Abstract
The inherently weak nature of chiral light–matter interactions can be enhanced by orders of magnitude utilizing artificially-engineered nanophotonic structures. These structures enable high spatial concentration of electromagnetic fields with controlled helicity and chirality. However, the effective design and optimization of nanostructures requires defining physical observables which quantify the degree of electromagnetic helicity and chirality. In this perspective, we discuss optical helicity, optical chirality, and their related conservation laws, describing situations in which each provides the most meaningful physical information in free space and in the context of chiral light–matter interactions. First, an instructive comparison is drawn to the concepts of momentum, force, and energy in classical mechanics. In free space, optical helicity closely parallels momentum, whereas optical chirality parallels force. In the presence of macroscopic matter, the optical helicity finds its optimal physical application in the case of lossless, dual-symmetric media, while, in contrast, the optical chirality provides physically observable information in the presence of lossy, dispersive media. Finally, based on numerical simulations of a gold and silicon nanosphere, we discuss how metallic and dielectric nanostructures can generate chiral electromagnetic fields upon interaction with chiral light, offering guidelines for the rational design of nanostructure-enhanced electromagnetic chirality.
Highlights
Chiral electromagnetic fields, exhibiting left- or right-handedness, have the ability to interact selectively with matter
We focus on chiral light–matter interactions in artificial nanostructures composed of linear, homogeneous, isotropic media, where material losses can play a significant role in the generation of chiral electromagnetic fields [66,67,83]
We applied time and parity symmetry relations to demonstrate how the optical chirality density and flux quantify the handedness of an electromagnetic field
Summary
Chiral electromagnetic fields, exhibiting left- or right-handedness, have the ability to interact selectively with matter. The optical helicity and the optical chirality differ in their physical meaning, and their application in the presence of matter is subtle yet distinct This perspective performs a comprehensive comparison of each quantity and its physical significance in free space and in the presence of matter. An additional, physically-relevant comparison to energy and momentum conservation in classical mechanics is provided and the physical observables arising from the conservation law of optical chirality in lossy, dispersive media are discussed We apply these observables to elucidate the physical mechanisms of chiral light–matter interactions in artificial nanostructures, where the distinct cases of metallic and dielectric nanoparticles are analyzed numerically. The chiral electromagnetic fields generated by gold and silicon nanospheres with 75 nm radius are considered, demonstrating in both cases that achiral, linearly polarized excitation does not yield a net electromagnetic chirality, while chiral excitation with left- and right-handed CPL results in mirror-symmetric optical chirality flux spectra
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