Toroidal optical transitions in hydrogen-like atoms

TL;DR

This paper explores the potential for a new spectroscopy method based on toroidal dipolar interactions in atoms, which are distinct from traditional electric and magnetic multipole interactions, especially considering relativistic effects.

Contribution

It introduces the concept of relativistic quantum mechanical toroidal dipolar interactions as a new avenue for atomic spectroscopy, expanding the understanding of light-matter interactions.

Findings

Toroidal transitions are odd under parity and time-reversal symmetries.

Relativistic effects enable the observation of toroidal dipolar interactions in atoms.

Toroidal interactions can be distinguished from electric and magnetic multipole transitions.

Abstract

It is commonly believed that electromagnetic spectra of atoms and molecules can be fully described by interactions of electric and magnetic multipoles. However, it has recently become clear that interactions between light and matter also involve toroidal multipoles - toroidal absorption lines have been observed in electromagnetic metamaterials. Here we show that a new type of spectroscopy of the hitherto largely neglected toroidal dipolar interaction becomes feasible if, apart from the classical r{\times}r{\times}p toroidal dipole density term responsible for the toroidal transitions in metamaterials, the spin-dependent r{\times}{\sigma} term (that only occurs in relativistic quantum mechanics) is taken into account. We show that toroidal transitions are odd under parity and time-reversal symmetries; they can therefore be observed and distinguished from electric multipole and magnetic dipole transitions.