**6. Conclusions**

In conclusion, this perspective provides insight on the physical applicability of the optical helicity and the optical chirality in free space and in the presence of matter. In free space, a qualitative parallel between momentum in classical mechanics and optical helicity in classical electrodynamics can be made; likewise, a parallel between force and optical chirality also exists. We applied time and parity symmetry relations to demonstrate how the optical chirality density and flux quantify the handedness of an electromagnetic field. When chiral light interacts with macroscopic matter, we then identified how the optical helicity provides useful physical information for the case of lossless, dual-symmetric media, while the optical chirality provides physically observable information in the case of lossy, dispersive media. Here, a comparison to energy and momentum conservation for the lossless, elastic collision and the lossy, inelastic collision of two moving objects provides insight on the applicability of optical helicity and optical chirality conservation in the presence of matter. Finally, we applied the conservation law of optical chirality to numerically simulate the optical chirality flux generated

by a gold and silicon nanosphere of 75 nm radius. While no optical chirality flux was generated upon linearly polarized excitation, left- and right-handed CPL resulted in mirror-symmetric optical chirality flux spectra in both cases. This effect can be further enhanced by tuning the geometry of the nanostructure; while metallic nanostructures with a chiral shape direct the currents arising from the surface-electron gas, the interplay between electric and magnetic dipole moments in dielectric nanostructures affects the generation of chiral light. This information provides a platform from which researchers can improve the rational design of nanophotonic structures for the optimized enhancement of chiral light–matter interactions.

**Author Contributions:** Conceptualization, L.V.P., J.A.D. and A.G.-E.; methodology, L.V.P., J.A.D. and A.G.-E.; software, L.V.P.; validation, L.V.P., J.A.D. and A.G.-E.; formal analysis, L.V.P. and A.G.-E.; investigation, L.V.P., J.A.D. and A.G.-E.; resources, L.V.P., J.A.D. and A.G.-E.; data curation, L.V.P.; writing—original draft preparation, L.V.P.; writing—review and editing, L.V.P., J.A.D. and A.G.-E.; visualization, L.V.P.; supervision, J.A.D. and A.G.-E.; project administration, J.A.D. and A.G.-E.; funding acquisition, L.V.P., J.A.D. and A.G.-E.

**Funding:** This research was supported by the Swiss National Science Foundation Early Postdoc.Mobility Fellowship, project number P2EZP2\_181595, Eusko Jaurlaritza, gran<sup>t</sup> numbers PI-2016-1-0041, KK-2017/00089, IT1164-19 and KK-2019/00101, Ministerio de Economia, Industria y Competitividad, Gobierno de Espana gran<sup>t</sup> number FIS2016-80174-P. A.G.-E. was funded by the Fellows Gipuzkoa fellowship of the Gipuzkoako Foru Aldundia through FEDER "Una Manera de hacer Europa".

**Acknowledgments:** The authors thank David J. Norris, Lukas Novotny, Christian Hafner, Mark Lawrence, John Abendroth, Michelle Solomon, Jack Hu, David R. Barton III, Elissa Klopfer and Shing-Shing Ho for helpful scientific feedback. We thank Shing-Shing Ho for contributions to artistic rendering.

**Conflicts of Interest:** The authors declare no conflict of interest.
