Chirality and angular momentum in optical radiation

TL;DR

This paper provides a quantum electrodynamic analysis of optical chirality density, clarifying its relation to angular momentum and helicity, and challenges the notion of 'superchirality' in chiroptical phenomena.

Contribution

It offers a rigorous quantum framework linking optical chirality to angular momentum and helicity, clarifying the physical basis of chiroptical effects and questioning the concept of superchirality.

Findings

Optical chirality density is related to photon populations with opposite angular momentum.

Only the spin angular momentum of light influences chiroptical measurements.

The concept of superchirality is redundant based on quantum analysis.

Abstract

This paper develops, in precise quantum electrodynamic terms, photonic attributes of the "optical chirality density", one of several measures long known to be conserved quantities for a vacuum electromagnetic field. The analysis lends insights into some recent interpretations of chiroptical experiments, in which this measure, and an associated chirality flux, have been treated as representing physically distinctive "superchiral" phenomena. In the fully quantized formalism the chirality density is promoted to operator status, whose exploration with reference to an arbitrary polarization basis reveals relationships to optical angular momentum and helicity operators. Analyzing multi-mode beams with complex wave-front structures, notably Laguerre-Gaussian modes, affords a deeper understanding of the interplay between optical chirality and optical angular momentum. By developing theory with due cognizance of the photonic character of light, it emerges that only the spin angular momentum of light is engaged in such observations. Furthermore, it is shown that these prominent measures of the helicity of chiral electromagnetic radiation have a common basis, in differences between the populations of optical modes associated with angular momenta of opposite sign. Using a calculation of the rate of circular dichroism as an example, with coherent states to model the electromagnetic field, it is discovered that two terms contribute to the differential effect. The primary contribution relates to the difference in left- and right- handed photon populations; the only other contribution, which displays a sinusoidal distance-dependence, corresponding to the claim of nodal enhancements, is connected with the quantum photon number-phase uncertainty relation. From the full analysis, it appears that the term "superchiral" can be considered redundant.