A unified formulation of dichroic signals using the Borrmann effect and twisted photon beams

TL;DR

This paper unifies the description of dichroic signals from the Borrmann effect and twisted photon beams, revealing their applications in probing electronic multipoles in various materials.

Contribution

It introduces a unified formulation for dichroic signals using an effective wavevector applicable to both magnetic and non-magnetic materials, incorporating multipole analysis.

Findings

Dichroic signals can be described with charge-like and magnetic multipoles.

Different polarization states of twisted beams probe distinct multipoles.

Practical considerations for spectroscopy using these effects are discussed.

Abstract

Dichroic signals derived from the Borrmann effect and a twisted photon beam with topological charge l = 1 are formulated with an effective wavevector. The unification applies for non-magnetic and magnetic materials. Electronic degrees of freedom associated with an ion are encapsulated in multipoles previously used to interpret conventional dichroism and Bragg diffraction enhanced by an atomic resonance. A dichroic signal exploiting the Borrmann effect with a linearly polarized beam presents charge-like multipoles that include a hexadecapole. A difference between dichroic signals obtained with a twisted beam carrying spin polarization (circular polarization) and opposite winding numbers presents charge-like atomic multipoles, whereas a twisted beam carrying linear polarization alone presents magnetic (time-odd) multipoles. Charge-like multipoles include a quadrupole, and magnetic multipoles include a dipole and an octupole. We discuss the practicalities and relative merits of spectroscopy exploiting the two remarkably closely-related processes.