Microscopic origin of the chiroptical response of optical media
Matthew S. Davis, Wenqi Zhu, Jay K. Lee, Henri J. Lezec, Amit K. Agrawal
The potential for enhancing the optical activity of natural chiral media using engineered nanophotonic components has been central in the quest towards developing next-generation circular-dichroism spectroscopic techniques. Through confinement and manipulation of optical fields at the nanoscale, ultrathin flat optical elements composed of an array of metallic or dielectric nanostructures have enabled a path towards achieving orders of magnitude enhancements in the chiroptical response. Here, we develop a theoretical framework based on coupled electron-oscillators to describe the underlying physics governing the origin of chiroptical response in optical media. The model identifies optical activity to fundamentally originate from electromagnetic coupling to the hybridized eigen-states of a coupled electron- oscillator system, whereas differential near-field absorption of opposite handedness light, though resulting in a far-field chiroptical response, is shown to have incorrectly been identified as optical activity. The model highlights the common microscopic origin of three distinct chiroptical phenomena, and unifies them under a single theoretical framework. We further validate the model predictions using experimental measurements, and show it to also be consistent with observations in the literature. The work provides a generalized theoretical framework for the design and study of chiroptical systems.