Superchiral photons unveil magnetic circular dichroism

TL;DR

This paper demonstrates that superchiral photon beams can enhance the detection of magnetic circular dichroism (MCD), revealing magnetic properties with improved sensitivity and providing a unified theoretical framework for related spectroscopies.

Contribution

It introduces an effective wavevector approach to unify and enhance the understanding of superchiral, twisted, and Borrmann effect spectroscopies, specifically improving MCD detection.

Findings

Superchiral beams significantly enhance MCD signals.

E2-E2 absorption processes show superior enhancement.

Atomic multipoles effectively interpret dichroic signals.

Abstract

Polarization-dependent photon spectroscopy (dichroism) using signal-enhancing superchiral beams is shown to be sensitive to magnetic properties of the sample, whereas previous investigations explored charge-like electronic properties of chiral samples. In the process of unveiling the potential to observe magnetic circular dichroism (MCD), we underline an affinity between spectroscopies using the Borrmann effect, twisted beams and superchiral beams. Use of an effective wavevector in a quantum-mechanical theory unites the aforementioned spectroscopies and vastly improves our understanding of their advantages. Exploiting an effective wavevector for superchiral beams, natural circular dichroism (NCD) is derived from electric dipole - magnetic dipole (E1-M1) and electric dipole - electric quadrupole (E1-E2) absorption events, and MCD is derived from electric quadrupole-electric quadrupole (E2-E2) absorption. Signal enhancement by superchiral beams is a straightforward gain for the user because NCD and MCD are otherwise precisely the same as for a circularly polarized beam, according to our calculations. Our analysis shows that enhancement of E2-E2 is superior to that available for parity-odd events under consideration. Electronic degrees of freedom in all dichroic signals are encapsulated in atomic multipoles that are frequently used in theoretical interpretations of several established experimental techniques.