Achiral, Helicity Preserving, and Resonant Structures for Enhanced Sensing of Chiral Molecules

We derive a set of design requirements that lead to structures suitable for molecular circular dichroism (CD) enhancement. Achirality of the structure and two suitably selected sequentially incident beams of opposite helicity ensures that the CD signal only depends on the chiral absorption properties of the molecules, and not on the achiral ones. Under this condition, a helicity preserving structure, which prevents the coupling of the two polarization handednesses, maximizes the enhancement of the CD signal for a given ability of the structure to enhance the field. When the achirality and helicity preservation requirements are met, the enhancement of the CD signal is directly related to the enhancement of the field. Next, we design an exemplary structure following the requirements. The considered system is a planar array of silicon cylinders under normally incident plane-wave illumination. Full-wave numerical calculations show that the enhancement of the transmission CD signal is between 6.5 and 3.75 for interaction lengths between 1.25 and 3 times the height of the cylinders.