Magnetic dipole and quadrupole dichroism induced by twisted light
Phys. Rev. A 113, 053102 – Published 5 May, 2026
DOI: https://doi.org/10.1103/s8cb-pqw4
Abstract
Molecular QED theory is used to calculate one-photon absorption probabilities in magnetic systems induced by structured light. Specifically, a circularly polarized Laguerre-Gaussian beam possessing a helical phase is applied to a microscopic particle characterized by magnetic dipole and magnetic quadrupole moments. This is inspired by the helical dichroism observed in recently fabricated propeller metamolecules that was attributed to the interference of magnetic dipole and quadrupole coupling terms. Observable properties computed include rates for circular dichroism (CD), vortex dichroism (VD), and circular-vortex dichroism (CVD), with the last of these arising from the coupling of both optical spin and orbital angular momenta. To ensure no possible dichroic effects are missed, the oft-ignored longitudinal component of the magnetic field and its gradient are properly included in addition to the usual transverse part. It is found that for isotropic purely magnetic dipole species, no CD, VD, or CVD arises from the transverse field alone, while only the longitudinal field gives rise to CD but no VD. For particles described by the magnetic dipole-quadrupole cross term, all three types of differential absorption are exhibited for both a fixed configuration and randomly oriented situations. Interestingly, the dichroic phenomena depend on the interference of transverse and longitudinal optical vortex field components to manifest.
