiber radius, with potential for significant oscillator strength enhancement.

Contribution

It provides a detailed calculation of quadrupole Rabi frequency and oscillator strength enhancements in ultrathin fibers, highlighting factors influencing these interactions and their dependence on fiber and atomic parameters.

Findings

Quadrupole Rabi frequency decreases rapidly with radial distance.

Oscillator strength varies slowly with radial distance.

Enhancement factors depend mainly on transition frequency, not internal atomic states.

Abstract

We investigate the electric quadrupole interaction of an alkali-metal atom with guided light in the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the quadrupole oscillator strength, and their enhancement factors. In the example of a rubidium-87 atom, we study the dependencies of the quadrupole Rabi frequency on the quantum numbers of the transition, the mode type, the phase circulation direction, the propagation direction, the orientation of the quantization axis, the position of the atom, and the fiber radius. We find that the root-mean-square (rms) quadrupole Rabi frequency reduces quickly but the quadrupole oscillator strength varies slowly with increasing radial distance. We show that the enhancement factors of the rms Rabi frequency and the oscillator strength do not depend on any characteristics of the internal atomic states except for the atomic transition frequency. The enhancement factor of the oscillator strength can be significant even when the atom is far away from the fiber. We show that, in the case where the atom is positioned on the fiber surface, the oscillator strength for the quasicircularly polarized fundamental mode HE$_{11}$ has a local minimum at the fiber radius $a\simeq 107$ nm, and is larger than that for quasicircularly polarized higher-order hybrid modes, TE modes, and TM modes in the region $a<498.2$ nm.