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The sensing of chiral molecules is important for chemical, pharmaceutical, and medical applications. The determination of the relative concentration of the two molecular mirror versions (enantiomers) in a given mixture is of particular importance for several reasons, in particular because the two enantiomers can have very different biological effects. This task can be achieved by circular dichroism (CD), the normalized difference between the absorption of incident left- and right-handed circularly polarized light. The molecular CD signal is typically weak, and many different kinds of nanostructures have been proposed for enhancing it. Most of them provide local enhancements only in electromagnetically small near-field regions attached to the material structures, resulting in vanishing total enhancements when experimentally meaningful analyte volumes are considered. In this talk, I will present the design of a cavity composed of two parallel arrays of silicon disks that allows to enhance the total CD signal by more than two orders of magnitude for a given molecule concentration and given thickness of the cell containing the molecules. I will show that the underlying principle is helicity-preserving first-order diffraction into helicity-preserving modes with large transverse momentum and long lifetimes. In sharp contrast, in a conventional Fabry-Perot cavity, each reflection flips the handedness of light, leading to large intensity enhancements inside the cavity, yet to smaller CD signals than without the cavity.
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