Chirality plays an essential role in life, providing unique functionalities to a wide range of biomolecules, chemicals and drugs, which makes chiral sensing and analysis critically important. Indeed, the detection and differentiation of enantiomers in small quantities are crucially important in many scientific fields, including biology, chemistry, and pharmacy. Chiral molecules manifest their handedness in their interaction with the chiral state of light (e.g., circularly polarized light), which is commonly leveraged in circular dichroism (CD) spectroscopy.
The wider application of chiral sensing continues to be constrained by the involved chiral signals being inherently weak. Compared to the linear refractive index, molecular chirality is extremely weak, resulting in low detection efficiencies. Recently, it has been shown that these weak chiroptical signals can be enhanced by increasing the optical chirality of the electromagnetic fields interacting with chiral samples.
Our ongoing research effort in this theme [1-3] is to conceive, design and demonstrate plasmonic [4] and dielectric nanostructures capable of detecting week chiral signals. We have recently reported the first analytical treatment of CD enhancement and extraction from a chiral bio-layer placed on top of a nanostructured substrate [1]. We have derived closed-form expressions of the CD and its functional dependence on the background-chiroptical response, substrate thickness and chirality, as well as on the optical chirality and intensity enhancement provided by the structure. Our results provide key insights into the tradeoffs that are to be accommodated in the design and conception of optimal nanophotonic structures for enhancing CD effects for chiral molecule detection. Based on our analysis, we introduced new, simple platforms for chiral sensing featuring large CD enhancements while exhibiting vanishing chiroptical background noise.Furthermore, we have shown numerically and analytically that dielectric structures can provide an optimum chiral sensing platform by offering uniform superchiral near-fields [2]. For instance, in simple dielectric dimers, circularly polarized light can induce parallel and out of phase electric and magnetic fields, but spectrally and spatially overlapped, thereby producing superchiral fields at the midpoint of the dimer. This behavior is in contrast to, for example, plasmonic dimers, where the optical chirality is limited by the electric dipolar field, which is not completely out of phase with the incident magnetic field. With the insights gained from this analysis, we developed [2] approaches for overlapping electric and magnetic fields in a single particle, based on Kerker effect. In particular, we introduced a Kerker-inspired metasurface consisting of holey dielectric disks, offering uniform and accessible superchiral near-fields with CD signal enhancements of nearly 24 times.
