The Role of Plasmon-Generated Near Fields for Enhanced Circular Dichroism Spectroscopy

Plasmon-enhanced circular dichroism has established itself as a promising candidate to push the limits of molecular handedness detection to the extremes, namely, toward a monolayer or even to a single molecule. A multitude of intricate mechanisms, both chemical and physical, have to contribute individually or in unison to an enhancement that is large enough that it may bridge the several orders of magnitude of lacking signal strength when detecting small analyte quantities in a circular dichroism scheme. Here, we assess in isolation the contribution arising from electromagnetic interactions between a homogeneous chiral medium and plasmonic structures. Using a suitably modified full-field electromagnetic simulation environment, we are able to investigate the viability of various canonical achiral and chiral plasmonic configurations for substrate-enhanced chiroptical spectroscopy. A clear hierarchy in enhancement factors is revealed that places achiral plasmonic gap antennas at the top, thus outperforming its chiral equivalent, the Born–Kuhn-type plasmonic dimer. Moreover, the importance of coplanarity of the incident rotating circular polarization field vector with the resonantly enhanced field vector in the plasmonic hot-spot is demonstrated. Taking everything into account, we obtain an enhancement of 3 orders of magnitude from purely electromagnetic interactions, thereby charting this part of the CD enhancement landscape.