Microscopic origin of the chiroptical response of optical media.

The potential for enhancing the optical activity of natural chiral media using engineered nanophotonic components has been central in the quest toward developing next-generation circular-dichroism spectroscopic techniques. Through confinement and manipulation of optical fields at the nanoscale, ultrathin optical elements have enabled a path toward achieving order-of-magnitude enhancements in the chiroptical response. Here, we develop a model framework to describe the underlying physics governing the origin of the chiroptical response in optical media. The model identifies optical activity to originate from electromagnetic coupling to the hybridized eigenstates of a coupled electron-oscillator system, whereas differential absorption of opposite handedness light, though resulting in a far-field chiroptical response, is shown to have incorrectly been identified as optical activity. We validate the model predictions using experimental measurements and show them to also be consistent with observations in the literature. The work provides a generalized framework for the design and study of chiroptical systems.