Superchiral photons unveil magnetic circular dichroism

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.

Structural chirality of β -Mn

“…Reduced radial matrix elements for Mn (3d 5 ) calculated from Cowan's program [43] are 1s|R|4p /a o = −0.003 54 and 1s|R 2 |3d /a o 2 = 0.000 95, where a o is the Bohr radius.…”

“…Optical activity is a very small effect. One proposal to improve its efficacy in labeling handed forms of compounds exploits superchiral beams of photons [4,5]. Many optically active molecules or crystals have enantiomers, or stereoisomers, whose atomic configurations are exact mirror images of each other and are thus handed.…”

Electronic multipoles in second harmonic generation and neutron scattering

“…Our use of them here to formulate response functions for SHG and neutron scattering has previously proved useful in several other photon scattering processes. Examples include x-ray magneto-optical sum rules [40][41][42], a unified formulation of dichroic signals using the Borrmann effect and twisted photon beams [43], resonant x-ray diffraction from chiral electric-polarization structures [44], and a demonstration that superchiral photons reveal magnetic circular dichroism [45]. But, our innovation for SHG starts by finding a representation for the response in terms of a spherical tensor operator.…”