Enantiospecific Optical Enhancement of Chiral Sensing and Separation with Dielectric Metasurfaces

Abstract

Circularly polarized light (CPL) exhibits an enantioselective interaction with chiral molecules, providing a pathway toward all-optical chiral resolution. High index dielectric nanoparticles have been shown to enhance this relationship, but with a spatially varying sign (or enantiospecificity) that yields a near zero spatially averaged enhancement. Using full field electromagnetic simulations, we design metasurfaces consisting of high index dielectric disks that provide large-volume, uniform-sign enhancements in both the optical density of chirality, C (the figure of merit for sensing and spectroscopy), and Kuhn’s dissymmetry factor, g (the figure of merit for separation). By varying disk radius, we achieve local enhancements in C and g up to 138-fold and 15-fold, respectively, as well as volumetric enhancements of 30-fold and 4.2-fold. The uniform-sign enhancements in C occur near the first Kerker condition, where overlapping electric and magnetic modes maximize field strength and preserve the π/2 phase lag between the electric and magnetic fields of CPL; in contrast, uniform-sign enhancements in g occur with spectrally separated modes, where fields and phase remain optimal without reduced molecular absorption. Using first-order kinetics of the molecule thiocamphor, we show how this optically enantiopure metasurface could enable 20% enantiomeric excesses with a >2000-fold increase in yield for a photoionization reaction compared to CPL alone.

Optical Helicity and Optical Chirality in Free Space and in the Presence of Matter

“…The interaction between chiral electromagnetic plane waves, such as circularly polarized light (CPL), and matter is inherently limited in sensitivity due to their bounded spatial distribution. Rapidly-evolving research efforts in the field of chiral nanophotonics aim to address this challenge by the tailored design of metallic [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and dielectric [29][30][31][32][33][34][35][36][37][38] nanostructures, arranged periodically in sub-wavelength metamaterials and metasurfaces, or in colloidal dispersions, achieving highly concentrated electromagnetic chirality in their evanescent field (see also review articles [25,[39][40][41][42]). However, the rational design of enhanced electromagnetic chirality in the presence of matter requires the definition of physical observables by which to quantify the chirality of light.…”

Helicity-Preserving Optical Cavity Modes for Enhanced Sensing of Chiral Molecules

“…11 also shows that the achiral nanostructure should be helicity-preserving, such that the near-field as well as the scattered far-field have the same handedness as the incident light. Along these lines, systems for enhancing CD signals have been reported [10][11][12][13][14]. However, the spatial region of enhancement was restricted to only parts of the optical near-field which extended over a few hundreds of nanometers or less.…”

Enantioselective manipulation of single chiral nanoparticles using optical tweezers

“…However, due to the weak magnitude of optical forces, optical enantioselective meth- * r.ali@if.ufrj.br ods are typically limited to particles larger than proteins and molecules of biological and pharmaceutical interest. Plasmonic tweezers and metasurfaces can increase the magnitude of optical forces and for this reason they were applied to achieve enantioselection of nanometer-sized particles [27,28]. Optical forces on chiral objects can also lead to interesting effects such as pulling forces [29,30] and left-handed torques [29].…”