Repulsive Casimir-Polder potentials of low-lying excited states of a multilevel alkali-metal atom near an optical nanofiber

TL;DR

This paper investigates the Casimir-Polder potential of low-lying excited states of a rubidium atom near an optical nanofiber, revealing conditions under which the potential becomes repulsive and oscillates, affecting atomic transition frequencies.

Contribution

It provides numerical analysis of the Casimir-Polder potentials for excited states near nanofibers, highlighting the possibility of repulsive interactions and frequency shifts.

Findings

Excited states can have positive, oscillating Casimir-Polder potentials.

Potential peaks of about 17 μK at 150 nm distance for the 8S_{1/2} state.

Transition frequency shifts oscillate and change sign with distance.

Abstract

We study the Casimir-Polder potential of a multilevel alkali-metal atom near an optical nanofiber. We calculate the mean potential of the atom in a fine-structure level. We perform numerical calculations for the Casimir-Polder potentials of the ground state and a few low-lying excited states of a rubidium atom. We show that, unlike the potential of the ground state, which is negative and attractive, the potential of a low-lying excited state may take positive values, oscillate around the zero value with a decaying amplitude, and become repulsive in some regions of atom-to-surface distances. We observe that, for a nanofiber with a radius of 200 nm, the potential for the state $8S_{1/2}$ of a rubidium atom achieves a positive peak value of about 17 $\mu$K at a distance of about 150 nm from the fiber surface, and becomes rather strongly repulsive in the region of distances from 150 to 400 nm. We also calculate the nanofiber-induced shifts of the transition frequencies of the atomic rubidium $D_2$ and $D_1$ lines. We find that the shifts are negative in the region of short distances, become positive, and oscillate around the zero value with a decaying amplitude in the region of large distances.